DEVELOPMENT OF AN ALGORITHM TO ESTIMATE EYE LENS DOSE IN TERMS OF OPERATIONAL QUANTITY Hp(3) USING HEAD TLD BADGE

DEVELOPMENT OF AN ALGORITHM TO ESTIMATE EYE LENS DOSE IN TERMS OF OPERATIONAL QUANTITY Hp(3)... Abstract In view of the recommendations of International Commission on Radiological Protection for reduction of the occupational annual dose limit for eye lens from 150 mSv to 20 mSv/y, questions have been raised on the adequacy of monitoring for the quantities Hp(10) and Hp(0.07). As an immediate requirement, in the present situation, where there is no exclusive eye lens dosemeter in India, the existing chest TLD badge was modified to be used as head badge (head dosemeter) by including a strap to enable wearing on the forehead. In order to estimate the eye lens dose in terms of the operational quantity Hp(3), the prevalent algorithm of chest badge was also modified. The modified algorithm was applied to estimate Hp(3) for dosemeters irradiated to various beta and photon radiations including mixtures. The Q values (estimated/delivered dose equivalent) were found to be within ±20% for most of the photon beams. INTRODUCTION In India, personnel monitoring of radiation workers against external radiation hazards of X, beta and gamma radiation is carried out using a thermoluminescence dosemeter (TLD) system based on CaSO4: Dy Teflon TLD disc.(1) This dosemeter can be used for estimation of doses in terms of the ICRU recommended quantities personal dose equivalent Hp(d) at 10 and 0.07 mm depth.(2, 3) In view of the fact that the International Commission on Radiological Protection (ICRP)(4) revised lifetime eye dose threshold for cataract induction downwards from 2000 to 500 mGy, and the occupational annual dose limit for eye lens from 150 to 20 mSv in April 2011, questions have been raised on the adequacy of monitoring for the quantities Hp(10) and Hp(0.07) without monitoring for Hp(3). This is especially true for occupational workers who could get a high dose to eye, working as interventional radiologist. In India, regulations have not yet been put in place to enforce the new limits. However, work is being carried out to assess and gather wide knowledge with respect to the workers eye lens dose, especially in medical interventional radiological facilities. The objectives include design of eye lens dosemeter, methodology for its implementation in routine service and appropriate line of action to be taken if the limit is crossed. At present in India, in routine monitoring, Hp(3) is not measured. Earlier, it was concluded that if Hp(0.07) and Hp(10) values are not exceeded, Hp(3) is considered to be within limits. Given the reduction in the dose limit for eye lens, the earlier assumption may not be valid.(4, 5) As an immediate requirement in the present situation, where there is no exclusive eye lens dosemeter, the existing chest TLD badge is modified to include a strap to enable the worker to wear it on the forehead. As the location on the forehead is in close proximity to eye, an assumption can be made that it represents the dose to the eye lens. However, the algorithm used for evaluation of the dose must be modified to represent the value of the operational quantity Hp(3). The present work was carried out to arrive at an estimate of Hp(3) with the use of the modified chest badge and algorithm. In the absence of any information in ISO(6) regarding the dimensions of the phantom to be used for Hp(3), water filled slab phantom was used. Later, the ORAMED(7) project, which addressed the implementation of the eye lens dose, has suggested a cylindrical phantom of ICRU tissue with 20 cm diameter and 20 cm height. However, in later papers(8, 9) it was also suggested that the change of phantom does not significantly affect the response for angles up to 45°. IEC 62 387-2012(10) has also recommended the use of slab phantom. In view of this the present study was carried out with ISO water slab phantom, considering that it will not significantly affect the results. MATERIAL AND METHODS The regular TL badge (dosemeter)(1, 3) used for personnel monitoring of radiation workers is modified to include a strap to enable wearing on the fore-head and is used for this study. The dosemeter comprises of three identical CaSO4:Dy Teflon discs each (13.6 mm diameter and thickness 0.8 mm) mechanically clipped over three circular holes each of diameter 12 mm on a nickel plated aluminum (Al) card of thickness 1 mm (TLD card) and TLD card is loaded in a plastic holder having three different filter regions on the front and rear side.(1, 3) The dosemeter disc 1 (D1) is under the metal filter (Cu + Al), disc 2 (D2) is under a Perspex filter and disc 3 (D3) is open having identification paper and polythene seal. TLD badge (dosemeter) is shown in the Figure 1. Figure 1. View largeDownload slide Schematic diagram of TLD badge. Figure 1. View largeDownload slide Schematic diagram of TLD badge. A set of 300 TLD cards from single batch having mean TL (light output in arbitrary unit) exposed to a test dose (3 mSv) for all three discs within ±5% was selected for the study. TLD card sealed in black polythene pouch and loaded in cassettes (TLD dosemeter) were irradiated to various photon beams as given in Table 1. All irradiations were carried out at 2 m distance as per ISO 4037.(6) During irradiations, the dosemeters were backed by an ISO water filled slab phantom having dimension 30 cm × 30 cm × 15 cm.(6) To account for field conditions which may include all directions of irradiation, exposures in both vertical and horizontal orientation of the dosemeter for the entire beam qualities were carried out. Since the dosemeter is asymmetric, for each angle of irradiation, four orientations were considered, two vertical and two horizontal. The angles of irradiation were 0°, 15°, 30°, 45°, 60° and 75° and for each angle, the irradiations were carried out in four orientations of the badge. Four dosemeters were irradiated at a time after ensuring that the df (approximate locus of the 98% isodose contour with respect to the dose in the center of the phantom) values given in ISO 4037(6) were not exceeded. The positioning of the dosemeters on phantom is shown in Figure 2. The delivered Hp(3) for each beam quality are also given in Table 1. It is assumed that the delivered Hp(3) is the true value and therefore no uncertainty is assigned to it. Table 1. Details of photon beams used for irradiation. Serial no.  Photon beams  Mean energy (keV)  Air kerma rate (mGy/h)  Conversion coefficient hpk (Sv/Gy) for normal incidence  Delivered Hp(3) (mSv)  1  N-40  33  45.74  1.29  3.55  2  N-60  48  51.84  1.63  2.60  3  N-100  83  12.52  1.81  3.10  4  N-120  100  11.85  0.74  3.05  5  N-150  118  49.88  1.66  4.25  6  N-250  208  20.78  1.46  2.80  7  S-Am  59.5  0.21  1.80  1.60  8  S-Cs  662  2.91  1.22  3.20  9  S-Co  1250  2.18  1.16  3.05  Serial no.  Photon beams  Mean energy (keV)  Air kerma rate (mGy/h)  Conversion coefficient hpk (Sv/Gy) for normal incidence  Delivered Hp(3) (mSv)  1  N-40  33  45.74  1.29  3.55  2  N-60  48  51.84  1.63  2.60  3  N-100  83  12.52  1.81  3.10  4  N-120  100  11.85  0.74  3.05  5  N-150  118  49.88  1.66  4.25  6  N-250  208  20.78  1.46  2.80  7  S-Am  59.5  0.21  1.80  1.60  8  S-Cs  662  2.91  1.22  3.20  9  S-Co  1250  2.18  1.16  3.05  N, narrow series; S, standard radionuclide source (ISO 4037-3: 1999(E)). Figure 2. View largeDownload slide Schematic of dosemeter position on the phantom for irradiation. Vertical orientation (1,3) for positive and negative angle and Horizontal orientation (2,4) for negative and positive angle. Figure 2. View largeDownload slide Schematic of dosemeter position on the phantom for irradiation. Vertical orientation (1,3) for positive and negative angle and Horizontal orientation (2,4) for negative and positive angle. The conversion coefficient hpk (3; N, α) from air kerma to the dose equivalent Hp(3) for the radiation quantities defined in ISO 4037-3(6) is taken from IEC 62 387-2012(10) for various angles. The exposed TLD cards were readout on a calibrated semi automatic TLD dosemeter reader.(11) Before reading, the reader was calibrated using ten dosemeters exposed on water filled ISO slab phantom to 3 mSv Hp(3) of 137Cs. Reader calibration factor (RCF) in terms of mSv/TL, was generated from the average output of these 10 TLD readings. For all read cards, the TL of each of the disc D1, D2, D3 was normalized to 137Cs photon readings. For normalizing, the net TL output of the discs exposed to various energies of photon, mixture of photon + beta and pure beta (reading—background) (or ND1, ND2, ND3) was multiplied with the (RCF). The normalized readings divided by the true dose equivalent Hp(3) is henceforth referred to as the response (TL/mSv) of the individual disc. Irradiations were also carried out to beta radiation using PTB beta secondary standard calibration system (BSS-2)(12, 13) which has 90Sr/90Ysource with calibrated output in terms of Hp(3).