PEDIATRIC HEAD CT EXPOSURE DOSES IN TUNISIA: A PILOT STUDY TOWARDS THE ESTABLISHMENT OF NATIONAL DIAGNOSTIC REFERENCE LEVELS

PEDIATRIC HEAD CT EXPOSURE DOSES IN TUNISIA: A PILOT STUDY TOWARDS THE ESTABLISHMENT OF NATIONAL... Abstract The purpose of this study was to assess and analyze the radiation doses during head pediatric CT from different CT units within six Tunisian hospitals representing different geographic regions in order to optimize the dose given and minimize the radiology risk to this category of patients and towards the derivation of national diagnostic reference levels. Patient data and exposure parameters were collected for four age groups (<1, 1–5, 5–10 and 10–15 y). Clinical protocols and exposure settings were analyzed. Doses were collected in terms of CTDIvol and DLP values. Effective and Organ doses to specific radiosensitive organs were estimated using the Monte Carlo simulation software ‘Impact CTDosimetry’. Results showed large variations in CT protocols and doses between different radiology departments. CTDIvol and DLP values demonstrated a broad range between the CT units and between the axial and helical scan techniques in the same unit. CTDI vol values were estimated to be 24.9, 31.7, 45.5 and 47.8 mGy for <1, 1–5, 5–10 and 10–15 y age groups, respectively. In term of DLP, median values were ~346, 528, 824, 897 mGy cm for the same age groups, respectively. Effective dose ranged from 1.4 to 5 mSv. Dose values were comparable with those reported in the literature. The study shows an evident need for continuous training of staff in radiation protection concepts, especially within the regional hospitals, emphasizes the importance of the production and the update of recommendations and good practice guidelines using interdisciplinary working groups and opens the way for the establishment of national DRLs. INTRODUCTION Multidetector row computed tomography (MDCT) scanners have contributed to a substantial increase in the diagnostic applications and frequency of computed tomography (CT) examinations. It was estimated that more than 62 million CT scans per year take place in the USA(1), including at least 4 million in pediatrics. Thus, higher doses in CT examinations compared with other X-ray diagnostic procedures have raised concerns about patient doses and safety. Justification and optimization principles of radiation protection are particularly important in pediatric patients, because children are more sensitive to radiation exposure than adults, and have a long potential lifespan in which radiation-induced diseases may appear decades later. As a result, the risk for developing a radiation related cancer can be several times higher for a young child compared with an adult exposed to an identical CT scan(2, 3). Therefore, it is important to evaluate radiation exposure in children in order to ensure that pediatric CT doses are kept to a minimum whilst maintaining the clinical effectiveness. At the core of optimization is the establishment of diagnostic reference levels (DRLs), first proposed by the International Commission on Radiation Protection (ICRP) in 1996(4) and subsequently introduced into European legislation(5) and International Basic Safety Standards(6). DRLs allow the identification of abnormally high dose levels by setting an upper threshold, which standard dose levels should not exceed when good practice is applied. Excessive doses in CT are not as readily identified through image quality affects, as in standard film-based radiography. Thus, an awareness of typical dose levels allows CT users to quickly identify and address any protocols which do not meet the ALARA (as low as reasonably achievable) principle, thus improving radiological practice(7). Current research recommends that specific protocols be tailored for pediatrics and that over scanning and multi-phase scanning must be avoided(8). Also, the improvement in patient dose optimization has been highlighted by the implementation of automatic exposure control systems in CT scanners(7). Recent years, radiation protection in pediatric CT received increased attention in international medical community. In Tunisia, the first actions were initiated under the IAEA project on strengthening of radiological protection of patients and medical exposure control. Nonetheless, the results of this study have mostly been interested with a global description of frequency of pediatric CT examinations and the magnitude of CT doses(9). Currently, there is no documented evidence related to Tunisian pediatric CT practice with respect to protocols and how these are applied; neither on the frequency of pediatric CT examinations or similar national DRLs and information as to how protocols are applied across different age groups for pediatric imaging. This lack of information regarding CT dose values is an obvious deficit regarding the CT exposure-associated risks in Tunisian children especially in the regional hospitals within the country that have fewer human resources specifically trained in terms of patient radiation protection compared to the university hospitals in the capital where we have some knowledge about local DRLs(10, 11). Head CT is the most frequent CT exam in pediatrics and most often in a traumatic context. During acquisition eye lens and thyroid, which are radiosensitive tissues, are irradiated directly and/or indirectly. An understanding of patient doses requires, likewise, the evaluation of organ and effective doses since they highlight the magnitude of risk in CT examination of children(12, 13). They are designed to provide a measure of overall radiation detriment due to stochastic effects and are to be used for prospective dose assessment to facilitate planning and optimization(12, 13). Thus, Monte Carlo (MC) simulations are often used to estimate dose distributions and assess organ doses for individual patients of various shapes and sizes because of an absence of techniques for measuring the dose directly from patient(14). For these reasons, we sought to assess the practice of pediatric CT in Tunisia in different hospitals within the country, including regional and university hospitals that perform pediatric head CT examinations. In this pilot study, pediatric head CT protocols were evaluated. CT dose index (CTDIvol), dose length product (DLP), effective doses as well as organ doses were assessed and the results were compared to those published from other countries. Detailed dosimetry data as presented in the current study are important for providing patient exposure data for optimization, formulation of national DRLs. MATERIALS AND METHODS Participating hospitals This pilot study was started at the end of 2015. The participating hospitals were selected from those from the public sector operating CT scanners. The Tunisian health infrastructure is divided into seven regional health administrations providing health care in the corresponding geographical regions of the country (Figure 1). The participating hospitals were equipped with modern MDCT scanners. Figure 1. View largeDownload slide Distribution of University hospitals and Regional hospitals in the country as well as the participating hospitals. Figure 1. View largeDownload slide Distribution of University hospitals and Regional hospitals in the country as well as the participating hospitals. The Tunisian University Hospitals are located mainly in two regions (Figure 1): surrounding the capital (district of Tunis) and in the center (Middle East) having a very high level of health care equipment and specialized human resources. Within the country, each regional hospital is equipped with a CT unit. According to the latest statistics of 2015 from the Tunisian Ministry of Health(15), there are 164 CT installations, 26% of which in the public sector. Taking into account the low number of pediatric CT examinations, this study was focused on head CT which is the most common one in the regional hospitals in the country with many clinical indications such as trauma, which is the most frequent one, epilepsy, hydrocephalus and other neurological pathologies. The study consisted of (1) the collection of patient data; (2) dose measurements and (3) dose simulation and data analysis. Patient data collection Due to the lack of medical physicists in the hospitals departments of diagnostic radiology, this study was conducted by the medical physicists of the National Center of Radiation Protection in collaboration with the University and pediatric radiologists in Children Hospital of the capital city. The study aimed firstly to gather the specifications of the CT equipment of the selected sites, Secondly, to collect CT review data for children of both genders after the hospital’s management authorization and to classify them in four age groups: <1, 1–5 , 5–10 and 10–15 y. Data collected for each patient were: the patient demographics (age, gender), technical settings (kVp, mAs, pitch, slice thickness, rotation time, scan length…) and the displayed CTDIvol and DLPs. CT dose measurement Dose measurements for the determination of CTDIvol were performed using a CT reference polymethylmethacrylate (PMMA) 16 cm diameter phantom representing the head of a child. The phantom was placed at the isocentre of the CT scanner together with a calibrated pencil-type ionization chamber RaySafe Xi with 10 cm sensitive length. Measurements were performed using a single axial slice with the clinical routine head examination protocol, following the recognized protocols for CT dosimetry(16, 17). The measured CTDIvol was compared to the displayed values. CT dose simulation CT organ doses and effective doses were obtained using the ImPACT CTDosimetry software package ver. 1.0.4 (27/05/2011)(18) which uses CT dosimetry data generated by the National Radiological Protection Board (NRPB)(19–21). Scanning parameters such as kV, mA, exposure time, pitch, slice thickness, and start and end positions of each scan were used as input data to the Impact CT spreadsheet in organ dose estimations.The effective dose was calculated using the tissue weighting factors from the Publication 103 of the ICRP(12). Matching the Neusoft and GE Brightspeed Excel scanner data in the ImPACT spreadsheet The ImPACT group has provided data for the commonly used models of CT scanners; for other scanners, the ImPACT developed a method by matching the dosimetric characteristics of the scanner models to those scanner models covered by the NRPB data sets(19, 20). In this study, the method of matching new models of CT scanners is used to add the Neusoft 64 slices (Neuviz) scanner manufacturer and the GE Brightspeed Excel (4 slices) scanner model, which are not included in the impact CT Dosimetry spreadsheet. The method of matching these two scanner models in ImPACT is based on the use of an Impact Factor (ImF) defined by the following equation(18):   ImF=a(CTDIcCTDIair)+b(CTDIpCTDIair)+c where the center and periphery CTDI values (CTDIc, CTDIp) are measured in the CT dosimetry phantoms at a particular tube voltage and a = 0,4738, b = 0,8045 and c = 0,0752 using a 10 mm beam collimation. The ImF is then matched to the closest ImF for a scanner model in the original Monte Carlo data set. For a new scanner manufacturer, the HVL is used as secondary matching criteria. Measured CTDI and NRPB MC data sets for the total normalized organ dose for the scanned volume were used to evaluate the CT organ doses and the effective dose. Validation of ImPACT CT Dosimetry software for pediatric patients The NRPB data sets utilized by ImPACT were generated using a mathematical phantom representing an adult. Pediatric modifying factors(22) derived from Monte Carlo modeling using the ORNL Cristy and Eckerman (1987) mathematical phantoms representing children were applied when calculating effective dose. On another hand, limited literature is available on the validation of the software for pediatric examinations. Most of them have been undertaken for adult CT examinations by using direct measurement methods on physical phantoms(23). For pediatric CT, Fuji et al.