MICRODOSIMETRIC MEASUREMENT OF SECONDARY RADIATION IN THE PASSIVE SCATTERED PROTON THERAPY ROOM OF iTHEMBA LABS USING A TISSUE-EQUIVALENT PROPORTIONAL COUNTER

MICRODOSIMETRIC MEASUREMENT OF SECONDARY RADIATION IN THE PASSIVE SCATTERED PROTON THERAPY ROOM... Abstract Measurements of the dose equivalent at different distances from the isocenter of the proton therapy center at iThemba LABS were previously performed with a tissue-equivalent proportional counter (TEPC). These measurements showed that the scattered radiation levels were one or two orders of magnitude higher in comparison to other passive scattering delivery systems. In order to reduce these radiation levels, additional shielding was installed shortly after the measurements were done. Therefore, the aim of this work is to quantify and assess the reduction of the secondary doses delivered in the proton therapy room at iThemba LABS after the installation of the additional shielding. This has been performed by measuring microdosimetric spectra with a TEPC at 11 locations around the isocenter when a clinical modulated beam of 200 MeV proton was impinging onto a water phantom placed at the isocenter. INTRODUCTION In the last decades proton therapy (PT) has become a popular technique worldwide for the treatment of cancer because it maximizes the tumor absorbed dose while decreasing the absorbed dose to the surrounding healthy tissues(1). However, some researchers are concerned that the quantification and understanding of the undesired radiation to these healthy tissues (out-of-field doses) and associated risk of inducing secondary tumors is not sufficiently complete to justify the use of PT, especially for treating pediatric tumors(2). This concern is due to the secondary particles, mainly neutrons, created during a PT treatment that contribute to the out-of-field doses and can potentially increase the risk of secondary cancers(3). Secondary neutrons are created as part of the interactions of the proton beam with the several components of the beamline (collimators, range modulator, nozzle, etc.) and inside the patient. Moreover, the neutron radiation field will be strongly dependent on whether the proton beam is delivered using a passive scattering or active scanning modality. On top of that, the resulting neutron fluence and neutron absorbed dose can vary largely between facilities. Therefore, assessment of neutron doses in each individual clinical PT facility is needed and requires special attention for radiation protection as part of prevention of secondary tumor induction. For instance, the secondary neutron absorbed doses as well as dose equivalent data produced in the passive beam delivery system of the iThemba LABS facility with a 200 MeV energy beam were measured in 1994 by means of a tissue-equivalent proportional counter (TEPC). They used a TEPC with an internal diameter of 1.27 cm (Model LET-SW1/2) manufactured by Far West Technology (FWT) and results were presented at the eighth Neutron Dosimetry Symposium in 1995(4). That study showed that the dose equivalent per therapeutic proton absorbed dose as given to the target (H/D) was in the order of 33 mSv Gy−1 at 120 cm from the isocenter at an angle of 90° with respect to the proton beam