(10) TLD cards were exposed to pure 90Sr/90Y beam at normal incidence using this system. During irradiations, the dosemeters were placed on a poly methyl metha acrylate (PMMA) slab phantom having dimension 30 cm × 30 cm × 5 cm. For studying the angular response, dosemeters were irradiated at horizontal and vertical orientations with phantom, angles at 0°, 15°, 30°, 45° and 60° with respect to the source. To check the validity of the algorithm in a mixed beta gamma field, some dosemeters were also irradiated to mixtures of 137Cs and 90Sr/90Y at normal incidence. Irradiations were also carried out to beta radiation with106Ru/106Rh source. In view of the fact that only Dp(0.07) values were available for this source for normal incidence, the conversion coefficients given in IEC(10) were used for estimating delivered doses in terms of Hp(3) when performing the irradiations. Studies similar to 90Sr/90Y were carried out both at normal and angular incidence. RESULTS AND DISCUSSION As it has been reported previously by other authors,(2, 3) normalized response of D1 with a filtration of 1060 mg/cm2 is not flat over the entire photon energy range. It is, however, equal to unity for energies >200 keV. Disc D2 with a filtration of 180 mg/cm2 of PMMA does not show any significant reduction in the response due to photons and is within 10% of disc D3 response for the entire photon energy range. Therefore, for all practical purposes it may be assumed that the ratio ND3/ND2 is nearly equal to 1 for all the photon energies. The energy dependent behavior is due to the non-tissue equivalence of the detector material (CaSO4:Dy) leading to higher response at lower energies. In actual field condition, the energy of the photon is not known. Therefore, for the identification of the photon energy, we have decided to use the ratio R12 (ratio of ND1 and D23 (average of ND2 and ND3)). The variation of R12 with energy is shown in Figure 3 for normal incidence. Figure 3. View largeDownload slide Plot of ratio ND1/D23 (R12) with effective energy of photon beams for normal incidence. Figure 3. View largeDownload slide Plot of ratio ND1/D23 (R12) with effective energy of photon beams for normal incidence. Due to the presence of metal filter Cu + Al of 1060 mg/cm2 thickness over the disc D1, it is expected that beta radiation does not contribute to the reading of D1 for mean beta energy ≤0.8 MeV. The disc D2 which is shielded by PMMA of thickness 180 mg/cm2 also shows a reduced response, whereas disc D3 with no filtration gives the maximum response for beta radiation. However, this response cannot be directly related to Hp(0.07) or Hp(3). The thickness of the disc of 0.8 mm modifies the beta response and therefore, correction is required in terms of reduction by an appropriate factor. In view of the observation that the ratio ND3/ND2 is nearly 1 for photon radiations, it can be assumed that the presence of beta is detected by a ratio ND3/ND2 of more than 1.3 (value based on previous results(2)). For pure 90Sr/90Y beta fields for normal incidence at various dose levels, ND3/mSv of delivered Hp(3) is shown in Table 2. It can be observed that ND3/mSv remains the same for doses as low as 1 mSv also. The ratio ND3/ND2 (R32) and ND3/mSv of delivered Hp(3) for both 90Sr/90Y and 106Ru/106Rh as a function of angle of incidence are shown in Table 3. Table 2. Variation of ND3/mSv of 90Sr/90Y beta radiation as a function of delivered Hp(3). Delivered Hp(3) (mSv)  ND3/mSv  1  2.82 ± 0.08  2.5  2.87 ± 0.07  5  2.84 ± 0.09  7.5  2.87 ± 0.04  10  2.84 ± 0.09  Delivered Hp(3) (mSv)  ND3/mSv  1  2.82 ± 0.08  2.5  2.87 ± 0.07  5  2.84 ± 0.09  7.5  2.87 ± 0.04  10  2.84 ± 0.09  Table 3. Ratio of disc response R32 (ND3/ND2) and response of disc D3/Hp(3) of 90Sr/90Y and 106Ru/106Rh beta sources for various angles. Beta source  Angle  R32  ND3/Hp(3) (mSv)  90Sr/90Y  0°  2.14 ± 0.06  2.87 ± 0.07  15°  2.07 ± 0.10  2.90 ± 0.11  30°  2.40 ± 0.26  3.25 ± 0.24  45°  2.90 ± 0.34  3.60 ± 0.25  60°  3.05 ± 0.47  3.20 ± 0.38  106Ru/106Rh  0°  1.37 ± 0.03  1.55 ± 0.04  15°  1.44 ± 0.07  1.55 ± 0.04  30°  1.62 ± 0.18  1.60 ± 0.10  45°  1.85 ± 0.14  1.70 ± 0.12  60°  1.95 ± 0.11  2.15 ± 0.18  Beta source  Angle  R32  ND3/Hp(3) (mSv)  90Sr/90Y  0°  2.14 ± 0.06  2.87 ± 0.07  15°  2.07 ± 0.10  2.90 ± 0.11  30°  2.40 ± 0.26  3.25 ± 0.24  45°  2.90 ± 0.34  3.60 ± 0.25  60°  3.05 ± 0.47  3.20 ± 0.38  106Ru/106Rh  0°  1.37 ± 0.03  1.55 ± 0.04  15°  1.44 ± 0.07  1.55 ± 0.04  30°  1.62 ± 0.18  1.60 ± 0.10  45°  1.85 ± 0.14  1.70 ± 0.12  60°  1.95 ± 0.11  2.15 ± 0.18  Hp(3) is the dose at 3 mm depth in tissue and theoretically the value of this quantity decreases with the increasing angle for photons. The decrease is probably due to increase in effective thickness of tissue at larger angles which affects lower energy photons to a greater extent than the higher energy photons. So, for higher photon energies, the decrease is almost negligible. Experimentally, it is observed that for the same air kerma value, there is a difference in response of dosemeter at various angles of incidence for 241Am photons as shown in Figure 4. At larger angles, the effective thickness of the filter for the incident photon increases. It is also possible that some incident radiation reaches the dosemeter without passing through the filter. Both of these reasons contribute to the observed variation in readings at larger angular incidence. The same variation is not observed for 60Co, since the high energy photon is not significantly attenuated by the metal filter. Figure 4. View largeDownload slide Variation of air kerma for photon beam at incidence angles. S, standard radionuclide source (ISO 4037-3:1999(E)). Figure 4. View largeDownload slide Variation of air kerma for photon beam at incidence angles. S, standard radionuclide source (ISO 4037-3:1999(E)). In view of the fact that variation is observed at angular irradiation (α) and energy (E) of the incident photon, it was considered more appropriate to use the angle-averaged value for each energy point to arrive at the algorithm. To arrive at the angle-averaged value, the normalized responses, i.e. ND1/mSv, ND2/mSv and ND3/mSv of Hp(3) were averaged over all angles of incidence and denoted as angle-averaged response. Figure 5 shows the variation of angle-averaged ratio R12 with energy of photon radiation. The angle average ratio R12 can be denoted as R12(E) is given by the following equation:   R12(E)=[[∑∝R∝12(E)]ver+[∑∝R∝12(E)]horiz]/12 (1)where, α = 0°, 15°, 30°, 45°, 60°, 75°. Figure 5. View largeDownload slide Plot of angle-averaged ratio ND1/D23 (R12) with effective energy of photon beams. Figure 5. View largeDownload slide Plot of angle-averaged ratio ND1/D23 (R12) with effective energy of photon beams. Development of an algorithm Various methods for development of algorithm are mentioned in the report of the Action Group ‘Harmonization and dosemetric quality assurance in individual monitoring for external radiation’ published by EURADOS.(14) In case of single energy independent detector, no algorithm is used. Dose is directly calculated from the background subtracted signal after applying a sensitivity correction factor (CF). In the case of multi-detector design, a linear combination of the signal of multiple detectors can be used to obtain the quantity. Depending on the ratio of different detectors response, a combination of formula can also be used with the help of branching condition ‘If-then’. In the present study, to develop an algorithm, polynomial branching algorithm methods were tried. The polynomial algorithm was developed by fitting a polynomial curve to the plot of angle-averaged R12 with average of net response ND2 and ND3 (D23). An alternate would be a polynomial curve to the plot of angle-averaged R12 with net response ND1 (shown as D1 in Figure 6). The polynomial fit for D23 is shown in Figure 6. However, since disc D1 is under a metal filter of 1060 mg/cm2, using it, as an estimation of dose at depth of 300 mg/cm2 was not considered suitable. The single second order polynomial equation generated from the average net response ND2 and ND3 was found to give the best fit and appears to be most suitable for the application of Hp(3) dose evaluation and is given below. The second order polynomial fit (D23) is shown in Figure 6.   