(24) performed dose measurements from both head and chest CT examinations using a radiophotoluminescence glass dosemeter (RGD) system placed on an anthropomorphic pediatric phantom of 1 year old. Similarly, McDermott et al.(25) used an anthropomorphic phantom representing a 5-year-old child to determine organ doses at specific surface and internal locations for head and chest CT protocols on a 64 MDCT scanner with TLDs placed in selected holes for each organ position. To evaluate and validate the efficiency of this software for pediatric patients, we performed a simulation of these measurements for pediatric head CT examination with ImPACT software using the same scan parameters which were undertaken in these studies. The organ doses were determined for the different sensitive organ belonged to the head scan field: eye lens, brain and thyroid. The results were then compared to those obtained by the direct measurements of these studies(24, 25). RESULTS By the end of 2016, six public institutions representing four out of all seven regional health administrations were audited (Figure 1). Three of them were university hospitals, and other three were regional. The private sector was not included in this pilot study. CT equipment data and verification of the CT dose displays The operated CT scanners were from three CT vendors (Siemens, GE and Neusoft) with an average age of 7 y (±4 y) from which 50% were 64-detector CTs, and all were equipped with automatic tube current modulation (ATM). In general, maintenance and CT image Quality control is provided by the local supplier under contract. For all the audited CT installations, the normalized measured CTDIvol (mGy/100 mAs) values and displayed values, as well as the ratio (measured/ displayed) are given in Table 1. The results demonstrate a good agreement between the displayed and the measured dosimetric quantities with a ratio close to 1. Table 1. Characteristic performance parameters of the CT systems used. Hospital  Scanner Model  Number of detector rows  ATM  Normalized CTDIvol (mGy/100 mAs)  Measured  Displayed  Ratio  UH1  SIEMENS Somatom Emotion  6  Yes  16,1  16,3  0,99  UH2  GE BrightSpeed Elite  16  Yes  14,4  15,6  0,93  UH3  SIEMENS Definition AS+  128  Yes  14,5  14,0  1,04  RH4  NEUSOFT  64  Yes  18,1  18,2  1,00  RH5  GE BrightSpeed Excel  4  Yes  18,6  19,6  0,95  RH6  NEUSOFT  64  Yes  20,2  19,9  1,02  Hospital  Scanner Model  Number of detector rows  ATM  Normalized CTDIvol (mGy/100 mAs)  Measured  Displayed  Ratio  UH1  SIEMENS Somatom Emotion  6  Yes  16,1  16,3  0,99  UH2  GE BrightSpeed Elite  16  Yes  14,4  15,6  0,93  UH3  SIEMENS Definition AS+  128  Yes  14,5  14,0  1,04  RH4  NEUSOFT  64  Yes  18,1  18,2  1,00  RH5  GE BrightSpeed Excel  4  Yes  18,6  19,6  0,95  RH6  NEUSOFT  64  Yes  20,2  19,9  1,02  Table 1. Characteristic performance parameters of the CT systems used. Hospital  Scanner Model  Number of detector rows  ATM  Normalized CTDIvol (mGy/100 mAs)  Measured  Displayed  Ratio  UH1  SIEMENS Somatom Emotion  6  Yes  16,1  16,3  0,99  UH2  GE BrightSpeed Elite  16  Yes  14,4  15,6  0,93  UH3  SIEMENS Definition AS+  128  Yes  14,5  14,0  1,04  RH4  NEUSOFT  64  Yes  18,1  18,2  1,00  RH5  GE BrightSpeed Excel  4  Yes  18,6  19,6  0,95  RH6  NEUSOFT  64  Yes  20,2  19,9  1,02  Hospital  Scanner Model  Number of detector rows  ATM  Normalized CTDIvol (mGy/100 mAs)  Measured  Displayed  Ratio  UH1  SIEMENS Somatom Emotion  6  Yes  16,1  16,3  0,99  UH2  GE BrightSpeed Elite  16  Yes  14,4  15,6  0,93  UH3  SIEMENS Definition AS+  128  Yes  14,5  14,0  1,04  RH4  NEUSOFT  64  Yes  18,1  18,2  1,00  RH5  GE BrightSpeed Excel  4  Yes  18,6  19,6  0,95  RH6  NEUSOFT  64  Yes  20,2  19,9  1,02  CT scan parameters Table 2 summarizes the CT acquisition parameters (kV, mAs, slice thickness, pitch, scan length) collected from a total of 482 pediatric CT head examinations. The table shows the number of patient data collected in each hospital (from 52 in UH3 to 151 in UH1). The number of patients in different age groups varied between minimum 11 and 60. Table 2. Summary of acquisition parameters for pediatric head CT examination in all participating hospitals. Hospital  Age group (y)  Number of patients  Technical parameters  Scan mode  kV  mAs  Scan length (cm)  Rotation time (s)  Slice thickness (mm)  Pitch  UH1  <1  22  Helical  110  (a) 114  14,5  0,6  3  0,8  1–5  60  134  17,7  5–10  41  144  17,7  10–15  28  152  17,7  UH2  <1  11  Axial Helical  120  (b) 149  13,5  1  5 1,25  0,562  1–5  27  161  13,8  5–10  20  169  15,5  10–15  19  217  15,5  UH3  <1  12  Axial Helical  120100  (a) 276  13,5  1  1,5–2–3  0,8  1–5  12  339  15,6  5–10  13  295  20,8  10–15  15  300  17,6  RH4  <1  —  helical  80-100-120  —  —  0,8–1  2–5  0,8  1–5  32  399,4  16,5  5–10  16  399,4  15,7  10–15  11  399,4  15,8  RH5  <1  11  Axial Helical  120–140  (b) 200  11,8  0,7  1,25–2,5–5  0,75  1–5  23  286  12,8  0,7–1  5–10  21  317  8,5  0,5–1  10–15  20  318  7,6  0,7–1  RH6  <1  13  Axial Helical  100–120  180  12  0,6–0,8  5 2–3  0,8–0,9  1–5  19  271  16  0,8–1  5–10  14  336  16,6  0,8–1  10–15  22  324  17,7  0,8–1  Hospital  Age group (y)  Number of patients  Technical parameters  Scan mode  kV  mAs  Scan length (cm)  Rotation time (s)  Slice thickness (mm)  Pitch  UH1  <1  22  Helical  110  (a) 114  14,5  0,6  3  0,8  1–5  60  134  17,7  5–10  41  144  17,7  10–15  28  152  17,7  UH2  <1  11  Axial Helical  120  (b) 149  13,5  1  5 1,25  0,562  1–5  27  161  13,8  5–10  20  169  15,5  10–15  19  217  15,5  UH3  <1  12  Axial Helical  120100  (a) 276  13,5  1  1,5–2–3  0,8  1–5  12  339  15,6  5–10  13  295  20,8  10–15  15  300  17,6  RH4  <1  —  helical  80-100-120  —  —  0,8–1  2–5  0,8  1–5  32  399,4  16,5  5–10  16  399,4  15,7  10–15  11  399,4  15,8  RH5  <1  11  Axial Helical  120–140  (b) 200  11,8  0,7  1,25–2,5–5  0,75  1–5  23  286  12,8  0,7–1  5–10  21  317  8,5  0,5–1  10–15  20  318  7,6  0,7–1  RH6  <1  13  Axial Helical  100–120  180  12  0,6–0,8  5 2–3  0,8–0,9  1–5  19  271  16  0,8–1  5–10  14  336  16,6  0,8–1  10–15  22  324  17,7  0,8–1  aEffective mAs. bmA. Table 2. Summary of acquisition parameters for pediatric head CT examination in all participating hospitals. Hospital  Age group (y)  Number of patients  Technical parameters  Scan mode  kV  mAs  Scan length (cm)  Rotation time (s)  Slice thickness (mm)  Pitch  UH1  <1  22  Helical  110  (a) 114  14,5  0,6  3  0,8  1–5  60  134  17,7  5–10  41  144  17,7  10–15  28  152  17,7  UH2  <1  11  Axial Helical  120  (b) 149  13,5  1  5 1,25  0,562  1–5  27  161  13,8  5–10  20  169  15,5  10–15  19  217  15,5  UH3  <1  12  Axial Helical  120100  (a) 276  13,5  1  1,5–2–3  0,8  1–5  12  339  15,6  5–10  13  295  20,8  10–15  15  300  17,6  RH4  <1  —  helical  80-100-120  —  —  0,8–1  2–5  0,8  1–5  32  399,4  16,5  5–10  16  399,4  15,7  10–15  11  399,4  15,8  RH5  <1  11  Axial Helical  120–140  (b) 200  11,8  0,7  1,25–2,5–5  0,75  1–5  23  286  12,8  0,7–1  5–10  21  317  8,5  0,5–1  10–15  20  318  7,6  0,7–1  RH6  <1  13  Axial Helical  100–120  180  12  0,6–0,8  5 2–3  0,8–0,9  1–5  19  271  16  0,8–1  5–10  14  336  16,6  0,8–1  10–15  22  324  17,7  0,8–1  Hospital  Age group (y)  Number of patients  Technical parameters  Scan mode  kV  mAs  Scan length (cm)  Rotation time (s)  Slice thickness (mm)  Pitch  UH1  <1  22  Helical  110  (a) 114  14,5  0,6  3  0,8  1–5  60  134  17,7  5–10  41  144  17,7  10–15  28  152  17,7  UH2  <1  11  Axial Helical  120  (b) 149  13,5  1  5 1,25  0,562  1–5  27  161  13,8  5–10  20  169  15,5  10–15  19  217  15,5  UH3  <1  12  Axial Helical  120100  (a) 276  13,5  1  1,5–2–3  0,8  1–5  12  339  15,6  5–10  13  295  20,8  10–15  15  300  17,6  RH4  <1  —  helical  80-100-120  —  —  0,8–1  2–5  0,8  1–5  32  399,4  16,5  5–10  16  399,4  15,7  10–15  11  399,4  15,8  RH5  <1  11  Axial Helical  120–140  (b) 200  11,8  0,7  1,25–2,5–5  0,75  1–5  23  286  12,8  0,7–1  5–10  21  317  8,5  0,5–1  10–15  20  318  7,6  0,7–1  RH6  <1  13  Axial Helical  100–120  180  12  0,6–0,8  5 2–3  0,8–0,9  1–5  19  271  16  0,8–1  5–10  14  336  16,6  0,8–1  10–15  22  324  17,7  0,8–1  aEffective mAs. bmA. For head CT for pediatric trauma and agitated children, helical scanning was predominantly used in all hospitals because of the need of a faster acquisition mode. But in UH2, UH3 and RH6 some pediatric head CT exams were performed with axial technique. Constant kV values of 110 and 120 were used across all age groups in UH1 and UH2 respectively, while a range of kV values (80–140) was used in the other hospitals. Table 2 also shows a significant difference between hospitals in term of tube current (mAs), pitch and slice thickness. The lowest range of mAs values was found in the UH1 and the highest one in the RH4 with a fixed value of 399.4 mAs applied for all children age. Patient dose Table 3 provides details of the descriptive statistics of median doses (CTDIvol, DLP) from head examinations for children arranged in four age groups: <1, 1–5, 5–10 and 10–15 y. Table 3. Statistics CTDIvol (mGy) and DLP (mGy cm) values from pediatric head CT examinations. Age group  Dose quantity  UH1  UH2  UH3  RH4  RH5  RH6  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  <1  CTDIvol  20,5  6,7–34,2  27,8  27,6–49,6  24,8  16,3–27,4  —  —  26,3  25,8–44,6  1 305 396  10,5–15,6  DLP  342  100–1007  473  331–1227  346  239–909      341  289–803    126–667  1–5  CTDIvol  25,1  11,2–34,8  31,2  23,0–70,1  32,1  26,1–47,3  52,8  11,8–59,4  43,1  35–101,7  20,2  14,8–48,7  DLP  474  405–1326  494  110–1263  767  466–1628  1015  497–2992  561  399–1249  398  247–1104  5–10  CTDIvol  27,1  22,1–33,9  37,3  28,7–81,8  42  30,4–48,5  52,8  11,8–59,4  75,4  62,2–83,9  49  34,6–56,3  DLP  529  378–1134  628  484–1575  822  681–1428  1103  248–2994  920  509–1487  825  597–1970  10–15  CTDIvol  28,9  23,2–32,8  46,8  29,7–84,7  46  45,4–47,3  52,8  47,6–52,8  75,4  61,0–93,7  48,7  34,6–56,3  DLP  547  422–848  740  475–1640  817  816–1470  1005  843–2049  977  380–2003  978  536–1148  Age group  Dose quantity  UH1  UH2  UH3  RH4  RH5  RH6  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  <1  CTDIvol  20,5  6,7–34,2  27,8  27,6–49,6  24,8  16,3–27,4  —  —  26,3  25,8–44,6  1 305 396  10,5–15,6  DLP  342  100–1007  473  331–1227  346  239–909      341  289–803    126–667  1–5  CTDIvol  25,1  11,2–34,8  31,2  23,0–70,1  32,1  26,1–47,3  52,8  11,8–59,4  43,1  35–101,7  20,2  14,8–48,7  DLP  474  405–1326  494  110–1263  767  466–1628  1015  497–2992  561  399–1249  398  247–1104  5–10  CTDIvol  27,1  22,1–33,9  37,3  28,7–81,8  42  30,4–48,5  52,8  11,8–59,4  75,4  62,2–83,9  49  34,6–56,3  DLP  529  378–1134  628  484–1575  822  681–1428  1103  248–2994  920  509–1487  825  597–1970  10–15  CTDIvol  28,9  23,2–32,8  46,8  29,7–84,7  46  45,4–47,3  52,8  47,6–52,8  75,4  61,0–93,7  48,7  34,6–56,3  DLP  547  422–848  740  475–1640  817  816–1470  1005  843–2049  977  380–2003  978  536–1148  Table 3. Statistics CTDIvol (mGy) and DLP (mGy cm) values from pediatric head CT examinations. Age group  Dose quantity  UH1  UH2  UH3  RH4  RH5  RH6  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  <1  CTDIvol  20,5  6,7–34,2  27,8  27,6–49,6  24,8  16,3–27,4  —  —  26,3  25,8–44,6  1 305 396  10,5–15,6  DLP  342  100–1007  473  331–1227  346  239–909      341  289–803    126–667  1–5  CTDIvol  25,1  11,2–34,8  31,2  23,0–70,1  32,1  26,1–47,3  52,8  11,8–59,4  43,1  35–101,7  20,2  14,8–48,7  DLP  474  405–1326  494  110–1263  767  466–1628  1015  497–2992  561  399–1249  398  247–1104  5–10  CTDIvol  27,1  22,1–33,9  37,3  28,7–81,8  42  30,4–48,5  52,8  11,8–59,4  75,4  62,2–83,9  49  34,6–56,3  DLP  529  