direction. Such radiation levels were high in comparison to other proton passive scattering facilities with similar irradiation set-ups which measured values in the range of 0.2–2.0 mSy Gy−1 at 100 cm from the isocenter (at 90°)(5–7). In order to reduce the neutron scattered radiation dose additional shielding was installed at iThemba proton treatment room afterwards, but experimental measurements to assess the neutron field could not be repeated at that time. Therefore, the aim of this paper is to quantify and assess the secondary radiation field during a clinical modulated 200 MeV proton beam impinging onto a water phantom at the iThemba LABS facility after the installation of the additional radiation shielding. Microdosimetric measurements were performed at different distances from the water phantom and at different angles (0°, 45° and 90°) with a TEPC to perform a full characterization of the stray radiation inside the room of the proton passive scattering system of iThemba LABS facility. MATERIALS AND METHODS The PT facility The measurements were carried out in the PT vault at the iThemba LABS facility in South Africa. Protons are accelerated to 200 MeV by a K-200 separated-sector cyclotron before being transported to the PT vault. The beam is fixed horizontally and it is laterally spread and flattened using a double scattering system. A 30 × 30 × 30 cm3 water phantom was positioned on the beam axis, with the upstream surface at 10 cm of the isocenter. It was irradiated with a beam degraded to a range of 100 mm in water, 30 mm diameter collimator aperture and a modulator wheel were used to generate a spread out Bragg peak (SOBP) width of 31 mm. More details of the beam delivery system can be found elsewhere(8). Tissue-equivalent proportional counter A commercial TEPC (LET SW5) built by FWT(9) was used in this work. It consists of a 2.13 mm thick-spherical shell made of tissue-equivalent plastic (A-150) with an internal diameter of 12.55 cm. It is filled with propane-based tissue-equivalent gas at a low gas pressure of 899 Pa to simulate 2.0 μm diameter sphere of unit density tissue. The counter was operated at 700 V to have enough gas gain and contains a built-in 244Cm source that emits alpha particles with an average energy of 5.8 MeV that is used for calibration purposes. The output signal of the TEPC is connected to a preamplifier (ORTEC Model 142PC) that converts the charge signal into a voltage, which is then sent to a digital acquisition system CAEN DT5780 that shapes the signal using a trapezoid filter to determine the pulse amplitude. The output signal of the preamplifier was amplified using two different electronic gains 1 and 8 simultaneously after the filtering of the digitizer and the resulting two spectra were joined into a single distribution by scaling the gain factor. The resulting microdosimetric spectrum covered a dynamic range of lineal energies between 1 and 2000 keV μm−1 necessary to measure in a mixed neutron-gamma field. Experimental setup The TEPC was placed in air at angles of 0°, 45°, 90° from the longitudinal direction of the proton beam and at different distances from 100 to 300 cm in the perpendicular direction and from 100 to 200 cm in the axis direction of the beam