CF=(13.44×R12×R12)−(24.85×R12)+12.41 (2)where CF is the relative response per mSv of delivered dose. Figure 6. View largeDownload slide Relative response per mSv of Hp(3) of ND1 and D23 with R12 for photons. Solid diamond for ND1 response and solid square for average response of D23. The solid line represent polynomial fit. Figure 6. View largeDownload slide Relative response per mSv of Hp(3) of ND1 and D23 with R12 for photons. Solid diamond for ND1 response and solid square for average response of D23. The solid line represent polynomial fit. In the case of an unknown reading, the algorithm for dose evaluation could be arrived at as follows:   DoseHp(3)=0.5×(ND2+ND3)1000CF (3) Validity check of algorithm To confirm the validity of the algorithm, Hp(3) was evaluated for all energies and angles of incidence of dosemeters exposed to photon beams using the Equations 2 and 3. The ratio (Q) of evaluated Hp(3) to delivered Hp(3) was also calculated. The Q value for normal incidence is given in Table 4. The Q value is given without any uncertainty as the delivered Hp(3) is considered to be the true value. Table 4. Ratio of estimated to delivered Hp(3) (Q values) for various photon beams for normal incidence. Photon beam  Effective energy (keV)  Delivered Hp(3) (mSv)  Estimated Hp(3) dose (mSv) by algorithm  Ratio (estimated dose/delivered dose) (Q)  N40  33  3.55  3.45 ± 0.17  0.98  N60  48  2.60  2.5 ± 0.09  0.95  N100  83  3.10  3.15 ± 0.07  1.01  N120  100  3.05  2.7 ± 0.29  0.90  N150  118  4.25  4.9 ± 0.68  1.16  N250  208  2.80  2.25 ± 0.20  0.81  241Am  59.5  1.60  1.57 ± 0.09  0.99  137Cs  662  3.20  3.38 ± 0.34  1.06  60Co  1250  3.05  3.15 ± 0.27  1.04  Photon beam  Effective energy (keV)  Delivered Hp(3) (mSv)  Estimated Hp(3) dose (mSv) by algorithm  Ratio (estimated dose/delivered dose) (Q)  N40  33  3.55  3.45 ± 0.17  0.98  N60  48  2.60  2.5 ± 0.09  0.95  N100  83  3.10  3.15 ± 0.07  1.01  N120  100  3.05  2.7 ± 0.29  0.90  N150  118  4.25  4.9 ± 0.68  1.16  N250  208  2.80  2.25 ± 0.20  0.81  241Am  59.5  1.60  1.57 ± 0.09  0.99  137Cs  662  3.20  3.38 ± 0.34  1.06  60Co  1250  3.05  3.15 ± 0.27  1.04  N, narrow series; S, standard radionuclide source (ISO 4037 -3:1999(E)). The Q value generated for photon beams and angle combination are plotted against the ratio R12 and is presented in Figure 7. It can be seen that the higher percentage variations (outliers) are for photon beams N150 and N250 for 75°. The Q values up to angles of ± 60° are all within the acceptable range of uncertainty (0.67–1.50)(15) Figure 7. View largeDownload slide Performance of dosemeter in terms of Hp(3) in pure photon field based on new algorithm. Hul, upper limit; Hll, lower limit. N250 and N150 represent X-ray beams narrow series of peak voltage 250 kV (effective energy: 208 KeV) and 150 kV (effective energy: 118 KeV). Figure 7. View largeDownload slide Performance of dosemeter in terms of Hp(3) in pure photon field based on new algorithm. Hul, upper limit; Hll, lower limit. N250 and N150 represent X-ray beams narrow series of peak voltage 250 kV (effective energy: 208 KeV) and 150 kV (effective energy: 118 KeV). The observed angular variation of Hp(3) with respect to normal incidence in the photon energy range of 33–1250 keV is plotted and are shown in Figure 8 and can be compared with the theoretical variation. Figure 8. View largeDownload slide Photon angle dependence of response/Hp(3) of the Head TLD dosemeter. N, narrow series; S, standard radionuclide source (ISO 4037-3:1999(E)). Figure 8. View largeDownload slide Photon angle dependence of response/Hp(3) of the Head TLD dosemeter. N, narrow series; S, standard radionuclide source (ISO 4037-3:1999(E)). In the horizontal orientation of the badge, the response increases with increasing angle as the dosemeter could see the radiation from the side of the badge, where there is an absence of metal filter and hence there is increase in the angular response. However, in the vertical orientation at higher angles, the beam sees a greater effective thickness of the metal filter resulting in decrease in the response. It has been reported(8, 9) that the Hp(3) values obtained from slab and ORAMED cylinder phantom are different at angles >45°. The use of the slab phantom could also be a reason for the observed variation at large angles. Hp(3) evaluation for beta radiation In a pure beta field, as seen in Table 2, the response of disc D3 overestimates the true dose by a factor of 2.85 for all doses for 90Sr/90Y, hence, for the estimation of Hp(3) through disc D3 response for 90Sr/90Y, a CF of 0.35(1/2.85) can be applied. However, in the field condition the dosemeter will face radiation from all angles. The ratio of ND3/ND2 in such a situation can vary as shown in Table 3. The values of ND3/mSv for 106Ru/106Rh source for various angles are also included in Table 3. To estimate the Hp(3) in practical situations, the variation in the ND3/mSv as a function of ratio R32 is plotted as shown in Figure 9. A general CF has been arrived at by fitting a straight line to the plot (Figure 9) and is given by the following equation:   CF=11270×R32−249.3 (4) Figure 9. View largeDownload slide Plot of ND3 (with reference to Hp(3)) as a function of R32 along with linear fitting. Figure 9. View largeDownload slide Plot of ND3 (with reference to Hp(3)) as a function of R32 along with linear fitting. The dose equivalent Hp(3) due to beta will be given by the following equation:   Hp(3)=(ND3−ND1)×CF (5) In the present study, high energy beta sources 90Sr/90Y (2.2 MeV) and 106Ru/106Rh (3.50 MeV) are only used as the lower energy sources such as 147Pm,85Kr would not contribute dose to eye lens at 3 mm depth. Though the dosemeter would respond to lower energies of beta radiation, Hp(3) should not be evaluated for such conditions. Therefore, a cutoff point is required for evaluation of Hp(3). This cutoff point can be obtained from the ratio of R32, which is highly dependent on the energy of the incident beta radiation. At lower energies, this ratio will be higher since the PMMA filter of thickness 1.6 mm would cut off lower energy betas. It is observed that for pure beta radiation and various angles, the maximum value of R32 is 3.05 for 90Sr/90Y or higher energies. Therefore, it is concluded that Hp(3) should be evaluated only when R32 ≤ 3.05. This ratio is also affected by the presence of gamma radiation. However, the increase in photon component will only further decrease the ratio since bothND2 and ND3 will be increased equally in the presence of photon radiation and the ratio will be highest only for pure beta radiation. It is noted that the ratio of R32 = 1.3 is the lower cut off for the identification of beta.(2) Equations 4 and 5 were applied for dosemeters which have been irradiated to pure beta radiations to evaluate Hp(3). The delivered Hp(3), estimated Hp(3) and Q value are given in Table 5. Table 5. Performance of the dosemeter for evaluation of Hp(3) in beta field for all angles. Irradiation quality  Angle  Delivered dose Hp(3) (mSv) (D)  Estimated dose Hp(3) (mSv) (E)  Q(E/D) Hp(3)  Beta  Beta  90Sr/90Y  0°  2.50  3.09 ± 0.11  1.24  90Sr/90Y  15°  2.50  3.03 ± 0.08  1.21  90Sr/90Y  30°  2.50  2.89 ± 0.15  1.16  90Sr/90Y  45°  2.50  2.59 ± 0.16  1.04  90Sr/90Y  60°  2.50  2.13 ± 0.14  0.85  106Ru/106Rh  0°  10.04  10.02 ± 0.48  0.99  106Ru/106Rh  15°  9.50  8.95 ± 0.33  0.94  106Ru/106Rh  30°  8.30  7.25 ± 0.53  0.87  106Ru/106Rh  45°  6.30  4.90 ± 0.19  0.80  106Ru/106Rh  60°  3.40  3.75 ± 0.19  1.10  Irradiation quality  Angle  Delivered dose Hp(3) (mSv) (D)  Estimated dose Hp(3) (mSv) (E)  Q(E/D) Hp(3)  Beta  Beta  90Sr/90Y  0°  2.50  3.09 ± 0.11  1.24  90Sr/90Y  15°  2.50  3.03 ± 0.08  1.21  90Sr/90Y  30°  2.50  2.89 ± 0.15  1.16  90Sr/90Y  45°  2.50  2.59 ± 0.16  1.04  90Sr/90Y  60°  2.50  2.13 ± 0.14  0.85  106Ru/106Rh  0°  10.04  10.02 ± 0.48  0.99  106Ru/106Rh  15°  9.50  8.95 ± 0.33  0.94  106Ru/106Rh  30°  8.30  7.25 ± 0.53  0.87  106Ru/106Rh  45°  6.30  4.90 ± 0.19  0.80  106Ru/106Rh  60°  3.40  3.75 ± 0.19  1.10  To test this algorithm in case of mixed photon and beta radiation, the responses ND1, ND2 and ND3 have been theoretically generated (Table 6). This is done by adding the response of the individual discs corresponding to 1 mSv Hp(3) of 90Sr/90Y and 1 mSv of 137Cs radiations in various proportions (Table 6). Further, for each combination of beta–gamma, the ratio of R32 and the corresponding estimated Hp(3) and Q values were generated. It can be seen that, irrespective of the different proportions of beta and gamma radiations, the Q values are satisfactory. (It may be noted that since the values are theoretically generated, no uncertainties are assigned). However, beta may not be identified if the gamma component is much higher (more than a factor of 3) than the beta component. To confirm the validity of these theoretical calculations, set of dosemeters were irradiated to beta–gamma mixtures (normal incidence) and the corresponding values of estimated Hp(3) and Q values are shown in Table 7. It can be observed that the theoretical Q values match reasonably well with the experimental Q values for the same proportions (rows 13 and 11 of Table 6 can be compared with rows 1 and 2 of Table 7). Table 6. Q values for mixed beta–gamma (137Cs + 90Sr/90Y) radiation for normal incidence (theoretical). Serial no.  