378–1134  628  484–1575  822  681–1428  1103  248–2994  920  509–1487  825  597–1970  10–15  CTDIvol  28,9  23,2–32,8  46,8  29,7–84,7  46  45,4–47,3  52,8  47,6–52,8  75,4  61,0–93,7  48,7  34,6–56,3  DLP  547  422–848  740  475–1640  817  816–1470  1005  843–2049  977  380–2003  978  536–1148  Age group  Dose quantity  UH1  UH2  UH3  RH4  RH5  RH6  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  <1  CTDIvol  20,5  6,7–34,2  27,8  27,6–49,6  24,8  16,3–27,4  —  —  26,3  25,8–44,6  1 305 396  10,5–15,6  DLP  342  100–1007  473  331–1227  346  239–909      341  289–803    126–667  1–5  CTDIvol  25,1  11,2–34,8  31,2  23,0–70,1  32,1  26,1–47,3  52,8  11,8–59,4  43,1  35–101,7  20,2  14,8–48,7  DLP  474  405–1326  494  110–1263  767  466–1628  1015  497–2992  561  399–1249  398  247–1104  5–10  CTDIvol  27,1  22,1–33,9  37,3  28,7–81,8  42  30,4–48,5  52,8  11,8–59,4  75,4  62,2–83,9  49  34,6–56,3  DLP  529  378–1134  628  484–1575  822  681–1428  1103  248–2994  920  509–1487  825  597–1970  10–15  CTDIvol  28,9  23,2–32,8  46,8  29,7–84,7  46  45,4–47,3  52,8  47,6–52,8  75,4  61,0–93,7  48,7  34,6–56,3  DLP  547  422–848  740  475–1640  817  816–1470  1005  843–2049  977  380–2003  978  536–1148  There is a broad range of dose indicators between hospitals. The UH1 presents the lowest CTDIvol value with a median values ranging from 20.5 mGy to 28.9 for all age groups, while the highest ones were observed for RH5 with median values ranging from 26.3 to 75.4 mGy. The RH4 presented a constant CTDIvol median value of 52.8 mGy for all age groups. In terms of DLP, this last hospital presents the highest values. As mentioned earlier, in UH2, UH3 ad RH6, axial and helical scan techniques were both used. Figure 2 shows a comparison of CTDIvol from axial and helical scan modes for all patient age groups within the same unit for the three hospitals. The results indicated that there is a wide range between the two techniques with higher values from the helical scan reaching 2 times those obtained from axial scan in both UH2 and RH6. Whereas, it was not the same for UH3 where doses from helical head scanning were lower. Figure 2. View largeDownload slide Obtained dose distribution (CTDIvol) in axial and helical scan mode within UH2, UH3 and RH6. Figure 2. View largeDownload slide Obtained dose distribution (CTDIvol) in axial and helical scan mode within UH2, UH3 and RH6. Estimation of effective dose and organ dose The effective doses per hospital and per age group estimated using the Impact CT software with the ICRP103 weighting factors are displayed in Table 4. The results show that the mean values per age group do not vary widely between hospitals, except for RH6 and RH4 which presented the highest effective doses for all age categories with a maximum median value 5 mSv. Table 4. Median values of patient effective dose (mSv) for different pediatric age groups with using the ICRP103 (E103) definition of effective dose. Age group  UH1  UH2  UH3  RH4  RH5  RH6  <1 y  1,8  2,4  1,9  —  2,5  3,3  1–5 y  2,0  2,2  2,6  5,0  2,5  2,0  5–10 y  1,9  2,2  4,0  4,6  2,7  3,0  10–15 y  1,4  1,9  2,4  2,9  2,6  2,9  Age group  UH1  UH2  UH3  RH4  RH5  RH6  <1 y  1,8  2,4  1,9  —  2,5  3,3  1–5 y  2,0  2,2  2,6  5,0  2,5  2,0  5–10 y  1,9  2,2  4,0  4,6  2,7  3,0  10–15 y  1,4  1,9  2,4  2,9  2,6  2,9  Table 4. Median values of patient effective dose (mSv) for different pediatric age groups with using the ICRP103 (E103) definition of effective dose. Age group  UH1  UH2  UH3  RH4  RH5  RH6  <1 y  1,8  2,4  1,9  —  2,5  3,3  1–5 y  2,0  2,2  2,6  5,0  2,5  2,0  5–10 y  1,9  2,2  4,0  4,6  2,7  3,0  10–15 y  1,4  1,9  2,4  2,9  2,6  2,9  Age group  UH1  UH2  UH3  RH4  RH5  RH6  <1 y  1,8  2,4  1,9  —  2,5  3,3  1–5 y  2,0  2,2  2,6  5,0  2,5  2,0  5–10 y  1,9  2,2  4,0  4,6  2,7  3,0  10–15 y  1,4  1,9  2,4  2,9  2,6  2,9  The comparison between organ doses from RGD(24) and TLD(25) and measurements compared to those obtained from the simulations in this study for pediatric head CT examination are presented in Table 5. Table 5. Organ doses calculated in this study from Impact simulations compared to Organ doses from RGD(24) and TLD(25) measurements from studies(24, 25).   Organ  Dorgan,TLD (mGy)(25)  Dorgan, this study (mGy)  Ratio  Dorgan,RGD (mGy)(24)  Dorgan, this study (mGy)  Ratio  Axial  Brain  32  39  0,82  31,9  32  0,91  Eye lens  39  40  0,98  35,8  39  0,92  Thyroid  46  43  1,07  1,2  1,4  0,8  Helical  Brain  32  38  0,84  —  —  —  Eye lens  33  39  0,85  —  —  —  Thyroid  43  42  1,02  —  —  —    Organ  Dorgan,TLD (mGy)(25)  Dorgan, this study (mGy)  Ratio  Dorgan,RGD (mGy)(24)  Dorgan, this study (mGy)  Ratio  Axial  Brain  32  39  0,82  31,9  32  0,91  Eye lens  39  40  0,98  35,8  39  0,92  Thyroid  46  43  1,07  1,2  1,4  0,8  Helical  Brain  32  38  0,84  —  —  —  Eye lens  33  39  0,85  —  —  —  Thyroid  43  42  1,02  —  —  —  Table 5. Organ doses calculated in this study from Impact simulations compared to Organ doses from RGD(24) and TLD(25) measurements from studies(24, 25).   Organ  Dorgan,TLD (mGy)(25)  Dorgan, this study (mGy)  Ratio  Dorgan,RGD (mGy)(24)  Dorgan, this study (mGy)  Ratio  Axial  Brain  32  39  0,82  31,9  32  0,91  Eye lens  39  40  0,98  35,8  39  0,92  Thyroid  46  43  1,07  1,2  1,4  0,8  Helical  Brain  32  38  0,84  —  —  —  Eye lens  33  39  0,85  —  —  —  Thyroid  43  42  1,02  —  —  —    Organ  Dorgan,TLD (mGy)(25)  Dorgan, this study (mGy)  Ratio  Dorgan,RGD (mGy)(24)  Dorgan, this study (mGy)  Ratio  Axial  Brain  32  39  0,82  31,9  32  0,91  Eye lens  39  40  0,98  35,8  39  0,92  Thyroid  46  43  1,07  1,2  1,4  0,8  Helical  Brain  32  38  0,84  —  —  —  Eye lens  33  39  0,85  —  —  —  Thyroid  43  42  1,02  —  —  —  Figure 3 presents the estimated organ doses for selected organs as eye lens, brain and thyroid in terms of median values per age groups for all participating hospitals. Large variations of organ doses exist among hospitals varying from 16 to 52 mGy for both eye lens and brain with outstanding values in the RH4 and lowest ones in UH1. Figure 3. View largeDownload slide Distribution of organ doses (mGy) per hospital for each patient age group. Figure 3. View largeDownload slide Distribution of organ doses (mGy) per hospital for each patient age group. Table 6 presents the dose indicators values of our study based on the 75th percentiles of the distribution of median values obtained from the facilities compared to those from international studies: IAEA Study(26), Portugal(27), Germany(28), UK(29), Belgium(30) and Switzerland(31). Table 6. Comparison of (CTDIvol, DLP) with previous studies.   Age (y)  This study  IAEA study(26)  Portugal(27)  Germany(28)  UK(29)  Belgium(30)  Switzerland(31)  CTDI (mGy)  <1  26  26  48  33  30  35  20  1–5  40  36  50  40  45  43  30  5–10  52  43  70  50  50  49  40  10–15  52  53  72  65  65  50  60  DLP (mGy cm)  <1  397  440  630  390  270  280  270  1–5  716  540  770  520  470  473  420  5–10  896  690  1100  710  620  637  560  10–15  978  840  1120  920  930  563  1000    Age (y)  This study  IAEA study(26)  Portugal(27)  Germany(28)  UK(29)  Belgium(30)  Switzerland(31)  CTDI (mGy)  <1  26  26  48  33  30  35  20  1–5  40  36  50  40  45  43  30  5–10  52  43  70  50  50  49  40  10–15  52  53  72  65  65  50  60  DLP (mGy cm)  <1  397  440  630  390  270  280  270  1–5  716  540  770  520  470  473  420  5–10  896  690  1100  710  620  637  560  10–15  978  840  1120  920  930  563  1000  Table 6. Comparison of (CTDIvol, DLP) with previous studies.   Age (y)  This study  IAEA study(26)  Portugal(27)  Germany(28)  UK(29)  Belgium(30)  Switzerland(31)  CTDI (mGy)  <1  26  26  48  33  30  35  20  1–5  40  36  50  40  45  43  30  5–10  52  43  70  50  50  49  40  10–15  52  53  72  65  65  50  60  DLP (mGy cm)  <1  397  440  630  390  270  280  270  1–5  716  540  770  520  470  473  420  5–10  896  690  1100  710  620  637  560  10–15  978  840  1120  920  930  563  1000    Age (y)  This study  IAEA study(26)  Portugal(27)  Germany(28)  UK(29)  Belgium(30)  Switzerland(31)  CTDI (mGy)  <1  26  26  48  33  30  35  20  1–5  40  36  50  40  45  43  30  5–10  52  43  70  50  50  49  40  10–15  52  53  72  65  65  50  60  DLP (mGy cm)  <1  397  440  630  390  270  280  270  1–5  716  540  770  520  470  473  420  5–10  896  690  1100  710  620  637  560  10–15  978  840  1120  920  930  563  1000  DISCUSSION In this work, pediatric head CT practices were evaluated in six Tunisian public regional and university hospitals dispersed in the whole regions (Figure 1). This is the first time pediatric CT dose assessment fully based on a nationwide pilot study. A unique feature of the data presented here is the approach for data collection. While most of the countries use questionnaires or web-based databases for analysis(27, 28), patient doses in this work were collected during on-site audits. An advantage of this approach is the verification of the CT scanners dose displays by measurements using the same phantom and dosimetry equipment. Despite the location of the regional CT installations away from the capital with a very low level of specialized human resources, the results showed a good agreement between displayed and measured CTDI due to the maintenance program carried out within the different sites by the CT manufacturer staff (Table 1). This methodology had the further advantage that the current practice could be discussed with the radiologists and the radiographers that provided a solid basis for the assessment of the national radiological practice. A limitation of the study was the restricted time duration of the audit. As a matter of fact, during audits CT scanners were blocked and clinical routine work was interrupted. Also, three regions are missing in this study (North West, Middle West, Middle East) due to the time-limitation and the workload of the team of medical physicists of the National Center of Radiation Protection who performed the audits. Radiation dose from CT mostly depends on the CT acquisition parameters, including tube potential (kVp), tube current (mAs), pitch and the scan length (L). This study revealed a significant variation in acquisition parameters among the radiology departments of each site. The lowest technical parameters were found in the UH1. These low values can be attributed to the fact that this radiology department belongs to a Children Hospital so that all practices and examinations are dedicated to pediatric patients who need special care and attention. It is also evident that significant variations in scan length existed between hospitals for given examinations. The large scan length leads to high doses in term of DLP and organ doses especially for organ positioned at the boundaries of the scan length. These variations were largely caused by the absence of standard imaging protocols, even when using a scanner from the same manufacturer, and by the many different practices involved in these hospitals. Moreover, not all sites in the study apply dedicated scan protocols for children, some of them use adult protocols for all pediatric head CT examination such as RH4. The details of the applied scan protocols indicated a lack of awareness of practitioners in some hospitals regarding exposure parameters and their impact on pediatric patient dose and emphasized the necessity to apply standard clinical guidelines relative to pediatrics CT practices. From the technical parameters that can be selected by the CT operator, those that have a high influence on dose and image quality are the mAs, tube voltage and pitch and they are the main factors of the wide variation in doses between different CT units. The highest values of CTDIvol observed in RH4 and RH5 (Table 3) might be due to the use of high exposure settings (mAs). Moreover, the constant CTDI value found in RH4 for all age groups could be attributed to the fact