as can be seen in Figure 1. Figure 1. View largeDownload slide Schematic layout of the experimental set up (top view). The small blue circles are the positions where the TEPC was placed. Figure 1. View largeDownload slide Schematic layout of the experimental set up (top view). The small blue circles are the positions where the TEPC was placed. The center of the TEPC was placed at the same height of the isocenter using a laboratory support. The beam intensity was lowered by a factor 100 relative to the values used in clinical practice in order to minimize pulse pile-up and dead time effects measured with the TEPC. Thus, using an absorbed dose rate of about 0.026 Gy min−1, no distortions were found in the measured pulse-height spectrum at the closest distance of 100 cm from the phantom. A total absorbed dose of 4 Gy was given to the center of the SOBP for each measurement location in order to get a statistical uncertainty of ≤1% on the measured absorbed dose with the counter. Data acquisition and data processing The ionization pulse-height spectrum collected with the TEPC is first calibrated in terms of lineal energy y using the internal alpha source. Due to the high uncertainties (~10%) related with the alpha calibration(10), the proton edge position was checked and eventually recalibrated in order to be at 136 keV μm−1 for a 2.0 μm simulated site size cavity(11). The pulse-height distributions were rebinned into 50 intervals per decade of lineal energy using an in-house MATLAB script based on the equations of appendix A of ICRU 36(12) to obtain the so-called microdosimetric spectrum. The absorbed dose DTEPC, quality factor Q¯ and dose equivalent H were determined from the frequency lineal energy distributions f(y) and dose lineal energy distributions d(y) using the following relations: DTEPC=C⋅y¯F⋅NTot=0.204⋅y¯F⋅NTotdg2 (1) D is in Gy. Where y¯F (keV μm−1) is the frequency–mean lineal energy, C is a factor depending on the geometry of the detector and Ntot is the total number of counts. For a spherical detector, like the one used in this study, C = 0.204 dg−2 being dg the diameter of the detector in μm units. In this study, two different approaches were used to calculate the quality factor q(y) based on ICRP 60(13) and ICRU 40(14) recommendations. The average quality factor Q¯ was calculated as follows: Q¯=∫q(y)⋅d(y)dy∫q(y)dy (2)q(y) is the quality factor as a function of y. Then, the dose equivalent is then obtained using equations (1) and (2): H=DTEPC⋅Q¯ (3) RESULTS AND DISCUSSION Figure 2a–d shows the single-event distributions measured with the TEPC at different positions around the water phantom. Three different contributions can be distinguished in the microdosimetric spectra, (i) events between ~1 and ~10 keV μm−1 are due mainly to gamma radiation and high energy neutrons that produce high energy recoil protons which have low LET values contributing to events up to ~10 keV μm−1, (ii) contribution from low energy neutrons that generate secondary low energy protons which have larger LET values up to the maximum stopping power of protons, the so-called proton edge at 136 keV μm−1 for a cavity of 2 μm(11). Events between ~10 and ~100 keV μm−1 correspond mainly to neutrons of several tens of MeV. Above