Delivered Hp(3) gamma + beta 90Sr/90Y  ND1  ND2  ND3  Ratio R32  Estimated Hp(3) (E) mSv  Q (E/D)  1  1 + 1  1025  2441  3783  1.550  2.63  1.315  2  1 + 2  1055  3863  6489  1.680  3.94  1.313  3  1 + 3  1085  5284  9195  1.740  5.22  1.305  4  1 + 4  1114  6705  11 901  1.775  6.49  1.299  5  1 + 5  1144  8127  14 607  1.797  7.76  1.294  6  1 + 6  1173  9548  17 313  1.813  9.03  1.290  7  1 + 7  1203  10 969  20 019  1.825  10.30  1.287  8  1 + 8  1233  12 391  22 724  1.834  11.57  1.285  9  1 + 9  1262  13 812  25 430  1.841  12.83  1.283  10  1 + 10  1292  15 233  28 136  1.847  14.10  1.282  Serial no.  Delivered Hp(3) gamma + beta 90Sr/90Y  ND1  ND2  ND3  Ratio R32  Estimated Hp(3) (E) mSv  Q (E/D)  1  1 + 1  1025  2441  3783  1.550  2.63  1.315  2  1 + 2  1055  3863  6489  1.680  3.94  1.313  3  1 + 3  1085  5284  9195  1.740  5.22  1.305  4  1 + 4  1114  6705  11 901  1.775  6.49  1.299  5  1 + 5  1144  8127  14 607  1.797  7.76  1.294  6  1 + 6  1173  9548  17 313  1.813  9.03  1.290  7  1 + 7  1203  10 969  20 019  1.825  10.30  1.287  8  1 + 8  1233  12 391  22 724  1.834  11.57  1.285  9  1 + 9  1262  13 812  25 430  1.841  12.83  1.283  10  1 + 10  1292  15 233  28 136  1.847  14.10  1.282    Beta + gamma  ND1  ND2  ND3  R32  E  Q(E/D)  11  1 + 1  1025  2441  3783  1.550  2.63  1.315  12  1 + 2  2021  3462  4861  1.404  3.87  1.291  13  1 + 3  3017  4482  5938  1.325  5.05*  1.264  14  1 + 4  4013  5502  7015  1.275  4.01*  0.803  15  1 + 5  5009  6522  8093  1.241  5.01*  0.835  16  1 + 6  6005  7542  9170  1.216  6.01*  0.858  17  1 + 7  7001  8562  10 247  1.197  7.00*  0.875  18  1 + 8  7997  9583  11 325  1.182  8.00*  0.889  19  1 + 9  8993  10 603  12 402  1.170  8.99*  0.899  20  1 + 10  9989  11 623  13 480  1.160  9.99*  0.908    Beta + gamma  ND1  ND2  ND3  R32  E  Q(E/D)  11  1 + 1  1025  2441  3783  1.550  2.63  1.315  12  1 + 2  2021  3462  4861  1.404  3.87  1.291  13  1 + 3  3017  4482  5938  1.325  5.05*  1.264  14  1 + 4  4013  5502  7015  1.275  4.01*  0.803  15  1 + 5  5009  6522  8093  1.241  5.01*  0.835  16  1 + 6  6005  7542  9170  1.216  6.01*  0.858  17  1 + 7  7001  8562  10 247  1.197  7.00*  0.875  18  1 + 8  7997  9583  11 325  1.182  8.00*  0.889  19  1 + 9  8993  10 603  12 402  1.170  8.99*  0.899  20  1 + 10  9989  11 623  13 480  1.160  9.99*  0.908  *Beta radiation is not identified. Hp(3) estimation is for pure photon radiation. Table 7. Q values for mixed beta–gamma radiation for normal incidence (experimental). Irradiation quality  Delivered dose, Hp(3) (mSv)(D)  Estimated dose, Hp(3) (mSv)(E)  Q (E/D)(Hp(3))  Gamma  Beta  Gamma  Beta  137Cs + 90Sr/90Y  3.20  1.00  2.99 ± 0.10  2.33 ± 0.01  1.27  137Cs + 90Sr/90Y  3.20  2.50  3.20 ± 0.10  4.07 ± 0.02  1.27  Irradiation quality  Delivered dose, Hp(3) (mSv)(D)  Estimated dose, Hp(3) (mSv)(E)  Q (E/D)(Hp(3))  Gamma  Beta  Gamma  Beta  137Cs + 90Sr/90Y  3.20  1.00  2.99 ± 0.10  2.33 ± 0.01  1.27  137Cs + 90Sr/90Y  3.20  2.50  3.20 ± 0.10  4.07 ± 0.02  1.27  In order to implement in the routine personnel dosimetry program, a flow chart for evaluation of Hp(3) for photon beam and mixed gamma–beta radiation is shown in Figure 10. Figure 10. View largeDownload slide Hp(3) dose evaluation flow chart. Figure 10. View largeDownload slide Hp(3) dose evaluation flow chart. CONCLUSIONS Estimation of Personal dose equivalent in terms of Hp(3) as per the international recommendation for both pure and mixed photon fields in the energy range of 30–1250 keV can be achieved in a conservative manner by using the existing TLD badge. An algorithm has been generated for the estimation of doses in terms of operational quantity Hp(3). This algorithm has also been extended to evaluate Hp(3) dose from beta sources of 90Sr/90Sr and 106Ru/106Rh. The algorithm designed based on experimental data will take into account the variation in response of three discs of TLD badge occurring due to variation in energies and angle of incidence. This work will help in accurate estimation of dose to eye lens received by radiation workers especially those working in interventional radiology and other work places where dose to the eye lens maybe significant. ACKNOWLEDGMENTS The authors are thankful to Dr Pradeepkumar K.S., Associate Director, HS&E Group for his encouragement in the work. REFERENCES 1 Vohra, K. G., Bhatt, R. C., Chandra, B., Pradhan, A. S., Lakshmanan, A. R. and Sastry, S. S. A personnel dosemeter TLD badge based on CaSO4:Dy teflon TLD discs. Health. Phys.  38, 193– 197 ( 1980). Google Scholar CrossRef Search ADS PubMed  2 Pradhan, S. M., Sneha, C., Chourasiya, G., Adtani, M. M., Tripathi, S. M. and Singh, S. K. Development of an algorithm for evaluating personal doses due to photon fields in terms of operational quantities for TLD badge system in India. Radiat. Prot. Dosimetry  136( 3), 176– 184 ( 2009). Google Scholar CrossRef Search ADS PubMed  3 Bakshi., A. K., Srivastava., K., Varadharajan., G., Pradhan., A. S. and Kher., R. K. Development of an algorithm for TLD badge system for dosimetry in the field of X and gamma radiation in terms of Hp(10). Radiat. Prot. Dosimetry  123( 2), 148– 155 ( 2007). Google Scholar CrossRef Search ADS PubMed  4 International Commission on Radiological Protection (ICRP). Statement on Tissue Reactions (2011). www.icrp.org//page.asp, retrieved 10-2011. 5 Behrens, R. et al.  . Intercomparison of eye lens dosemeters. Radiat. Prot. Dosimetry  ncw051 ( 2016) doi:10:1093/rpd/ncw051. 6 International Organisation for Standardisation. Calibration of area and personal dosemeters and the measurement of their response as a function of energy and angle of incidence. Part-3, ISO-4037-3 (Geneva:ISO) (1999). 7 Vanhavere, F. et al.  . Measurements of eye lens doses in interventional radiology and cardiology: final results of the ORAMED project. Radiat. Meas.  46, 1243– 1247 ( 2011). Google Scholar CrossRef Search ADS   8 Behrens, R. and Hupe, O. Influence of the phantom shape (slab, cylinder or alderson) on the performance of an Hp(3) eye dosemeter. Radiat. Prot. Dosimetry  168( no 44), 441– 449 ( 2016). Google Scholar CrossRef Search ADS PubMed  9 Behrens, R. On the necessity of a new cylinder phantom for eye lens dosimetry. J. Radiol. Prot.  32, 455– 464 ( 2012). Google Scholar CrossRef Search ADS PubMed  10 Radiation protection instrumentation—passive integrating dosimetry systems for personal and environmental of photon and beta radiation. IEC-62387-2012, Edition 1.0 2012-12. 11 Kulkarni, M. S., Pradeep, R. and Kannan, S. A new PC based semi-automatic TLD badge reader system for personnel monitoring. Proceedings of the 10th International Congress of the International Radiation Protection Association on Harmonization of Radiation, Human Life and the Ecosystem (IRPA-10), (p. 1 v) No. P-3b-167, May 2000, Tokyo, Japan: Japan Health Physics Society (2000). 12 Ambrosi, P., Buchholz, G. and Helmstädter, K. Physikalisch-TechnischeBundesanstalt, Bundesallee 100, D-38116 Braunschweig, Germany. Published by institute of Physics publishing and sissa, November 8, 2007. The PTB Beta Secondary Standard BSS2 for radiation protection. 13 Bakshi, A. K., Vandana, S., PalaniSelvam, T., Chougaonkar, M. P. and Mayya, Y. S. Measurement of the output of ISO recommended beta sources with an extrapolation chamber. Radiat. Meas.  53–54, 50– 55 ( 2013). Google Scholar CrossRef Search ADS   14 EURADOS Report of the Action Group. Harmonization and dosimetric quality assurance in individual monitoring for external radiation. Part 2, A catalogue of dosemeters and dosimetric services within EU Member States and Switzerland able to estimate external radiation doses as personal dose equivalent. Edited by J.W.E. van Dijk, J.M. Bordy, F. Vanhavere, C. Wernli and M. Zamani-Valasiadou. European Radiation Dosimetry Group (1999). 15 ISO 14146. Radiation protection—criteria and performance limits for the periodic evaluation of processors of personal dosemeters for X and gamma radiation (2000). © The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

DEVELOPMENT OF AN ALGORITHM TO ESTIMATE EYE LENS DOSE IN TERMS OF OPERATIONAL QUANTITY Hp(3) USING HEAD TLD BADGE