that technologists have used the same imaging protocol for all children age which is almost that of adult brain CT without making the necessary adjustment of scanning parameters, despite the existence of a ‘Baby brain protocol’ among the brain protocols settings of the Neusoft scanner. In the other hospitals, an increase of CTDI with patient age is noted, this is due to the occasional slight increase in kV and mAs values with children age. Despite the fact that all scanners are equipped with ATM (Table 1), not all hospitals used it for children. Also, even if it is used in some hospitals, for instance RH3, doses remained high. Preliminary studies that applied current modulation with children showed a significant dose reduction of ~30%, depending on the scanned patient region(32). Although these promising developments suggest that CT doses to children could be reduced, the effect in clinical routine is still unclear. Here is manifested the role of the manufacturer in the education and the training of CT practitioners in the capabilities of CT machines. In terms of DLP, as mentioned earlier, the variations between hospitals were largely contributed by inherent differences in the equipment and different scanning protocols used among hospitals. This increase in DLP is due to the use of large scan length by technologists in some hospitals. Since the high head CT examination percentage in the country is due to trauma from the high rate of motor accidents, technologists and radiologists in UH1 usually tend to increase the scan length up to the cervical vertebrae (C1, C2) for clinical needs in case of pediatric head trauma in order to maintain accurate diagnosis. The high DLP values are also due to the effect of the over-ranging which applies only to helical scanning and is taken to be the difference between the exposed scan length and the planned scan length. Over-ranging is the increase in DLP due to the additional rotations at the beginning and at the end of a spiral scan required for data interpolation to reconstruct the first and the last slice of the imaged region. The two most important parameters that affect over ranging are the beam width and the pitch(33). In fact, for MDCT scanners, the number of additional rotations is strongly pitch dependent, with a typical increase in irradiation length of 1.5 times the total nominal beam width(34). Consequently, the setting of strict scan lengths would be required for the reduction of unnecessary exposure to pediatric patients in CT scans(29, 35) with a large beam width and a large pitch factor. It is commonly known that the helical mode leads to higher doses. However, in Figure 2, UH3 presented lower doses from helical scanning compared to axial technique. This could be explained by the fact that the practitioners used practically the same scan parameters for the two scan modes but they tended to increase the kV values from 100 kV for helical examinations to 120 kV for axial ones. In fact, an increase of kV increases the dose by ≈kV2. For pediatric patients, axial scanning is preferred over helical scanning. But if helical scanning mode is required for clinical needs, a careful selection of technical parameters is recommended. In term of effective dose, the high values due to the high DLP values discussed in previous section since there is a strong correlation between the effective dose and DLP which takes into account CT scanning parameters. Our results also show that there is no significant difference in effective dose for all ages of children. According to Huda et al.(36), the variation of pediatric head size is not as readily evident and rises slowly with patient age, except for the youngest infant in the first 2 years of life where head increases rapidly. From the results of Table 5 we can conclude that ImPACT software could be useful to simulate organ doses from head pediatric CT examinations and these doses could be directly determined from the adult phantom of ImPACT software without any corrections. It is expected that some variation in the calculated doses will be evident and unavoidable across the different organ dosimetry methods. Indeed, there are several important influencing factors on the measured and calculated absorbed doses that need to be considered. These include the models used to represent the organs and their position in the body, the type/model of CT scanner being used and/or modeled in the calculation and the scan parameters used in simulating the X-ray exposure. Brady et al.(37) have previously found that for the CT brain examination, the measured and computed doses in brain were all within ~30%. They affirmed that the clinical conditions could not be matched exactly in the computational methods and therefore, various assumptions and approximations had to be made. Thus, conditions that have been used for each simulation provide the best match to the clinical scenario. The anatomical scan regions suggest radiation risk exposure to sensitive organs such as the eye lens and thyroid gland, which increases the probability of eye cataracts and cancer. The eye lens and the brain are usually included either totally or partially in the irradiation field in brain CT examination and as known organs within the scan volume receive higher absorbed doses than those outside the scan volume. To avoid irradiation of the eyes, the head CT examination is normally planned to exclude the eyes from the imaged area. Controlling radiation exposure to the eyes is, hence, important especially in patients who require multiple scans. In this study, the brain dose is specifically higher for RH5 due to the practice itself. The operators performed head CT examination in two or three series including the brain at each irradiation. This practice demonstrates the lack of the operator’s training in radiation protection and specific pediatric protocols. Furthermore, organs on the periphery of the scan volume can have a significant variation in absorbed dose across the organ due to partial irradiation. The highest thyroid absorbed dose is dependent on each individual scan and the selection of collimation as the thyroid may or may not be within the scan volume. Even when it is outside the scan volume it is likely to receive a dose from scattered radiation and over-ranging. As it is shown in Figure 3 thyroid receives a dose during head CT examination within all hospitals, with a median value ranging from 0.5 mGy to a maximum value of 34 mGy. This is mainly due to the large scan length used in some units sometimes for clinical needs as is the case in UH1 and to the over ranging which contributed to the increase in doses to organs and tissues positioned at the boundaries of the scan length(38). The over ranging could easily be avoided by changing the scanning protocol from helical to axial mode. As an overall trend, the CTDI values of the present study are comparable to the data reported from other studies(26, 28–31) and clearly lower than those of the Portuguese study(27). In terms of DLP, our values are comparable the German values(28), lower than the Portuguese ones(27) and slightly higher than those of other countries(29–31). This is mainly due to the values obtained in the regional hospitals. From the whole results of this study, we can deduce that there is a wide variation of the CT doses between the regional hospitals within the country and the university hospitals located at the capital region. This variation is greater if technologists and radiologists are insufficiently knowledgeable in the capabilities of CT machines and in radiation risk due to the scarcity and lack of regional coordination in the delivery of continuous training courses on radiation protection for health professionals using ionizing radiation. This study emphasizes the importance of the production and the update of recommendations and good practice guidelines using interdisciplinary working groups and promotes the establishment and the use of national DRLs based on a national survey including all the regions of the country. CONCLUSION In this pilot study, pediatric radiation dose was investigated for head CT procedures. A large variation is observed among and within hospitals. The main contributor to these variations was the use of different techniques and protocols, for adults in some cases, which shows the importance of using only pediatric protocols for CT examinations in children. Our results were in good compliance with some previous studies and slightly higher than other ones. These variations suggest that pediatric patients are still exposed to a large amount of unnecessary radiation and optimization is not fulfilled. Optimization of protection for pediatric CT procedures should be given a priority and the recommendation of standardizing CT pediatric protocols and image quality needs to be studied at the national level. This reinforces the role of the medical physicists in the field of patient protection and safety during medical exposure. A regular survey and determination of the DRL should be done especially if there are more and new installations are distributed all across the country. The time for survey should allow the implementation of corrective actions to take place after the initial DRLs. A graded approach may be used to select procedures for which DRLs are to be established for children. These actions should be a joint effort among all the stakeholders including health authorities, radiation protection regulators, professional societies and universities and researchers in radiation protection in medicine. Acknowledgements The work was encouraged by the IAEA through the TC project RAF/9/053 ‘Strengthening Technical Capabilities for Patient and Occupational Radiation Protection in Member States’. The authors are grateful to the Abdu Salam International Center for Theoretical Physics (ICTP) in Trieste, Italy, for training the first author in Computed Tomography: Quality Control, Dosimetry and Optimization. The authors express their gratitude to Ms Jenia Vassileva, IAEA’s Radiation Protection Expert, for her encouragement, help and article reviewing. 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IRPA14 Proceedings, pp. 541–544 ( 2017) 12 International Commission on Radiological Protection (ICRP). The 2007 recommendations of the international commission on radiological protection, ICRP Publication 103. Ann. ICRP  37, 1– 332 ( 2007). 13 International Commission on Radiological Protection (ICRP). Radiological protection in medicine, ICRP Publication 105. Ann. ICRP  37( 6), 1– 64 ( 2007). CrossRef Search ADS   14 Xu, G. X. and Eckerman, K. F. Handbook of Anatomical Models for Radiation Dosimetry  ( Florida: CRC Press) ( 2010). 15 Ministry of Public Health, Tunisia. Sanitary Map of Tunisia, 2015 (June 2016) [PDF file]. Available on: http://www.santetunisie.rns.tn/images/docs/anis/stat/cartesanitaire2015.pdf. 16 ImPACT CT Scanner Dose Survey. Measurement Protocol, Version 5.0. St. George’s Healthcare NHS Trust, London, UK ( 1997). 17 International Atomic Energy Agency (IAEA). Dosimetry in Diagnostic Radiology: An International Code of Practice, TRS N° 457, Vienna ( 2007). 18 ImPACT CT Dosimetry Calculator. Version 1.0.4. St. George’s Healthcare NHS Trust, London, UK, 27 May 2011. 19 Jones, D. G. and Shrimpton, P. C. NRPB-R250: Survey of CT practice in the UK. Part 3: Normalised organ doses calculated using Monte Carlo techniques. National Radiological Protection Board, Didcot, Oxforshire, UK, Report SR250 ( 1991). 20 Jones, D. G. and Shrimpton, P. C. NRPB-SR250: Normalised Organ Doses for X-Ray Computed Tomography Calculated Using Monte Carlo Technique  ( Didcot, Oxforshire, UK: National Radiological Protection Board) ( 1993). 21 Shrimpton, P. C., Jones, D. G., Hillier, M. C., Wall, B. F., Le Heron, J. C. and Faulkner, K. NRPB-R249: survey of CT practice in the UK. Part 2: Dosimetric Aspects. National Radiological Protection Board, Didcot, Oxforshire, UK, Report SR250 ( 1991). 22 Khursheed, A., Hillier, M. C., Shrimpton, P. C. and Wall, B. F. 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CT paediatric doses in Belgium: a multi-centre study—results from a dosimetry audit in 2007–2009. Belgium Federal Agency of Nuclear Control ( 2010). 31 Verdun, F., Gutierrez, D., Vader, J., Aroua, A., Alamo-Maestre, L., Bochud, F. and Gudinchet, F. CT radiation dose in children: a survey to establish age-based diagnostic reference levels in Switzerland. Eur. Radiol.  18, 1980– 1986 ( 2008). Google Scholar CrossRef Search ADS PubMed  32 Greess, H., Wolf, H., Kalender, W. A. and Bautz, W. Dose reduction in CT examination of children by an attenuation-based on-line modulation of tube current (CARE Dose). Eur. Radiol.  12( 6), 1571– 1576 ( 2002). Google Scholar CrossRef Search ADS PubMed  33 Tzedakis, A., Damilakis, J., Perisinakis, K., Karantanas, A., Karabekios, S. and Gourtsoyiannis, N. Influence of z overscanning on normalized effective doses calculated for pediatric patients undergoing multidetector CT examinations. Med. Phys.  34, 1163– 117 ( 2007). 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Google Scholar CrossRef Search ADS PubMed  38 Fujii, K., Aoyama, T., Yamauchi-Kawaura, C., Koyama, S., Yamauchi, M., Ko, S., Akahane, K. and Nishizaw, K. Radiation dose evaluation in 64-slice CT examinations with adult and paediatric anthropomorphic phantoms. Br. J. Radiol.  82( 984), 1010– 1018 ( 2009). Google Scholar CrossRef Search ADS PubMed  © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