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Abstract

Abstract In view of the recommendations of International Commission on Radiological Protection for reduction of the occupational annual dose limit for eye lens from 150 mSv to 20 mSv/y, questions have been raised on the adequacy of monitoring for the quantities Hp(10) and Hp(0.07). As an immediate requirement, in the present situation, where there is no exclusive eye lens dosemeter in India, the existing chest TLD badge was modified to be used as head badge (head dosemeter) by including a strap to enable wearing on the forehead. In order to estimate the eye lens dose in terms of the operational quantity Hp(3), the prevalent algorithm of chest badge was also modified. The modified algorithm was applied to estimate Hp(3) for dosemeters irradiated to various beta and photon radiations including mixtures. The Q values (estimated/delivered dose equivalent) were found to be within ±20% for most of the photon beams. INTRODUCTION In India, personnel monitoring of radiation workers against external radiation hazards of X, beta and gamma radiation is carried out using a thermoluminescence dosemeter (TLD) system based on CaSO4: Dy Teflon TLD disc.(1) This dosemeter can be used for estimation of doses in terms of the ICRU recommended quantities personal dose equivalent Hp(d) at 10 and 0.07 mm depth.(2, 3) In view of the fact that the International Commission on Radiological Protection (ICRP)(4) revised lifetime eye dose threshold for cataract induction downwards from 2000 to 500 mGy, and the occupational annual dose limit for eye lens from 150 to 20 mSv in April 2011, questions have been raised on the adequacy of monitoring for the quantities Hp(10) and Hp(0.07) without monitoring for Hp(3). This is especially true for occupational workers who could get a high dose to eye, working as interventional radiologist. In India, regulations have not yet been put in place to enforce the new limits. However, work is being carried out to assess and gather wide knowledge with respect to the workers eye lens dose, especially in medical interventional radiological facilities. The objectives include design of eye lens dosemeter, methodology for its implementation in routine service and appropriate line of action to be taken if the limit is crossed. At present in India, in routine monitoring, Hp(3) is not measured. Earlier, it was concluded that if Hp(0.07) and Hp(10) values are not exceeded, Hp(3) is considered to be within limits. Given the reduction in the dose limit for eye lens, the earlier assumption may not be valid.(4, 5) As an immediate requirement in the present situation, where there is no exclusive eye lens dosemeter, the existing chest TLD badge is modified to include a strap to enable the worker to wear it on the forehead. As the location on the forehead is in close proximity to eye, an assumption can be made that it represents the dose to the eye lens. However, the algorithm used for evaluation of the dose must be modified to represent the value of the operational quantity Hp(3). The present work was carried out to arrive at an estimate of Hp(3) with the use of the modified chest badge and algorithm. In the absence of any information in ISO(6) regarding the dimensions of the phantom to be used for Hp(3), water filled slab phantom was used. Later, the ORAMED(7) project, which addressed the implementation of the eye lens dose, has suggested a cylindrical phantom of ICRU tissue with 20 cm diameter and 20 cm height. However, in later papers(8, 9) it was also suggested that the change of phantom does not significantly affect the response for angles up to 45°. IEC 62 387-2012(10) has also recommended the use of slab phantom. In view of this the present study was carried out with ISO water slab phantom, considering that it will not significantly affect the results. MATERIAL AND METHODS The regular TL badge (dosemeter)(1, 3) used for personnel monitoring of radiation workers is modified to include a strap to enable wearing on the fore-head and is used for this study. The dosemeter comprises of three identical CaSO4:Dy Teflon discs each (13.6 mm diameter and thickness 0.8 mm) mechanically clipped over three circular holes each of diameter 12 mm on a nickel plated aluminum (Al) card of thickness 1 mm (TLD card) and TLD card is loaded in a plastic holder having three different filter regions on the front and rear side.(1, 3) The dosemeter disc 1 (D1) is under the metal filter (Cu + Al), disc 2 (D2) is under a Perspex filter and disc 3 (D3) is open having identification paper and polythene seal. TLD badge (dosemeter) is shown in the Figure 1. Figure 1. View largeDownload slide Schematic diagram of TLD badge. Figure 1. View largeDownload slide Schematic diagram of TLD badge. A set of 300 TLD cards from single batch having mean TL (light output in arbitrary unit) exposed to a test dose (3 mSv) for all three discs within ±5% was selected for the study. TLD card sealed in black polythene pouch and loaded in cassettes (TLD dosemeter) were irradiated to various photon beams as given in Table 1. All irradiations were carried out at 2 m distance as per ISO 4037.(6) During irradiations, the dosemeters were backed by an ISO water filled slab phantom having dimension 30 cm × 30 cm × 15 cm.(6) To account for field conditions which may include all directions of irradiation, exposures in both vertical and horizontal orientation of the dosemeter for the entire beam qualities were carried out. Since the dosemeter is asymmetric, for each angle of irradiation, four orientations were considered, two vertical and two horizontal. The angles of irradiation were 0°, 15°, 30°, 45°, 60° and 75° and for each angle, the irradiations were carried out in four orientations of the badge. Four dosemeters were irradiated at a time after ensuring that the df (approximate locus of the 98% isodose contour with respect to the dose in the center of the phantom) values given in ISO 4037(6) were not exceeded. The positioning of the dosemeters on phantom is shown in Figure 2. The delivered Hp(3) for each beam quality are also given in Table 1. It is assumed that the delivered Hp(3) is the true value and therefore no uncertainty is assigned to it. Table 1. Details of photon beams used for irradiation. Serial no.  Photon beams  Mean energy (keV)  Air kerma rate (mGy/h)  Conversion coefficient hpk (Sv/Gy) for normal incidence  Delivered Hp(3) (mSv)  1  N-40  33  45.74  1.29  3.55  2  N-60  48  51.84  1.63  2.60  3  N-100  83  12.52  1.81  3.10  4  N-120  100  11.85  0.74  3.05  5  N-150  118  49.88  1.66  4.25  6  N-250  208  20.78  1.46  2.80  7  S-Am  59.5  0.21  1.80  1.60  8  S-Cs  662  2.91  1.22  3.20  9  S-Co  1250  2.18  1.16  3.05  Serial no.  Photon beams  Mean energy (keV)  Air kerma rate (mGy/h)  Conversion coefficient hpk (Sv/Gy) for normal incidence  Delivered Hp(3) (mSv)  1  N-40  33  45.74  1.29  3.55  2  N-60  48  51.84  1.63  2.60  3  N-100  83  12.52  1.81  3.10  4  N-120  100  11.85  0.74  3.05  5  N-150  118  49.88  1.66  4.25  6  N-250  208  20.78  1.46  2.80  7  S-Am  59.5  0.21  1.80  1.60  8  S-Cs  662  2.91  1.22  3.20  9  S-Co  1250  2.18  1.16  3.05  N, narrow series; S, standard radionuclide source (ISO 4037-3: 1999(E)). Figure 2. View largeDownload slide Schematic of dosemeter position on the phantom for irradiation. Vertical orientation (1,3) for positive and negative angle and Horizontal orientation (2,4) for negative and positive angle. Figure 2. View largeDownload slide Schematic of dosemeter position on the phantom for irradiation. Vertical orientation (1,3) for positive and negative angle and Horizontal orientation (2,4) for negative and positive angle. The conversion coefficient hpk (3; N, α) from air kerma to the dose equivalent Hp(3) for the radiation quantities defined in ISO 4037-3(6) is taken from IEC 62 387-2012(10) for various angles. The exposed TLD cards were readout on a calibrated semi automatic TLD dosemeter reader.(11) Before reading, the reader was calibrated using ten dosemeters exposed on water filled ISO slab phantom to 3 mSv Hp(3) of 137Cs. Reader calibration factor (RCF) in terms of mSv/TL, was generated from the average output of these 10 TLD readings. For all read cards, the TL of each of the disc D1, D2, D3 was normalized to 137Cs photon readings. For normalizing, the net TL output of the discs exposed to various energies of photon, mixture of photon + beta and pure beta (reading—background) (or ND1, ND2, ND3) was multiplied with the (RCF). The normalized readings divided by the true dose equivalent Hp(3) is henceforth referred to as the response (TL/mSv) of the individual disc. Irradiations were also carried out to beta radiation using PTB beta secondary standard calibration system (BSS-2)(12, 13) which has 90Sr/90Ysource with calibrated output in terms of Hp(3).(10) TLD cards were exposed to pure 90Sr/90Y beam at normal incidence using this system. During irradiations, the dosemeters were placed on a poly methyl metha acrylate (PMMA) slab phantom having dimension 30 cm × 30 cm × 5 cm. For studying the angular response, dosemeters were irradiated at horizontal and vertical orientations with phantom, angles at 0°, 15°, 30°, 45° and 60° with respect to the source. To check the validity of the algorithm in a mixed beta gamma field, some dosemeters were also irradiated to mixtures of 137Cs and 90Sr/90Y at normal incidence. Irradiations were also carried out to beta radiation with106Ru/106Rh source. In view of the fact that only Dp(0.07) values were available for this source for normal incidence, the conversion coefficients given in IEC(10) were used for estimating delivered doses in terms of Hp(3) when performing the irradiations. Studies similar to 90Sr/90Y were carried out both at normal and angular incidence. RESULTS AND DISCUSSION As it has been reported previously by other authors,(2, 3) normalized response of D1 with a filtration of 1060 mg/cm2 is not flat over the entire photon energy range. It is, however, equal to unity for energies >200 keV. Disc D2 with a filtration of 180 mg/cm2 of PMMA does not show any significant reduction in the response due to photons and is within 10% of disc D3 response for the entire photon energy range. Therefore, for all practical purposes it may be assumed that the ratio ND3/ND2 is nearly equal to 1 for all the photon energies. The energy dependent behavior is due to the non-tissue equivalence of the detector material (CaSO4:Dy) leading to higher response at lower energies. In actual field condition, the energy of the photon is not known. Therefore, for the identification of the photon energy, we have decided to use the ratio R12 (ratio of ND1 and D23 (average of ND2 and ND3)). The variation of R12 with energy is shown in Figure 3 for normal incidence. Figure 3. View largeDownload slide Plot of ratio ND1/D23 (R12) with effective energy of photon beams for normal incidence. Figure 3. View largeDownload slide Plot of ratio ND1/D23 (R12) with effective energy of photon beams for normal incidence. Due to the presence of metal filter Cu + Al of 1060 mg/cm2 thickness over the disc D1, it is expected that beta radiation does not contribute to the reading of D1 for mean beta energy ≤0.8 MeV. The disc D2 which is shielded by PMMA of thickness 180 mg/cm2 also shows a reduced response, whereas disc D3 with no filtration gives the maximum response for beta radiation. However, this response cannot be directly related to Hp(0.07) or Hp(3). The thickness of the disc of 0.8 mm modifies the beta response and therefore, correction is required in terms of reduction by an appropriate factor. In view of the observation that the ratio ND3/ND2 is nearly 1 for photon radiations, it can be assumed that the presence of beta is detected by a ratio ND3/ND2 of more than 1.3 (value based on previous results(2)). For pure 90Sr/90Y beta fields for normal incidence at various dose levels, ND3/mSv of delivered Hp(3) is shown in Table 2. It can be observed that ND3/mSv remains the same for doses as low as 1 mSv also. The ratio ND3/ND2 (R32) and ND3/mSv of delivered Hp(3) for both 90Sr/90Y and 106Ru/106Rh as a function of angle of incidence are shown in Table 3. Table 2. Variation of ND3/mSv of 90Sr/90Y beta radiation as a function of delivered Hp(3). Delivered Hp(3) (mSv)  ND3/mSv  1  2.82 ± 0.08  2.5  2.87 ± 0.07  5  2.84 ± 0.09  7.5  2.87 ± 0.04  10  2.84 ± 0.09  Delivered Hp(3) (mSv)  ND3/mSv  1  2.82 ± 0.08  2.5  2.87 ± 0.07  5  2.84 ± 0.09  7.5  2.87 ± 0.04  10  2.84 ± 0.09  Table 3. Ratio of disc response R32 (ND3/ND2) and response of disc D3/Hp(3) of 90Sr/90Y and 106Ru/106Rh beta sources for various angles. Beta source  Angle  R32  ND3/Hp(3) (mSv)  90Sr/90Y  0°  2.14 ± 0.06  2.87 ± 0.07  15°  2.07 ± 0.10  2.90 ± 0.11  30°  2.40 ± 0.26  3.25 ± 0.24  45°  2.90 ± 0.34  3.60 ± 0.25  60°  3.05 ± 0.47  3.20 ± 0.38  106Ru/106Rh  0°  1.37 ± 0.03  1.55 ± 0.04  15°  1.44 ± 0.07  1.55 ± 0.04  30°  1.62 ± 0.18  1.60 ± 0.10  45°  1.85 ± 0.14  1.70 ± 0.12  60°  1.95 ± 0.11  2.15 ± 0.18  Beta source  Angle  R32  ND3/Hp(3) (mSv)  90Sr/90Y  0°  2.14 ± 0.06  2.87 ± 0.07  15°  2.07 ± 0.10  2.90 ± 0.11  30°  2.40 ± 0.26  3.25 ± 0.24  45°  2.90 ± 0.34  3.60 ± 0.25  60°  3.05 ± 0.47  3.20 ± 0.38  106Ru/106Rh  0°  1.37 ± 0.03  1.55 ± 0.04  15°  1.44 ± 0.07  1.55 ± 0.04  30°  1.62 ± 0.18  1.60 ± 0.10  45°  1.85 ± 0.14  1.70 ± 0.12  60°  1.95 ± 0.11  2.15 ± 0.18  Hp(3) is the dose at 3 mm depth in tissue and theoretically the value of this quantity decreases with the increasing angle for photons. The decrease is probably due to increase in effective thickness of tissue at larger angles which affects lower energy photons to a greater extent than the higher energy photons. So, for higher photon energies, the decrease is almost negligible. Experimentally, it is observed that for the same air kerma value, there is a difference in response of dosemeter at various angles of incidence for 241Am photons as shown in Figure 4. At larger angles, the effective thickness of the filter for the incident photon increases. It is also possible that some incident radiation reaches the dosemeter without passing through the filter. Both of these reasons contribute to the observed variation in readings at larger angular incidence. The same variation is not observed for 60Co, since the high energy photon is not significantly attenuated by the metal filter. Figure 4. View largeDownload slide Variation of air kerma for photon beam at incidence angles. S, standard radionuclide source (ISO 4037-3:1999(E)). Figure 4. View largeDownload slide Variation of air kerma for photon beam at incidence angles. S, standard radionuclide source (ISO 4037-3:1999(E)). In view of the fact that variation is observed at angular irradiation (α) and energy (E) of the incident photon, it was considered more appropriate to use the angle-averaged value for each energy point to arrive at the algorithm. To arrive at the angle-averaged value, the normalized responses, i.e. ND1/mSv, ND2/mSv and ND3/mSv of Hp(3) were averaged over all angles of incidence and denoted as angle-averaged response. Figure 5 shows the variation of angle-averaged ratio R12 with energy of photon radiation. The angle average ratio R12 can be denoted as R12(E) is given by the following equation:   R12(E)=[[∑∝R∝12(E)]ver+[∑∝R∝12(E)]horiz]/12 (1)where, α = 0°, 15°, 30°, 45°, 60°, 75°. Figure 5. View largeDownload slide Plot of angle-averaged ratio ND1/D23 (R12) with effective energy of photon beams. Figure 5. View largeDownload slide Plot of angle-averaged ratio ND1/D23 (R12) with effective energy of photon beams. Development of an algorithm Various methods for development of algorithm are mentioned in the report of the Action Group ‘Harmonization and dosemetric quality assurance in individual monitoring for external radiation’ published by EURADOS.(14) In case of single energy independent detector, no algorithm is used. Dose is directly calculated from the background subtracted signal after applying a sensitivity correction factor (CF). In the case of multi-detector design, a linear combination of the signal of multiple detectors can be used to obtain the quantity. Depending on the ratio of different detectors response, a combination of formula can also be used with the help of branching condition ‘If-then’. In the present study, to develop an algorithm, polynomial branching algorithm methods were tried. The polynomial algorithm was developed by fitting a polynomial curve to the plot of angle-averaged R12 with average of net response ND2 and ND3 (D23). An alternate would be a polynomial curve to the plot of angle-averaged R12 with net response ND1 (shown as D1 in Figure 6). The polynomial fit for D23 is shown in Figure 6. However, since disc D1 is under a metal filter of 1060 mg/cm2, using it, as an estimation of dose at depth of 300 mg/cm2 was not considered suitable. The single second order polynomial equation generated from the average net response ND2 and ND3 was found to give the best fit and appears to be most suitable for the application of Hp(3) dose evaluation and is given below. The second order polynomial fit (D23) is shown in Figure 6.   