PEDIATRIC HEAD CT EXPOSURE DOSES IN TUNISIA: A PILOT STUDY TOWARDS THE ESTABLISHMENT OF NATIONAL DIAGNOSTIC REFERENCE LEVELS

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Abstract

Abstract The purpose of this study was to assess and analyze the radiation doses during head pediatric CT from different CT units within six Tunisian hospitals representing different geographic regions in order to optimize the dose given and minimize the radiology risk to this category of patients and towards the derivation of national diagnostic reference levels. Patient data and exposure parameters were collected for four age groups (<1, 1–5, 5–10 and 10–15 y). Clinical protocols and exposure settings were analyzed. Doses were collected in terms of CTDIvol and DLP values. Effective and Organ doses to specific radiosensitive organs were estimated using the Monte Carlo simulation software ‘Impact CTDosimetry’. Results showed large variations in CT protocols and doses between different radiology departments. CTDIvol and DLP values demonstrated a broad range between the CT units and between the axial and helical scan techniques in the same unit. CTDI vol values were estimated to be 24.9, 31.7, 45.5 and 47.8 mGy for <1, 1–5, 5–10 and 10–15 y age groups, respectively. In term of DLP, median values were ~346, 528, 824, 897 mGy cm for the same age groups, respectively. Effective dose ranged from 1.4 to 5 mSv. Dose values were comparable with those reported in the literature. The study shows an evident need for continuous training of staff in radiation protection concepts, especially within the regional hospitals, emphasizes the importance of the production and the update of recommendations and good practice guidelines using interdisciplinary working groups and opens the way for the establishment of national DRLs. INTRODUCTION Multidetector row computed tomography (MDCT) scanners have contributed to a substantial increase in the diagnostic applications and frequency of computed tomography (CT) examinations. It was estimated that more than 62 million CT scans per year take place in the USA(1), including at least 4 million in pediatrics. Thus, higher doses in CT examinations compared with other X-ray diagnostic procedures have raised concerns about patient doses and safety. Justification and optimization principles of radiation protection are particularly important in pediatric patients, because children are more sensitive to radiation exposure than adults, and have a long potential lifespan in which radiation-induced diseases may appear decades later. As a result, the risk for developing a radiation related cancer can be several times higher for a young child compared with an adult exposed to an identical CT scan(2, 3). Therefore, it is important to evaluate radiation exposure in children in order to ensure that pediatric CT doses are kept to a minimum whilst maintaining the clinical effectiveness. At the core of optimization is the establishment of diagnostic reference levels (DRLs), first proposed by the International Commission on Radiation Protection (ICRP) in 1996(4) and subsequently introduced into European legislation(5) and International Basic Safety Standards(6). DRLs allow the identification of abnormally high dose levels by setting an upper threshold, which standard dose levels should not exceed when good practice is applied. Excessive doses in CT are not as readily identified through image quality affects, as in standard film-based radiography. Thus, an awareness of typical dose levels allows CT users to quickly identify and address any protocols which do not meet the ALARA (as low as reasonably achievable) principle, thus improving radiological practice(7). Current research recommends that specific protocols be tailored for pediatrics and that over scanning and multi-phase scanning must be avoided(8). Also, the improvement in patient dose optimization has been highlighted by the implementation of automatic exposure control systems in CT scanners(7). Recent years, radiation protection in pediatric CT received increased attention in international medical community. In Tunisia, the first actions were initiated under the IAEA project on strengthening of radiological protection of patients and medical exposure control. Nonetheless, the results of this study have mostly been interested with a global description of frequency of pediatric CT examinations and the magnitude of CT doses(9). Currently, there is no documented evidence related to Tunisian pediatric CT practice with respect to protocols and how these are applied; neither on the frequency of pediatric CT examinations or similar national DRLs and information as to how protocols are applied across different age groups for pediatric imaging. This lack of information regarding CT dose values is an obvious deficit regarding the CT exposure-associated risks in Tunisian children especially in the regional hospitals within the country that have fewer human resources specifically trained in terms of patient radiation protection compared to the university hospitals in the capital where we have some knowledge about local DRLs(10, 11). Head CT is the most frequent CT exam in pediatrics and most often in a traumatic context. During acquisition eye lens and thyroid, which are radiosensitive tissues, are irradiated directly and/or indirectly. An understanding of patient doses requires, likewise, the evaluation of organ and effective doses since they highlight the magnitude of risk in CT examination of children(12, 13). They are designed to provide a measure of overall radiation detriment due to stochastic effects and are to be used for prospective dose assessment to facilitate planning and optimization(12, 13). Thus, Monte Carlo (MC) simulations are often used to estimate dose distributions and assess organ doses for individual patients of various shapes and sizes because of an absence of techniques for measuring the dose directly from patient(14). For these reasons, we sought to assess the practice of pediatric CT in Tunisia in different hospitals within the country, including regional and university hospitals that perform pediatric head CT examinations. In this pilot study, pediatric head CT protocols were evaluated. CT dose index (CTDIvol), dose length product (DLP), effective doses as well as organ doses were assessed and the results were compared to those published from other countries. Detailed dosimetry data as presented in the current study are important for providing patient exposure data for optimization, formulation of national DRLs. MATERIALS AND METHODS Participating hospitals This pilot study was started at the end of 2015. The participating hospitals were selected from those from the public sector operating CT scanners. The Tunisian health infrastructure is divided into seven regional health administrations providing health care in the corresponding geographical regions of the country (Figure 1). The participating hospitals were equipped with modern MDCT scanners. Figure 1. View largeDownload slide Distribution of University hospitals and Regional hospitals in the country as well as the participating hospitals. Figure 1. View largeDownload slide Distribution of University hospitals and Regional hospitals in the country as well as the participating hospitals. The Tunisian University Hospitals are located mainly in two regions (Figure 1): surrounding the capital (district of Tunis) and in the center (Middle East) having a very high level of health care equipment and specialized human resources. Within the country, each regional hospital is equipped with a CT unit. According to the latest statistics of 2015 from the Tunisian Ministry of Health(15), there are 164 CT installations, 26% of which in the public sector. Taking into account the low number of pediatric CT examinations, this study was focused on head CT which is the most common one in the regional hospitals in the country with many clinical indications such as trauma, which is the most frequent one, epilepsy, hydrocephalus and other neurological pathologies. The study consisted of (1) the collection of patient data; (2) dose measurements and (3) dose simulation and data analysis. Patient data collection Due to the lack of medical physicists in the hospitals departments of diagnostic radiology, this study was conducted by the medical physicists of the National Center of Radiation Protection in collaboration with the University and pediatric radiologists in Children Hospital of the capital city. The study aimed firstly to gather the specifications of the CT equipment of the selected sites, Secondly, to collect CT review data for children of both genders after the hospital’s management authorization and to classify them in four age groups: <1, 1–5 , 5–10 and 10–15 y. Data collected for each patient were: the patient demographics (age, gender), technical settings (kVp, mAs, pitch, slice thickness, rotation time, scan length…) and the displayed CTDIvol and DLPs. CT dose measurement Dose measurements for the determination of CTDIvol were performed using a CT reference polymethylmethacrylate (PMMA) 16 cm diameter phantom representing the head of a child. The phantom was placed at the isocentre of the CT scanner together with a calibrated pencil-type ionization chamber RaySafe Xi with 10 cm sensitive length. Measurements were performed using a single axial slice with the clinical routine head examination protocol, following the recognized protocols for CT dosimetry(16, 17). The measured CTDIvol was compared to the displayed values. CT dose simulation CT organ doses and effective doses were obtained using the ImPACT CTDosimetry software package ver. 1.0.4 (27/05/2011)(18) which uses CT dosimetry data generated by the National Radiological Protection Board (NRPB)(19–21). Scanning parameters such as kV, mA, exposure time, pitch, slice thickness, and start and end positions of each scan were used as input data to the Impact CT spreadsheet in organ dose estimations.The effective dose was calculated using the tissue weighting factors from the Publication 103 of the ICRP(12). Matching the Neusoft and GE Brightspeed Excel scanner data in the ImPACT spreadsheet The ImPACT group has provided data for the commonly used models of CT scanners; for other scanners, the ImPACT developed a method by matching the dosimetric characteristics of the scanner models to those scanner models covered by the NRPB data sets(19, 20). In this study, the method of matching new models of CT scanners is used to add the Neusoft 64 slices (Neuviz) scanner manufacturer and the GE Brightspeed Excel (4 slices) scanner model, which are not included in the impact CT Dosimetry spreadsheet. The method of matching these two scanner models in ImPACT is based on the use of an Impact Factor (ImF) defined by the following equation(18):   ImF=a(CTDIcCTDIair)+b(CTDIpCTDIair)+c where the center and periphery CTDI values (CTDIc, CTDIp) are measured in the CT dosimetry phantoms at a particular tube voltage and a = 0,4738, b = 0,8045 and c = 0,0752 using a 10 mm beam collimation. The ImF is then matched to the closest ImF for a scanner model in the original Monte Carlo data set. For a new scanner manufacturer, the HVL is used as secondary matching criteria. Measured CTDI and NRPB MC data sets for the total normalized organ dose for the scanned volume were used to evaluate the CT organ doses and the effective dose. Validation of ImPACT CT Dosimetry software for pediatric patients The NRPB data sets utilized by ImPACT were generated using a mathematical phantom representing an adult. Pediatric modifying factors(22) derived from Monte Carlo modeling using the ORNL Cristy and Eckerman (1987) mathematical phantoms representing children were applied when calculating effective dose. On another hand, limited literature is available on the validation of the software for pediatric examinations. Most of them have been undertaken for adult CT examinations by using direct measurement methods on physical phantoms(23). For pediatric CT, Fuji et al.(24) performed dose measurements from both head and chest CT examinations using a radiophotoluminescence glass dosemeter (RGD) system placed on an anthropomorphic pediatric phantom of 1 year old. Similarly, McDermott et al.(25) used an anthropomorphic phantom representing a 5-year-old child to determine organ doses at specific surface and internal locations for head and chest CT protocols on a 64 MDCT scanner with TLDs placed in selected holes for each organ position. To evaluate and validate the efficiency of this software for pediatric patients, we performed a simulation of these measurements for pediatric head CT examination with ImPACT software using the same scan parameters which were undertaken in these studies. The organ doses were determined for the different sensitive organ belonged to the head scan field: eye lens, brain and thyroid. The results were then compared to those obtained by the direct measurements of these studies(24, 25). RESULTS By the end of 2016, six public institutions representing four out of all seven regional health administrations were audited (Figure 1). Three of them were university hospitals, and other three were regional. The private sector was not included in this pilot study. CT equipment data and verification of the CT dose displays The operated CT scanners were from three CT vendors (Siemens, GE and Neusoft) with an average age of 7 y (±4 y) from which 50% were 64-detector CTs, and all were equipped with automatic tube current modulation (ATM). In general, maintenance and CT image Quality control is provided by the local supplier under contract. For all the audited CT installations, the normalized measured CTDIvol (mGy/100 mAs) values and displayed values, as well as the ratio (measured/ displayed) are given in Table 1. The results demonstrate a good agreement between the displayed and the measured dosimetric quantities with a ratio close to 1. Table 1. Characteristic performance parameters of the CT systems used. Hospital  Scanner Model  Number of detector rows  ATM  Normalized CTDIvol (mGy/100 mAs)  Measured  Displayed  Ratio  UH1  SIEMENS Somatom Emotion  6  Yes  16,1  16,3  0,99  UH2  GE BrightSpeed Elite  16  Yes  14,4  15,6  0,93  UH3  SIEMENS Definition AS+  128  Yes  14,5  14,0  1,04  RH4  NEUSOFT  64  Yes  18,1  18,2  1,00  RH5  GE BrightSpeed Excel  4  Yes  18,6  19,6  0,95  RH6  NEUSOFT  64  Yes  20,2  19,9  1,02  Hospital  Scanner Model  Number of detector rows  ATM  Normalized CTDIvol (mGy/100 mAs)  Measured  Displayed  Ratio  UH1  SIEMENS Somatom Emotion  6  Yes  16,1  16,3  0,99  UH2  GE BrightSpeed Elite  16  Yes  14,4  15,6  0,93  UH3  SIEMENS Definition AS+  128  Yes  14,5  14,0  1,04  RH4  NEUSOFT  64  Yes  18,1  18,2  1,00  RH5  GE BrightSpeed Excel  4  Yes  18,6  19,6  0,95  RH6  NEUSOFT  64  Yes  20,2  19,9  1,02  Table 1. Characteristic performance parameters of the CT systems used. Hospital  Scanner Model  Number of detector rows  ATM  Normalized CTDIvol (mGy/100 mAs)  Measured  Displayed  Ratio  UH1  SIEMENS Somatom Emotion  6  Yes  16,1  16,3  0,99  UH2  GE BrightSpeed Elite  16  Yes  14,4  15,6  0,93  UH3  SIEMENS Definition AS+  128  Yes  14,5  14,0  1,04  RH4  NEUSOFT  64  Yes  18,1  18,2  1,00  RH5  GE BrightSpeed Excel  4  Yes  18,6  19,6  0,95  RH6  NEUSOFT  64  Yes  20,2  19,9  1,02  Hospital  Scanner Model  Number of detector rows  ATM  Normalized CTDIvol (mGy/100 mAs)  Measured  Displayed  Ratio  UH1  SIEMENS Somatom Emotion  6  Yes  16,1  16,3  0,99  UH2  GE BrightSpeed Elite  16  Yes  14,4  15,6  0,93  UH3  SIEMENS Definition AS+  128  Yes  14,5  14,0  1,04  RH4  NEUSOFT  64  Yes  18,1  18,2  1,00  RH5  GE BrightSpeed Excel  4  Yes  18,6  19,6  0,95  RH6  NEUSOFT  64  Yes  20,2  19,9  1,02  CT scan parameters Table 2 summarizes the CT acquisition parameters (kV, mAs, slice thickness, pitch, scan length) collected from a total of 482 pediatric CT head examinations. The table shows the number of patient data collected in each hospital (from 52 in UH3 to 151 in UH1). The number of patients in different age groups varied between minimum 11 and 60. Table 2. Summary of acquisition parameters for pediatric head CT examination in all participating hospitals. Hospital  Age group (y)  Number of patients  Technical parameters  Scan mode  kV  mAs  Scan length (cm)  Rotation time (s)  Slice thickness (mm)  Pitch  UH1  <1  22  Helical  110  (a) 114  14,5  0,6  3  0,8  1–5  60  134  17,7  5–10  41  144  17,7  10–15  28  152  17,7  UH2  <1  11  Axial Helical  120  (b) 149  13,5  1  5 1,25  0,562  1–5  27  161  13,8  5–10  20  169  15,5  10–15  19  217  15,5  UH3  <1  12  Axial Helical  120100  (a) 276  13,5  1  1,5–2–3  0,8  1–5  12  339  15,6  5–10  13  295  20,8  10–15  15  300  17,6  RH4  <1  —  helical  80-100-120  —  —  0,8–1  2–5  0,8  1–5  32  399,4  16,5  5–10  16  399,4  15,7  10–15  11  399,4  15,8  RH5  <1  11  Axial Helical  120–140  (b) 200  11,8  0,7  1,25–2,5–5  0,75  1–5  23  286  12,8  0,7–1  5–10  21  317  8,5  0,5–1  10–15  20  318  7,6  0,7–1  RH6  <1  13  Axial Helical  100–120  180  12  0,6–0,8  5 2–3  0,8–0,9  1–5  19  271  16  0,8–1  5–10  14  336  16,6  0,8–1  10–15  22  324  17,7  0,8–1  Hospital  Age group (y)  Number of patients  Technical parameters  Scan mode  kV  mAs  Scan length (cm)  Rotation time (s)  Slice thickness (mm)  Pitch  UH1  <1  22  Helical  110  (a) 114  14,5  0,6  3  0,8  1–5  60  134  17,7  5–10  41  144  17,7  10–15  28  152  17,7  UH2  <1  11  Axial Helical  120  (b) 149  13,5  1  5 1,25  0,562  1–5  27  161  13,8  5–10  20  169  15,5  10–15  19  217  15,5  UH3  <1  12  Axial Helical  120100  (a) 276  13,5  1  1,5–2–3  0,8  1–5  12  339  15,6  5–10  13  295  20,8  10–15  15  300  17,6  RH4  <1  —  helical  80-100-120  —  —  0,8–1  2–5  0,8  1–5  32  399,4  16,5  5–10  16  399,4  15,7  10–15  11  399,4  15,8  RH5  <1  11  Axial Helical  120–140  (b) 200  11,8  0,7  1,25–2,5–5  0,75  1–5  23  286  12,8  0,7–1  5–10  21  317  8,5  0,5–1  10–15  20  318  7,6  0,7–1  RH6  <1  13  Axial Helical  100–120  180  12  0,6–0,8  5 2–3  0,8–0,9  1–5  19  271  16  0,8–1  5–10  14  336  16,6  0,8–1  10–15  22  324  17,7  0,8–1  aEffective mAs. bmA. Table 2. Summary of acquisition parameters for pediatric head CT examination in all participating hospitals. Hospital  Age group (y)  Number of patients  Technical parameters  Scan mode  kV  mAs  Scan length (cm)  Rotation time (s)  Slice thickness (mm)  Pitch  UH1  <1  22  Helical  110  (a) 114  14,5  0,6  3  0,8  1–5  60  134  17,7  5–10  41  144  17,7  10–15  28  152  17,7  UH2  <1  11  Axial Helical  120  (b) 149  13,5  1  5 1,25  0,562  1–5  27  161  13,8  5–10  20  169  15,5  10–15  19  217  15,5  UH3  <1  12  Axial Helical  120100  (a) 276  13,5  1  1,5–2–3  0,8  1–5  12  339  15,6  5–10  13  295  20,8  10–15  15  300  17,6  RH4  <1  —  helical  80-100-120  —  —  0,8–1  2–5  0,8  1–5  32  399,4  16,5  5–10  16  399,4  15,7  10–15  11  399,4  15,8  RH5  <1  11  Axial Helical  120–140  (b) 200  11,8  0,7  1,25–2,5–5  0,75  1–5  23  286  12,8  0,7–1  5–10  21  317  8,5  0,5–1  10–15  20  318  7,6  0,7–1  RH6  <1  13  Axial Helical  100–120  180  12  0,6–0,8  5 2–3  0,8–0,9  1–5  19  271  16  0,8–1  5–10  14  336  16,6  0,8–1  10–15  22  324  17,7  0,8–1  Hospital  Age group (y)  Number of patients  Technical parameters  Scan mode  kV  mAs  Scan length (cm)  Rotation time (s)  Slice thickness (mm)  Pitch  UH1  <1  22  Helical  110  (a) 114  14,5  0,6  3  0,8  1–5  60  134  17,7  5–10  41  144  17,7  10–15  28  152  17,7  UH2  <1  11  Axial Helical  120  (b) 149  13,5  1  5 1,25  0,562  1–5  27  161  13,8  5–10  20  169  15,5  10–15  19  217  15,5  UH3  <1  12  Axial Helical  120100  (a) 276  13,5  1  1,5–2–3  0,8  1–5  12  339  15,6  5–10  13  295  20,8  10–15  15  300  17,6  RH4  <1  —  helical  80-100-120  —  —  0,8–1  2–5  0,8  1–5  32  399,4  16,5  5–10  16  399,4  15,7  10–15  11  399,4  15,8  RH5  <1  11  Axial Helical  120–140  (b) 200  11,8  0,7  1,25–2,5–5  0,75  1–5  23  286  12,8  0,7–1  5–10  21  317  8,5  0,5–1  10–15  20  318  7,6  0,7–1  RH6  <1  13  Axial Helical  100–120  180  12  0,6–0,8  5 2–3  0,8–0,9  1–5  19  271  16  0,8–1  5–10  14  336  16,6  0,8–1  10–15  22  324  17,7  0,8–1  aEffective mAs. bmA. For head CT for pediatric trauma and agitated children, helical scanning was predominantly used in all hospitals because of the need of a faster acquisition mode. But in UH2, UH3 and RH6 some pediatric head CT exams were performed with axial technique. Constant kV values of 110 and 120 were used across all age groups in UH1 and UH2 respectively, while a range of kV values (80–140) was used in the other hospitals. Table 2 also shows a significant difference between hospitals in term of tube current (mAs), pitch and slice thickness. The lowest range of mAs values was found in the UH1 and the highest one in the RH4 with a fixed value of 399.4 mAs applied for all children age. Patient dose Table 3 provides details of the descriptive statistics of median doses (CTDIvol, DLP) from head examinations for children arranged in four age groups: <1, 1–5, 5–10 and 10–15 y. Table 3. Statistics CTDIvol (mGy) and DLP (mGy cm) values from pediatric head CT examinations. Age group  Dose quantity  UH1  UH2  UH3  RH4  RH5  RH6  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  <1  CTDIvol  20,5  6,7–34,2  27,8  27,6–49,6  24,8  16,3–27,4  —  —  26,3  25,8–44,6  1 305 396  10,5–15,6  DLP  342  100–1007  473  331–1227  346  239–909      341  289–803    126–667  1–5  CTDIvol  25,1  11,2–34,8  31,2  23,0–70,1  32,1  26,1–47,3  52,8  11,8–59,4  43,1  35–101,7  20,2  14,8–48,7  DLP  474  405–1326  494  110–1263  767  466–1628  1015  497–2992  561  399–1249  398  247–1104  5–10  CTDIvol  27,1  22,1–33,9  37,3  28,7–81,8  42  30,4–48,5  52,8  11,8–59,4  75,4  62,2–83,9  49  34,6–56,3  DLP  529  378–1134  628  484–1575  822  681–1428  1103  248–2994  920  509–1487  825  597–1970  10–15  CTDIvol  28,9  23,2–32,8  46,8  29,7–84,7  46  45,4–47,3  52,8  47,6–52,8  75,4  61,0–93,7  48,7  34,6–56,3  DLP  547  422–848  740  475–1640  817  816–1470  1005  843–2049  977  380–2003  978  536–1148  Age group  Dose quantity  UH1  UH2  UH3  RH4  RH5  RH6  