CF=(13.44×R12×R12)−(24.85×R12)+12.41 (2)where CF is the relative response per mSv of delivered dose. Figure 6. View largeDownload slide Relative response per mSv of Hp(3) of ND1 and D23 with R12 for photons. Solid diamond for ND1 response and solid square for average response of D23. The solid line represent polynomial fit. Figure 6. View largeDownload slide Relative response per mSv of Hp(3) of ND1 and D23 with R12 for photons. Solid diamond for ND1 response and solid square for average response of D23. The solid line represent polynomial fit. In the case of an unknown reading, the algorithm for dose evaluation could be arrived at as follows:   DoseHp(3)=0.5×(ND2+ND3)1000CF (3) Validity check of algorithm To confirm the validity of the algorithm, Hp(3) was evaluated for all energies and angles of incidence of dosemeters exposed to photon beams using the Equations 2 and 3. The ratio (Q) of evaluated Hp(3) to delivered Hp(3) was also calculated. The Q value for normal incidence is given in Table 4. The Q value is given without any uncertainty as the delivered Hp(3) is considered to be the true value. Table 4. Ratio of estimated to delivered Hp(3) (Q values) for various photon beams for normal incidence. Photon beam  Effective energy (keV)  Delivered Hp(3) (mSv)  Estimated Hp(3) dose (mSv) by algorithm  Ratio (estimated dose/delivered dose) (Q)  N40  33  3.55  3.45 ± 0.17  0.98  N60  48  2.60  2.5 ± 0.09  0.95  N100  83  3.10  3.15 ± 0.07  1.01  N120  100  3.05  2.7 ± 0.29  0.90  N150  118  4.25  4.9 ± 0.68  1.16  N250  208  2.80  2.25 ± 0.20  0.81  241Am  59.5  1.60  1.57 ± 0.09  0.99  137Cs  662  3.20  3.38 ± 0.34  1.06  60Co  1250  3.05  3.15 ± 0.27  1.04  Photon beam  Effective energy (keV)  Delivered Hp(3) (mSv)  Estimated Hp(3) dose (mSv) by algorithm  Ratio (estimated dose/delivered dose) (Q)  N40  33  3.55  3.45 ± 0.17  0.98  N60  48  2.60  2.5 ± 0.09  0.95  N100  83  3.10  3.15 ± 0.07  1.01  N120  100  3.05  2.7 ± 0.29  0.90  N150  118  4.25  4.9 ± 0.68  1.16  N250  208  2.80  2.25 ± 0.20  0.81  241Am  59.5  1.60  1.57 ± 0.09  0.99  137Cs  662  3.20  3.38 ± 0.34  1.06  60Co  1250  3.05  3.15 ± 0.27  1.04  N, narrow series; S, standard radionuclide source (ISO 4037 -3:1999(E)). The Q value generated for photon beams and angle combination are plotted against the ratio R12 and is presented in Figure 7. It can be seen that the higher percentage variations (outliers) are for photon beams N150 and N250 for 75°. The Q values up to angles of ± 60° are all within the acceptable range of uncertainty (0.67–1.50)(15) Figure 7. View largeDownload slide Performance of dosemeter in terms of Hp(3) in pure photon field based on new algorithm. Hul, upper limit; Hll, lower limit. N250 and N150 represent X-ray beams narrow series of peak voltage 250 kV (effective energy: 208 KeV) and 150 kV (effective energy: 118 KeV). Figure 7. View largeDownload slide Performance of dosemeter in terms of Hp(3) in pure photon field based on new algorithm. Hul, upper limit; Hll, lower limit. N250 and N150 represent X-ray beams narrow series of peak voltage 250 kV (effective energy: 208 KeV) and 150 kV (effective energy: 118 KeV). The observed angular variation of Hp(3) with respect to normal incidence in the photon energy range of 33–1250 keV is plotted and are shown in Figure 8 and can be compared with the theoretical variation. Figure 8. View largeDownload slide Photon angle dependence of response/Hp(3) of the Head TLD dosemeter. N, narrow series; S, standard radionuclide source (ISO 4037-3:1999(E)). Figure 8. View largeDownload slide Photon angle dependence of response/Hp(3) of the Head TLD dosemeter. N, narrow series; S, standard radionuclide source (ISO 4037-3:1999(E)). In the horizontal orientation of the badge, the response increases with increasing angle as the dosemeter could see the radiation from the side of the badge, where there is an absence of metal filter and hence there is increase in the angular response. However, in the vertical orientation at higher angles, the beam sees a greater effective thickness of the metal filter resulting in decrease in the response. It has been reported(8, 9) that the Hp(3) values obtained from slab and ORAMED cylinder phantom are different at angles >45°. The use of the slab phantom could also be a reason for the observed variation at large angles. Hp(3) evaluation for beta radiation In a pure beta field, as seen in Table 2, the response of disc D3 overestimates the true dose by a factor of 2.85 for all doses for 90Sr/90Y, hence, for the estimation of Hp(3) through disc D3 response for 90Sr/90Y, a CF of 0.35(1/2.85) can be applied. However, in the field condition the dosemeter will face radiation from all angles. The ratio of ND3/ND2 in such a situation can vary as shown in Table 3. The values of ND3/mSv for 106Ru/106Rh source for various angles are also included in Table 3. To estimate the Hp(3) in practical situations, the variation in the ND3/mSv as a function of ratio R32 is plotted as shown in Figure 9. A general CF has been arrived at by fitting a straight line to the plot (Figure 9) and is given by the following equation:   CF=11270×R32−249.3 (4) Figure 9. View largeDownload slide Plot of ND3 (with reference to Hp(3)) as a function of R32 along with linear fitting. Figure 9. View largeDownload slide Plot of ND3 (with reference to Hp(3)) as a function of R32 along with linear fitting. The dose equivalent Hp(3) due to beta will be given by the following equation:   Hp(3)=(ND3−ND1)×CF (5) In the present study, high energy beta sources 90Sr/90Y (2.2 MeV) and 106Ru/106Rh (3.50 MeV) are only used as the lower energy sources such as 147Pm,85Kr would not contribute dose to eye lens at 3 mm depth. Though the dosemeter would respond to lower energies of beta radiation, Hp(3) should not be evaluated for such conditions. Therefore, a cutoff point is required for evaluation of Hp(3). This cutoff point can be obtained from the ratio of R32, which is highly dependent on the energy of the incident beta radiation. At lower energies, this ratio will be higher since the PMMA filter of thickness 1.6 mm would cut off lower energy betas. It is observed that for pure beta radiation and various angles, the maximum value of R32 is 3.05 for 90Sr/90Y or higher energies. Therefore, it is concluded that Hp(3) should be evaluated only when R32 ≤ 3.05. This ratio is also affected by the presence of gamma radiation. However, the increase in photon component will only further decrease the ratio since bothND2 and ND3 will be increased equally in the presence of photon radiation and the ratio will be highest only for pure beta radiation. It is noted that the ratio of R32 = 1.3 is the lower cut off for the identification of beta.(2) Equations 4 and 5 were applied for dosemeters which have been irradiated to pure beta radiations to evaluate Hp(3). The delivered Hp(3), estimated Hp(3) and Q value are given in Table 5. Table 5. Performance of the dosemeter for evaluation of Hp(3) in beta field for all angles. Irradiation quality  Angle  Delivered dose Hp(3) (mSv) (D)  Estimated dose Hp(3) (mSv) (E)  Q(E/D) Hp(3)  Beta  Beta  90Sr/90Y  0°  2.50  3.09 ± 0.11  1.24  90Sr/90Y  15°  2.50  3.03 ± 0.08  1.21  90Sr/90Y  30°  2.50  2.89 ± 0.15  1.16  90Sr/90Y  45°  2.50  2.59 ± 0.16  1.04  90Sr/90Y  60°  2.50  2.13 ± 0.14  0.85  106Ru/106Rh  0°  10.04  10.02 ± 0.48  0.99  106Ru/106Rh  15°  9.50  8.95 ± 0.33  0.94  106Ru/106Rh  30°  8.30  7.25 ± 0.53  0.87  106Ru/106Rh  45°  6.30  4.90 ± 0.19  0.80  106Ru/106Rh  60°  3.40  3.75 ± 0.19  1.10  Irradiation quality  Angle  Delivered dose Hp(3) (mSv) (D)  Estimated dose Hp(3) (mSv) (E)  Q(E/D) Hp(3)  Beta  Beta  90Sr/90Y  0°  2.50  3.09 ± 0.11  1.24  90Sr/90Y  15°  2.50  3.03 ± 0.08  1.21  90Sr/90Y  30°  2.50  2.89 ± 0.15  1.16  90Sr/90Y  45°  2.50  2.59 ± 0.16  1.04  90Sr/90Y  60°  2.50  2.13 ± 0.14  0.85  106Ru/106Rh  0°  10.04  10.02 ± 0.48  0.99  106Ru/106Rh  15°  9.50  8.95 ± 0.33  0.94  106Ru/106Rh  30°  8.30  7.25 ± 0.53  0.87  106Ru/106Rh  45°  6.30  4.90 ± 0.19  0.80  106Ru/106Rh  60°  3.40  3.75 ± 0.19  1.10  To test this algorithm in case of mixed photon and beta radiation, the responses ND1, ND2 and ND3 have been theoretically generated (Table 6). This is done by adding the response of the individual discs corresponding to 1 mSv Hp(3) of 90Sr/90Y and 1 mSv of 137Cs radiations in various proportions (Table 6). Further, for each combination of beta–gamma, the ratio of R32 and the corresponding estimated Hp(3) and Q values were generated. It can be seen that, irrespective of the different proportions of beta and gamma radiations, the Q values are satisfactory. (It may be noted that since the values are theoretically generated, no uncertainties are assigned). However, beta may not be identified if the gamma component is much higher (more than a factor of 3) than the beta component. To confirm the validity of these theoretical calculations, set of dosemeters were irradiated to beta–gamma mixtures (normal incidence) and the corresponding values of estimated Hp(3) and Q values are shown in Table 7. It can be observed that the theoretical Q values match reasonably well with the experimental Q values for the same proportions (rows 13 and 11 of Table 6 can be compared with rows 1 and 2 of Table 7). Table 6. Q values for mixed beta–gamma (137Cs + 90Sr/90Y) radiation for normal incidence (theoretical). Serial no.  