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  <1  CTDIvol  20,5  6,7–34,2  27,8  27,6–49,6  24,8  16,3–27,4  —  —  26,3  25,8–44,6  1 305 396  10,5–15,6  DLP  342  100–1007  473  331–1227  346  239–909      341  289–803    126–667  1–5  CTDIvol  25,1  11,2–34,8  31,2  23,0–70,1  32,1  26,1–47,3  52,8  11,8–59,4  43,1  35–101,7  20,2  14,8–48,7  DLP  474  405–1326  494  110–1263  767  466–1628  1015  497–2992  561  399–1249  398  247–1104  5–10  CTDIvol  27,1  22,1–33,9  37,3  28,7–81,8  42  30,4–48,5  52,8  11,8–59,4  75,4  62,2–83,9  49  34,6–56,3  DLP  529  378–1134  628  484–1575  822  681–1428  1103  248–2994  920  509–1487  825  597–1970  10–15  CTDIvol  28,9  23,2–32,8  46,8  29,7–84,7  46  45,4–47,3  52,8  47,6–52,8  75,4  61,0–93,7  48,7  34,6–56,3  DLP  547  422–848  740  475–1640  817  816–1470  1005  843–2049  977  380–2003  978  536–1148  Table 3. Statistics CTDIvol (mGy) and DLP (mGy cm) values from pediatric head CT examinations. Age group  Dose quantity  UH1  UH2  UH3  RH4  RH5  RH6  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  <1  CTDIvol  20,5  6,7–34,2  27,8  27,6–49,6  24,8  16,3–27,4  —  —  26,3  25,8–44,6  1 305 396  10,5–15,6  DLP  342  100–1007  473  331–1227  346  239–909      341  289–803    126–667  1–5  CTDIvol  25,1  11,2–34,8  31,2  23,0–70,1  32,1  26,1–47,3  52,8  11,8–59,4  43,1  35–101,7  20,2  14,8–48,7  DLP  474  405–1326  494  110–1263  767  466–1628  1015  497–2992  561  399–1249  398  247–1104  5–10  CTDIvol  27,1  22,1–33,9  37,3  28,7–81,8  42  30,4–48,5  52,8  11,8–59,4  75,4  62,2–83,9  49  34,6–56,3  DLP  529  378–1134  628  484–1575  822  681–1428  1103  248–2994  920  509–1487  825  597–1970  10–15  CTDIvol  28,9  23,2–32,8  46,8  29,7–84,7  46  45,4–47,3  52,8  47,6–52,8  75,4  61,0–93,7  48,7  34,6–56,3  DLP  547  422–848  740  475–1640  817  816–1470  1005  843–2049  977  380–2003  978  536–1148  Age group  Dose quantity  UH1  UH2  UH3  RH4  RH5  RH6  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  Median  Range  <1  CTDIvol  20,5  6,7–34,2  27,8  27,6–49,6  24,8  16,3–27,4  —  —  26,3  25,8–44,6  1 305 396  10,5–15,6  DLP  342  100–1007  473  331–1227  346  239–909      341  289–803    126–667  1–5  CTDIvol  25,1  11,2–34,8  31,2  23,0–70,1  32,1  26,1–47,3  52,8  11,8–59,4  43,1  35–101,7  20,2  14,8–48,7  DLP  474  405–1326  494  110–1263  767  466–1628  1015  497–2992  561  399–1249  398  247–1104  5–10  CTDIvol  27,1  22,1–33,9  37,3  28,7–81,8  42  30,4–48,5  52,8  11,8–59,4  75,4  62,2–83,9  49  34,6–56,3  DLP  529  378–1134  628  484–1575  822  681–1428  1103  248–2994  920  509–1487  825  597–1970  10–15  CTDIvol  28,9  23,2–32,8  46,8  29,7–84,7  46  45,4–47,3  52,8  47,6–52,8  75,4  61,0–93,7  48,7  34,6–56,3  DLP  547  422–848  740  475–1640  817  816–1470  1005  843–2049  977  380–2003  978  536–1148  There is a broad range of dose indicators between hospitals. The UH1 presents the lowest CTDIvol value with a median values ranging from 20.5 mGy to 28.9 for all age groups, while the highest ones were observed for RH5 with median values ranging from 26.3 to 75.4 mGy. The RH4 presented a constant CTDIvol median value of 52.8 mGy for all age groups. In terms of DLP, this last hospital presents the highest values. As mentioned earlier, in UH2, UH3 ad RH6, axial and helical scan techniques were both used. Figure 2 shows a comparison of CTDIvol from axial and helical scan modes for all patient age groups within the same unit for the three hospitals. The results indicated that there is a wide range between the two techniques with higher values from the helical scan reaching 2 times those obtained from axial scan in both UH2 and RH6. Whereas, it was not the same for UH3 where doses from helical head scanning were lower. Figure 2. View largeDownload slide Obtained dose distribution (CTDIvol) in axial and helical scan mode within UH2, UH3 and RH6. Figure 2. View largeDownload slide Obtained dose distribution (CTDIvol) in axial and helical scan mode within UH2, UH3 and RH6. Estimation of effective dose and organ dose The effective doses per hospital and per age group estimated using the Impact CT software with the ICRP103 weighting factors are displayed in Table 4. The results show that the mean values per age group do not vary widely between hospitals, except for RH6 and RH4 which presented the highest effective doses for all age categories with a maximum median value 5 mSv. Table 4. Median values of patient effective dose (mSv) for different pediatric age groups with using the ICRP103 (E103) definition of effective dose. Age group  UH1  UH2  UH3  RH4  RH5  RH6  <1 y  1,8  2,4  1,9  —  2,5  3,3  1–5 y  2,0  2,2  2,6  5,0  2,5  2,0  5–10 y  1,9  2,2  4,0  4,6  2,7  3,0  10–15 y  1,4  1,9  2,4  2,9  2,6  2,9  Age group  UH1  UH2  UH3  RH4  RH5  RH6  <1 y  1,8  2,4  1,9  —  2,5  3,3  1–5 y  2,0  2,2  2,6  5,0  2,5  2,0  5–10 y  1,9  2,2  4,0  4,6  2,7  3,0  10–15 y  1,4  1,9  2,4  2,9  2,6  2,9  Table 4. Median values of patient effective dose (mSv) for different pediatric age groups with using the ICRP103 (E103) definition of effective dose. Age group  UH1  UH2  UH3  RH4  RH5  RH6  <1 y  1,8  2,4  1,9  —  2,5  3,3  1–5 y  2,0  2,2  2,6  5,0  2,5  2,0  5–10 y  1,9  2,2  4,0  4,6  2,7  3,0  10–15 y  1,4  1,9  2,4  2,9  2,6  2,9  Age group  UH1  UH2  UH3  RH4  RH5  RH6  <1 y  1,8  2,4  1,9  —  2,5  3,3  1–5 y  2,0  2,2  2,6  5,0  2,5  2,0  5–10 y  1,9  2,2  4,0  4,6  2,7  3,0  10–15 y  1,4  1,9  2,4  2,9  2,6  2,9  The comparison between organ doses from RGD(24) and TLD(25) and measurements compared to those obtained from the simulations in this study for pediatric head CT examination are presented in Table 5. Table 5. Organ doses calculated in this study from Impact simulations compared to Organ doses from RGD(24) and TLD(25) measurements from studies(24, 25).   Organ  Dorgan,TLD (mGy)(25)  Dorgan, this study (mGy)  Ratio  Dorgan,RGD (mGy)(24)  Dorgan, this study (mGy)  Ratio  Axial  Brain  32  39  0,82  31,9  32  0,91  Eye lens  39  40  0,98  35,8  39  0,92  Thyroid  46  43  1,07  1,2  1,4  0,8  Helical  Brain  32  38  0,84  —  —  —  Eye lens  33  39  0,85  —  —  —  Thyroid  43  42  1,02  —  —  —    Organ  Dorgan,TLD (mGy)(25)  Dorgan, this study (mGy)  Ratio  Dorgan,RGD (mGy)(24)  Dorgan, this study (mGy)  Ratio  Axial  Brain  32  39  0,82  31,9  32  0,91  Eye lens  39  40  0,98  35,8  39  0,92  Thyroid  46  43  1,07  1,2  1,4  0,8  Helical  Brain  32  38  0,84  —  —  —  Eye lens  33  39  0,85  —  —  —  Thyroid  43  42  1,02  —  —  —  Table 5. Organ doses calculated in this study from Impact simulations compared to Organ doses from RGD(24) and TLD(25) measurements from studies(24, 25).   Organ  Dorgan,TLD (mGy)(25)  Dorgan, this study (mGy)  Ratio  Dorgan,RGD (mGy)(24)  Dorgan, this study (mGy)  Ratio  Axial  Brain  32  39  0,82  31,9  32  0,91  Eye lens  39  40  0,98  35,8  39  0,92  Thyroid  46  43  1,07  1,2  1,4  0,8  Helical  Brain  32  38  0,84  —  —  —  Eye lens  33  39  0,85  —  —  —  Thyroid  43  42  1,02  —  —  —    Organ  Dorgan,TLD (mGy)(25)  Dorgan, this study (mGy)  Ratio  Dorgan,RGD (mGy)(24)  Dorgan, this study (mGy)  Ratio  Axial  Brain  32  39  0,82  31,9  32  0,91  Eye lens  39  40  0,98  35,8  39  0,92  Thyroid  46  43  1,07  1,2  1,4  0,8  Helical  Brain  32  38  0,84  —  —  —  Eye lens  33  39  0,85  —  —  —  Thyroid  43  42  1,02  —  —  —  Figure 3 presents the estimated organ doses for selected organs as eye lens, brain and thyroid in terms of median values per age groups for all participating hospitals. Large variations of organ doses exist among hospitals varying from 16 to 52 mGy for both eye lens and brain with outstanding values in the RH4 and lowest ones in UH1. Figure 3. View largeDownload slide Distribution of organ doses (mGy) per hospital for each patient age group. Figure 3. View largeDownload slide Distribution of organ doses (mGy) per hospital for each patient age group. Table 6 presents the dose indicators values of our study based on the 75th percentiles of the distribution of median values obtained from the facilities compared to those from international studies: IAEA Study(26), Portugal(27), Germany(28), UK(29), Belgium(30) and Switzerland(31). Table 6. Comparison of (CTDIvol, DLP) with previous studies.   Age (y)  This study  IAEA study(26)  Portugal(27)  Germany(28)  UK(29)  Belgium(30)  Switzerland(31)  CTDI (mGy)  <1  26  26  48  33  30  35  20  1–5  40  36  50  40  45  43  30  5–10  52  43  70  50  50  49  40  10–15  52  53  72  65  65  50  60  DLP (mGy cm)  <1  397  440  630  390  270  280  270  1–5  716  540  770  520  470  473  420  5–10  896  690  1100  710  620  637  560  10–15  978  840  1120  920  930  563  1000    Age (y)  This study  IAEA study(26)  Portugal(27)  Germany(28)  UK(29)  Belgium(30)  Switzerland(31)  CTDI (mGy)  <1  26  26  48  33  30  35  20  1–5  40  36  50  40  45  43  30  5–10  52  43  70  50  50  49  40  10–15  52  53  72  65  65  50  60  DLP (mGy cm)  <1  397  440  630  390  270  280  270  1–5  716  540  770  520  470  473  420  5–10  896  690  1100  710  620  637  560  10–15  978  840  1120  920  930  563  1000  Table 6. Comparison of (CTDIvol, DLP) with previous studies.   Age (y)  This study  IAEA study(26)  Portugal(27)  Germany(28)  UK(29)  Belgium(30)  Switzerland(31)  CTDI (mGy)  <1  26  26  48  33  30  35  20  1–5  40  36  50  40  45  43  30  5–10  52  43  70  50  50  49  40  10–15  52  53  72  65  65  50  60  DLP (mGy cm)  <1  397  440  630  390  270  280  270  1–5  716  540  770  520  470  473  420  5–10  896  690  1100  710  620  637  560  10–15  978  840  1120  920  930  563  1000    Age (y)  This study  IAEA study(26)  Portugal(27)  Germany(28)  UK(29)  Belgium(30)  Switzerland(31)  CTDI (mGy)  <1  26  26  48  33  30  35  20  1–5  40  36  50  40  45  43  30  5–10  52  43  70  50  50  49  40  10–15  52  53  72  65  65  50  60  DLP (mGy cm)  <1  397  440  630  390  270  280  270  1–5  716  540  770  520  470  473  420  5–10  896  690  1100  710  620  637  560  10–15  978  840  1120  920  930  563  1000  DISCUSSION In this work, pediatric head CT practices were evaluated in six Tunisian public regional and university hospitals dispersed in the whole regions (Figure 1). This is the first time pediatric CT dose assessment fully based on a nationwide pilot study. A unique feature of the data presented here is the approach for data collection. While most of the countries use questionnaires or web-based databases for analysis(27, 28), patient doses in this work were collected during on-site audits. An advantage of this approach is the verification of the CT scanners dose displays by measurements using the same phantom and dosimetry equipment. Despite the location of the regional CT installations away from the capital with a very low level of specialized human resources, the results showed a good agreement between displayed and measured CTDI due to the maintenance program carried out within the different sites by the CT manufacturer staff (Table 1). This methodology had the further advantage that the current practice could be discussed with the radiologists and the radiographers that provided a solid basis for the assessment of the national radiological practice. A limitation of the study was the restricted time duration of the audit. As a matter of fact, during audits CT scanners were blocked and clinical routine work was interrupted. Also, three regions are missing in this study (North West, Middle West, Middle East) due to the time-limitation and the workload of the team of medical physicists of the National Center of Radiation Protection who performed the audits. Radiation dose from CT mostly depends on the CT acquisition parameters, including tube potential (kVp), tube current (mAs), pitch and the scan length (L). This study revealed a significant variation in acquisition parameters among the radiology departments of each site. The lowest technical parameters were found in the