Delivered Hp(3) gamma + beta 90Sr/90Y  ND1  ND2  ND3  Ratio R32  Estimated Hp(3) (E) mSv  Q (E/D)  1  1 + 1  1025  2441  3783  1.550  2.63  1.315  2  1 + 2  1055  3863  6489  1.680  3.94  1.313  3  1 + 3  1085  5284  9195  1.740  5.22  1.305  4  1 + 4  1114  6705  11 901  1.775  6.49  1.299  5  1 + 5  1144  8127  14 607  1.797  7.76  1.294  6  1 + 6  1173  9548  17 313  1.813  9.03  1.290  7  1 + 7  1203  10 969  20 019  1.825  10.30  1.287  8  1 + 8  1233  12 391  22 724  1.834  11.57  1.285  9  1 + 9  1262  13 812  25 430  1.841  12.83  1.283  10  1 + 10  1292  15 233  28 136  1.847  14.10  1.282  Serial no.  Delivered Hp(3) gamma + beta 90Sr/90Y  ND1  ND2  ND3  Ratio R32  Estimated Hp(3) (E) mSv  Q (E/D)  1  1 + 1  1025  2441  3783  1.550  2.63  1.315  2  1 + 2  1055  3863  6489  1.680  3.94  1.313  3  1 + 3  1085  5284  9195  1.740  5.22  1.305  4  1 + 4  1114  6705  11 901  1.775  6.49  1.299  5  1 + 5  1144  8127  14 607  1.797  7.76  1.294  6  1 + 6  1173  9548  17 313  1.813  9.03  1.290  7  1 + 7  1203  10 969  20 019  1.825  10.30  1.287  8  1 + 8  1233  12 391  22 724  1.834  11.57  1.285  9  1 + 9  1262  13 812  25 430  1.841  12.83  1.283  10  1 + 10  1292  15 233  28 136  1.847  14.10  1.282    Beta + gamma  ND1  ND2  ND3  R32  E  Q(E/D)  11  1 + 1  1025  2441  3783  1.550  2.63  1.315  12  1 + 2  2021  3462  4861  1.404  3.87  1.291  13  1 + 3  3017  4482  5938  1.325  5.05*  1.264  14  1 + 4  4013  5502  7015  1.275  4.01*  0.803  15  1 + 5  5009  6522  8093  1.241  5.01*  0.835  16  1 + 6  6005  7542  9170  1.216  6.01*  0.858  17  1 + 7  7001  8562  10 247  1.197  7.00*  0.875  18  1 + 8  7997  9583  11 325  1.182  8.00*  0.889  19  1 + 9  8993  10 603  12 402  1.170  8.99*  0.899  20  1 + 10  9989  11 623  13 480  1.160  9.99*  0.908    Beta + gamma  ND1  ND2  ND3  R32  E  Q(E/D)  11  1 + 1  1025  2441  3783  1.550  2.63  1.315  12  1 + 2  2021  3462  4861  1.404  3.87  1.291  13  1 + 3  3017  4482  5938  1.325  5.05*  1.264  14  1 + 4  4013  5502  7015  1.275  4.01*  0.803  15  1 + 5  5009  6522  8093  1.241  5.01*  0.835  16  1 + 6  6005  7542  9170  1.216  6.01*  0.858  17  1 + 7  7001  8562  10 247  1.197  7.00*  0.875  18  1 + 8  7997  9583  11 325  1.182  8.00*  0.889  19  1 + 9  8993  10 603  12 402  1.170  8.99*  0.899  20  1 + 10  9989  11 623  13 480  1.160  9.99*  0.908  *Beta radiation is not identified. Hp(3) estimation is for pure photon radiation. Table 7. Q values for mixed beta–gamma radiation for normal incidence (experimental). Irradiation quality  Delivered dose, Hp(3) (mSv)(D)  Estimated dose, Hp(3) (mSv)(E)  Q (E/D)(Hp(3))  Gamma  Beta  Gamma  Beta  137Cs + 90Sr/90Y  3.20  1.00  2.99 ± 0.10  2.33 ± 0.01  1.27  137Cs + 90Sr/90Y  3.20  2.50  3.20 ± 0.10  4.07 ± 0.02  1.27  Irradiation quality  Delivered dose, Hp(3) (mSv)(D)  Estimated dose, Hp(3) (mSv)(E)  Q (E/D)(Hp(3))  Gamma  Beta  Gamma  Beta  137Cs + 90Sr/90Y  3.20  1.00  2.99 ± 0.10  2.33 ± 0.01  1.27  137Cs + 90Sr/90Y  3.20  2.50  3.20 ± 0.10  4.07 ± 0.02  1.27  In order to implement in the routine personnel dosimetry program, a flow chart for evaluation of Hp(3) for photon beam and mixed gamma–beta radiation is shown in Figure 10. Figure 10. View largeDownload slide Hp(3) dose evaluation flow chart. Figure 10. View largeDownload slide Hp(3) dose evaluation flow chart. CONCLUSIONS Estimation of Personal dose equivalent in terms of Hp(3) as per the international recommendation for both pure and mixed photon fields in the energy range of 30–1250 keV can be achieved in a conservative manner by using the existing TLD badge. An algorithm has been generated for the estimation of doses in terms of operational quantity Hp(3). This algorithm has also been extended to evaluate Hp(3) dose from beta sources of 90Sr/90Sr and 106Ru/106Rh. The algorithm designed based on experimental data will take into account the variation in response of three discs of TLD badge occurring due to variation in energies and angle of incidence. This work will help in accurate estimation of dose to eye lens received by radiation workers especially those working in interventional radiology and other work places where dose to the eye lens maybe significant. ACKNOWLEDGMENTS The authors are thankful to Dr Pradeepkumar K.S., Associate Director, HS&E Group for his encouragement in the work. REFERENCES 1 Vohra, K. G., Bhatt, R. C., Chandra, B., Pradhan, A. S., Lakshmanan, A. R. and Sastry, S. S. A personnel dosemeter TLD badge based on CaSO4:Dy teflon TLD discs. Health. Phys.  38, 193– 197 ( 1980). Google Scholar CrossRef Search ADS PubMed  2 Pradhan, S. M., Sneha, C., Chourasiya, G., Adtani, M. M., Tripathi, S. M. and Singh, S. K. Development of an algorithm for evaluating personal doses due to photon fields in terms of operational quantities for TLD badge system in India. Radiat. Prot. Dosimetry  136( 3), 176– 184 ( 2009). Google Scholar CrossRef Search ADS PubMed  3 Bakshi., A. K., Srivastava., K., Varadharajan., G., Pradhan., A. S. and Kher., R. K. Development of an algorithm for TLD badge system for dosimetry in the field of X and gamma radiation in terms of Hp(10). Radiat. Prot. Dosimetry  123( 2), 148– 155 ( 2007). Google Scholar CrossRef Search ADS PubMed  4 International Commission on Radiological Protection (ICRP). Statement on Tissue Reactions (2011). www.icrp.org//page.asp, retrieved 10-2011. 5 Behrens, R. et al.  . Intercomparison of eye lens dosemeters. Radiat. Prot. Dosimetry  ncw051 ( 2016) doi:10:1093/rpd/ncw051. 6 International Organisation for Standardisation. Calibration of area and personal dosemeters and the measurement of their response as a function of energy and angle of incidence. Part-3, ISO-4037-3 (Geneva:ISO) (1999). 7 Vanhavere, F. et al.  . Measurements of eye lens doses in interventional radiology and cardiology: final results of the ORAMED project. Radiat. Meas.  46, 1243– 1247 ( 2011). Google Scholar CrossRef Search ADS   8 Behrens, R. and Hupe, O. Influence of the phantom shape (slab, cylinder or alderson) on the performance of an Hp(3) eye dosemeter. Radiat. Prot. Dosimetry  168( no 44), 441– 449 ( 2016). Google Scholar CrossRef Search ADS PubMed  9 Behrens, R. On the necessity of a new cylinder phantom for eye lens dosimetry. J. Radiol. Prot.  32, 455– 464 ( 2012). Google Scholar CrossRef Search ADS PubMed  10 Radiation protection instrumentation—passive integrating dosimetry systems for personal and environmental of photon and beta radiation. IEC-62387-2012, Edition 1.0 2012-12. 11 Kulkarni, M. S., Pradeep, R. and Kannan, S. A new PC based semi-automatic TLD badge reader system for personnel monitoring. Proceedings of the 10th International Congress of the International Radiation Protection Association on Harmonization of Radiation, Human Life and the Ecosystem (IRPA-10), (p. 1 v) No. P-3b-167, May 2000, Tokyo, Japan: Japan Health Physics Society (2000). 12 Ambrosi, P., Buchholz, G. and Helmstädter, K. Physikalisch-TechnischeBundesanstalt, Bundesallee 100, D-38116 Braunschweig, Germany. Published by institute of Physics publishing and sissa, November 8, 2007. The PTB Beta Secondary Standard BSS2 for radiation protection. 13 Bakshi, A. K., Vandana, S., PalaniSelvam, T., Chougaonkar, M. P. and Mayya, Y. S. Measurement of the output of ISO recommended beta sources with an extrapolation chamber. Radiat. Meas.  53–54, 50– 55 ( 2013). Google Scholar CrossRef Search ADS   14 EURADOS Report of the Action Group. Harmonization and dosimetric quality assurance in individual monitoring for external radiation. Part 2, A catalogue of dosemeters and dosimetric services within EU Member States and Switzerland able to estimate external radiation doses as personal dose equivalent. Edited by J.W.E. van Dijk, J.M. Bordy, F. Vanhavere, C. Wernli and M. Zamani-Valasiadou. European Radiation Dosimetry Group (1999). 15 ISO 14146. Radiation protection—criteria and performance limits for the periodic evaluation of processors of personal dosemeters for X and gamma radiation (2000). © The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com

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Radiation Protection DosimetryOxford University Press

Published: Mar 1, 2018

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