UH1. These low values can be attributed to the fact that this radiology department belongs to a Children Hospital so that all practices and examinations are dedicated to pediatric patients who need special care and attention. It is also evident that significant variations in scan length existed between hospitals for given examinations. The large scan length leads to high doses in term of DLP and organ doses especially for organ positioned at the boundaries of the scan length. These variations were largely caused by the absence of standard imaging protocols, even when using a scanner from the same manufacturer, and by the many different practices involved in these hospitals. Moreover, not all sites in the study apply dedicated scan protocols for children, some of them use adult protocols for all pediatric head CT examination such as RH4. The details of the applied scan protocols indicated a lack of awareness of practitioners in some hospitals regarding exposure parameters and their impact on pediatric patient dose and emphasized the necessity to apply standard clinical guidelines relative to pediatrics CT practices. From the technical parameters that can be selected by the CT operator, those that have a high influence on dose and image quality are the mAs, tube voltage and pitch and they are the main factors of the wide variation in doses between different CT units. The highest values of CTDIvol observed in RH4 and RH5 (Table 3) might be due to the use of high exposure settings (mAs). Moreover, the constant CTDI value found in RH4 for all age groups could be attributed to the fact that technologists have used the same imaging protocol for all children age which is almost that of adult brain CT without making the necessary adjustment of scanning parameters, despite the existence of a ‘Baby brain protocol’ among the brain protocols settings of the Neusoft scanner. In the other hospitals, an increase of CTDI with patient age is noted, this is due to the occasional slight increase in kV and mAs values with children age. Despite the fact that all scanners are equipped with ATM (Table 1), not all hospitals used it for children. Also, even if it is used in some hospitals, for instance RH3, doses remained high. Preliminary studies that applied current modulation with children showed a significant dose reduction of ~30%, depending on the scanned patient region(32). Although these promising developments suggest that CT doses to children could be reduced, the effect in clinical routine is still unclear. Here is manifested the role of the manufacturer in the education and the training of CT practitioners in the capabilities of CT machines. In terms of DLP, as mentioned earlier, the variations between hospitals were largely contributed by inherent differences in the equipment and different scanning protocols used among hospitals. This increase in DLP is due to the use of large scan length by technologists in some hospitals. Since the high head CT examination percentage in the country is due to trauma from the high rate of motor accidents, technologists and radiologists in UH1 usually tend to increase the scan length up to the cervical vertebrae (C1, C2) for clinical needs in case of pediatric head trauma in order to maintain accurate diagnosis. The high DLP values are also due to the effect of the over-ranging which applies only to helical scanning and is taken to be the difference between the exposed scan length and the planned scan length. Over-ranging is the increase in DLP due to the additional rotations at the beginning and at the end of a spiral scan required for data interpolation to reconstruct the first and the last slice of the imaged region. The two most important parameters that affect over ranging are the beam width and the pitch(33). In fact, for MDCT scanners, the number of additional rotations is strongly pitch dependent, with a typical increase in irradiation length of 1.5 times the total nominal beam width(34). Consequently, the setting of strict scan lengths would be required for the reduction of unnecessary exposure to pediatric patients in CT scans(29, 35) with a large beam width and a large pitch factor. It is commonly known that the helical mode leads to higher doses. However, in Figure 2, UH3 presented lower doses from helical scanning compared to axial technique. This could be explained by the fact that the practitioners used practically the same scan parameters for the two scan modes but they tended to increase the kV values from 100 kV for helical examinations to 120 kV for axial ones. In fact, an increase of kV increases the dose by ≈kV2. For pediatric patients, axial scanning is preferred over helical scanning. But if helical scanning mode is required for clinical needs, a careful selection of technical parameters is recommended. In term of effective dose, the high values due to the high DLP values discussed in previous section since there is a strong correlation between the effective dose and DLP which takes into account CT scanning parameters. Our results also show that there is no significant difference in effective dose for all ages of children. According to Huda et al.(36), the variation of pediatric head size is not as readily evident and rises slowly with patient age, except for the youngest infant in the first 2 years of life where head increases rapidly. From the results of Table 5 we can conclude that ImPACT software could be useful to simulate organ doses from head pediatric CT examinations and these doses could be directly determined from the adult phantom of ImPACT software without any corrections. It is expected that some variation in the calculated doses will be evident and unavoidable across the different organ dosimetry methods. Indeed, there are several important influencing factors on the measured and calculated absorbed doses that need to be considered. These include the models used to represent the organs and their position in the body, the type/model of CT scanner being used and/or modeled in the calculation and the scan parameters used in simulating the X-ray exposure. Brady et al.(37) have previously found that for the CT brain examination, the measured and computed doses in brain were all within ~30%. They affirmed that the clinical conditions could not be matched exactly in the computational methods and therefore, various assumptions and approximations had to be made. Thus, conditions that have been used for each simulation provide the best match to the clinical scenario. The anatomical scan regions suggest radiation risk exposure to sensitive organs such as the eye lens and thyroid gland, which increases the probability of eye cataracts and cancer. The eye lens and the brain are usually included either totally or partially in the irradiation field in brain CT examination and as known organs within the scan volume receive higher absorbed doses than those outside the scan volume. To avoid irradiation of the eyes, the head CT examination is normally planned to exclude the eyes from the imaged area. Controlling radiation exposure to the eyes is, hence, important especially in patients who require multiple scans. In this study, the brain dose is specifically higher for RH5 due to the practice itself. The operators performed head CT examination in two or three series including the brain at each irradiation. This practice demonstrates the lack of the operator’s training in radiation protection and specific pediatric protocols. Furthermore, organs on the periphery of the scan volume can have a significant variation in absorbed dose across the organ due to partial irradiation. The highest thyroid absorbed dose is dependent on each individual scan and the selection of collimation as the thyroid may or may not be within the scan volume. Even when it is outside the scan volume it is likely to receive a dose from scattered radiation and over-ranging. As it is shown in Figure 3 thyroid receives a dose during head CT examination within all hospitals, with a median value ranging from 0.5 mGy to a maximum value of 34 mGy. This is mainly due to the large scan length used in some units sometimes for clinical needs as is the case in UH1 and to the over ranging which contributed to the increase in doses to organs and tissues positioned at the boundaries of the scan length(38). The over ranging could easily be avoided by changing the scanning protocol from helical to axial mode. As an overall trend, the CTDI values of the present study are comparable to the data reported from other studies(26, 28–31) and clearly lower than those of the Portuguese study(27). In terms of DLP, our values are comparable the German values(28), lower than the Portuguese ones(27) and slightly higher than those of other countries(29–31). This is mainly due to the values obtained in the regional hospitals. From the whole results of this study, we can deduce that there is a wide variation of the CT doses between the regional hospitals within the country and the university hospitals located at the capital region. This variation is greater if technologists and radiologists are insufficiently knowledgeable in the capabilities of CT machines and in radiation risk due to the scarcity and lack of regional coordination in the delivery of continuous training courses on radiation protection for health professionals using ionizing radiation. This study emphasizes the importance of the production and the update of recommendations and good practice guidelines using interdisciplinary working groups and promotes the establishment and the use of national DRLs based on a national survey including all the regions of the country. CONCLUSION In this pilot study, pediatric radiation dose was investigated for head CT procedures. A large variation is observed among and within hospitals. The main contributor to these variations was the use of different techniques and protocols, for adults in some cases, which shows the importance of using only pediatric protocols for CT examinations in children. Our results were in good compliance with some previous studies and slightly higher than other ones. These variations suggest that pediatric patients are still exposed to a large amount of unnecessary radiation and optimization is not fulfilled. Optimization of protection for pediatric CT procedures should be given a priority and the recommendation of standardizing CT pediatric protocols and image quality needs to be studied at the national level. This reinforces the role of the medical physicists in the field of patient protection and safety during medical exposure. A regular survey and determination of the DRL should be done especially if there are more and new installations are distributed all across the country. The time for survey should allow the implementation of corrective actions to take place after the initial DRLs. A graded approach may be used to select procedures for which DRLs are to be established for children. These actions should be a joint effort among all the stakeholders including health authorities, radiation protection regulators, professional societies and universities and researchers in radiation protection in medicine. Acknowledgements The work was encouraged by the IAEA through the TC project RAF/9/053 ‘Strengthening Technical Capabilities for Patient and Occupational Radiation Protection in Member States’. The authors are grateful to the Abdu Salam International Center for Theoretical Physics (ICTP) in Trieste, Italy, for training the first author in Computed Tomography: Quality Control, Dosimetry and Optimization. The authors express their gratitude to Ms Jenia Vassileva, IAEA’s Radiation Protection Expert, for her encouragement, help and article reviewing. 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Radiation Protection DosimetryOxford University Press

Published: Apr 18, 2018

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