the proton edge, the (iii) contribution to the absorbed dose is due mainly to heavy recoil ions (He, N, C and O) produced by neutrons with an energy high enough to produce fissions. The relative contribution of each type of radiation depends on the position and can be observed in these graphs. Figure 2. View largeDownload slide (a) Microdosimetric spectra at different distances on the forward direction (b) on the perpendicular direction to the beam and (c) at 45°. (d) Microdosimetric spectra measured at 0°, 45° and 90° at 1 m from the isocenter. Figure 2. View largeDownload slide (a) Microdosimetric spectra at different distances on the forward direction (b) on the perpendicular direction to the beam and (c) at 45°. (d) Microdosimetric spectra measured at 0°, 45° and 90° at 1 m from the isocenter. In the forward direction (Figure 2a) the quality of the radiation does not change from 100 until 250 cm. In Figure 2b the distributions measured on the perpendicular direction to the beam are compared. It can be observed that at 300 cm from the target there is a higher contribution from low LET radiation and a lower contribution of low energy neutrons but the heavy recoil contribution from high energy neutrons remains almost unchanged at different distances. Figure 2c shows the distributions at 45° from the longitudinal direction of the proton beam. Here the contribution of low neutron energies increases from distance 100 to 200 cm increasing the quality of the radiation (Table 1). Microdosimetric spectra measured at 0°, 45° and 90° at 100 cm from the isocenter (Figure 2d) show that the low energy neutron component is slightly higher at 90° with respect to the forward radiation and this can also be seen on the averaged values indicated in Table 1. Table 1. DTEPC/D is the absorbed dose measured with the TEPC divided by absorbed dose measured with the ionization chamber at the isocentre. HICRP60/D and HICRU40/D values are obtained with the quality factors defined in ICRP 60 and ICRU 40, respectively. Q factor obtained with recommendations from ICRP 60 and ICRU 40 and averaged microdosimetric quantities determined at different distances from the phantom. ϕ (deg) Distance XY (cm, cm) DTEPC/D (mGy Gy−1) HICRP 60/D (mSv Gy−1) HICRU 40/D (mSv Gy−1) QICRP60 QICRU 40 y¯F (keV μm−1) y¯D (keV μm−1) 0 000, 100 0.021 ± 0.001 0.20 ± 0.02 0.22 ± 0.02 9.5 10.6 10.32 ± 0.52 59.8 ± 3.2 000, 150 0.019 ± 0.001 0.19 ± 0.02 0.21 ± 0.02 9.6 10.6 10.49 ± 0.53 57.5 ± 3.1 000, 200 0.018 ± 0.001 0.17 ± 0.02 0.19 ± 0.02 9.6 10.6 10.54 ± 0.53 55.6 ± 3.0 45 100, 100 0.036 ± 0.002 0.30 ± 0.04 0.33 ± 0.04 8.4 9.4 9.10 ± 0.46 51.1 ± 2.7 150, 150 0.026 ± 0.001 0.23 ± 0.03 0.26 ± 0.03 9.0 10.0 9.80 ± 0.49 53.0 ± 2.8 200, 200 0.022 ± 0.001 0.20 ± 0.02 0.22 ± 0.03 9.1 10.1 9.90 ± 0.50 54.1 ± 2.9 90 100, 000 0.047 ± 0.002 0.44 ± 0.05 0.48 ± 0.06 9.2 10.2 10.11 ± 0.51 57.0 ± 2.9 150, 000 0.039 ± 0.002 0.37 ± 0.04 0.41 ± 0.05 9.4 10.5 10.42 ± 0.52 57.5 ± 3.0 200, 000 0.035 ± 0.002 0.33 ± 0.04 0.37 ± 0.04 9.5 10.5 10.52 ± 0.53 58.1 ± 3.0 250, 000 0.040 ± 0.002 0.35 ± 0.04 0.39 ± 0.05 8.7 9.8 9.48 ± 0.47 55.5 ± 2.9 300, 000 0.061 ± 0.002 0.41 ± 0.06 0.47 ± 0.06 6.9 7.8 7.86 ± 0.39 46.4 ± 2.7 ϕ (deg) Distance XY (cm, cm) DTEPC/D (mGy Gy−1) HICRP 60/D (mSv Gy−1) HICRU 40/D (mSv Gy−1) QICRP60 QICRU 40 y¯F (keV μm−1) y¯D (keV μm−1) 0 000, 100 0.021 ± 0.001 0.20 ± 0.02 0.22 ± 0.02 9.5 10.6 10.32 ± 0.52 59.8 ± 3.2 000, 150 0.019 ± 0.001 0.19 ± 0.02 0.21 ± 0.02 9.6 10.6 10.49 ± 0.53 57.5 ± 3.1 000, 200 0.018 ± 0.001 0.17 ± 0.02 0.19 ± 0.02 9.6 10.6 10.54 ± 0.53 55.6 ± 3.0 45 100, 100 0.036 ± 0.002 0.30 ± 0.04 0.33 ± 0.04 8.4 9.4 9.10 ± 0.46 51.1 ± 2.7 150, 150 0.026 ± 0.001 0.23 ± 0.03 0.26 ± 0.03 9.0 10.0 9.80 ± 0.49 53.0 ± 2.8 200, 200 0.022 ± 0.001 0.20 ± 0.02 0.22 ± 0.03 9.1 10.1 9.90 ± 0.50 54.1 ± 2.9 90 100, 000 0.047 ± 0.002 0.44 ± 0.05 0.48 ± 0.06 9.2 10.2 10.11 ± 0.51 57.0 ± 2.9 150, 000 0.039 ± 0.002 0.37 ± 0.04 0.41 ± 0.05 9.4 10.5 10.42 ± 0.52 57.5 ± 3.0 200, 000 0.035 ± 0.002 0.33 ± 0.04 0.37 ± 0.04 9.5 10.5 10.52 ± 0.53 58.1 ± 3.0 250, 000 0.040 ± 0.002 0.35 ± 0.04 0.39 ± 0.05 8.7 9.8 9.48 ± 0.47 55.5 ± 2.9 300, 000 0.061 ± 0.002 0.41 ± 0.06 0.47 ± 0.06 6.9 7.8 7.86 ± 0.39 46.4 ± 2.7 Table 1. DTEPC/D is the absorbed dose measured with the TEPC divided by absorbed dose measured with the ionization chamber at the isocentre. HICRP60/D and HICRU40/D values are obtained with the quality factors defined in ICRP 60 and ICRU 40, respectively. Q factor obtained with recommendations from ICRP 60 and ICRU 40 and averaged microdosimetric quantities determined at different distances from the phantom. ϕ (deg) Distance XY (cm, cm) DTEPC/D (mGy Gy−1) HICRP 60/D (mSv Gy−1) HICRU 40/D (mSv Gy−1) QICRP60 QICRU 40 y¯F (keV μm−1) y¯D (keV μm−1) 0 000, 100 0.021 ± 0.001 0.20 ± 0.02 0.22 ± 0.02 9.5 10.6 10.32 ± 0.52 59.8 ± 3.2 000, 150 0.019 ± 0.001 0.19 ± 0.02 0.21 ± 0.02 9.6 10.6 10.49 ± 0.53 57.5 ± 3.1 000, 200 0.018 ± 0.001 0.17 ± 0.02 0.19 ± 0.02 9.6 10.6 10.54 ± 0.53 55.6 ± 3.0 45 100, 100 0.036 ± 0.002 0.30 ± 0.04 0.33 ± 0.04 8.4 9.4 9.10 ± 0.46 51.1 ± 2.7 150, 150 0.026 ± 0.001 0.23 ± 0.03 0.26 ± 0.03 9.0 10.0 9.80 ± 0.49 53.0 ± 2.8 200, 200 0.022 ± 0.001 0.20 ± 0.02 0.22 ± 0.03 9.1 10.1 9.90 ± 0.50 54.1 ± 2.9 90 100, 000 0.047 ± 0.002 0.44 ± 0.05 0.48 ± 0.06 9.2 10.2 10.11 ± 0.51 57.0 ± 2.9 150, 000 0.039 ± 0.002 0.37 ± 0.04 0.41 ± 0.05 9.4 10.5 10.42 ± 0.52 57.5 ± 3.0 200, 000 0.035 ± 0.002 0.33 ± 0.04 0.37 ± 0.04 9.5 10.5 10.52 ± 0.53 58.1 ± 3.0 250, 000 0.040 ± 0.002 0.35 ± 0.04 0.39 ± 0.05 8.7 9.8 9.48 ± 0.47 55.5 ± 2.9 300, 000 0.061 ± 0.002 0.41 ± 0.06 0.47 ± 0.06 6.9 7.8 7.86 ± 0.39 46.4 ± 2.7 ϕ (deg) Distance XY (cm, cm) DTEPC/D (mGy Gy−1) HICRP 60/D (mSv Gy−1) HICRU 40/D (mSv Gy−1) QICRP60 QICRU 40 y¯F (keV μm−1) y¯D (keV μm−1) 0 000, 100 0.021 ± 0.001 0.20 ± 0.02 0.22 ± 0.02 9.5 10.6 10.32 ± 0.52 59.8 ± 3.2 000, 150 0.019 ± 0.001 0.19 ± 0.02 0.21 ± 0.02 9.6 10.6 10.49 ± 0.53 57.5 ± 3.1 000, 200 0.018 ± 0.001 0.17 ± 0.02 0.19 ± 0.02 9.6 10.6 10.54 ± 0.53 55.6 ± 3.0 45 100, 100 0.036 ± 0.002 0.30 ± 0.04 0.33 ± 0.04 8.4 9.4 9.10 ± 0.46 51.1 ± 2.7 150, 150 0.026 ± 0.001 0.23 ± 0.03 0.26 ± 0.03 9.0 10.0 9.80 ± 0.49 53.0 ± 2.8 200, 200 0.022 ± 0.001 0.20 ± 0.02 0.22 ± 0.03 9.1 10.1 9.90 ± 0.50 54.1 ± 2.9 90 100, 000 0.047 ± 0.002 0.44 ± 0.05 0.48 ± 0.06 9.2 10.2 10.11 ± 0.51 57.0 ± 2.9 150, 000 0.039 ± 0.002 0.37 ± 0.04 0.41 ± 0.05 9.4 10.5 10.42 ± 0.52 57.5 ± 3.0 200, 000 0.035 ± 0.002 0.33 ± 0.04 0.37 ± 0.04 9.5 10.5 10.52 ± 0.53 58.1 ± 3.0 250, 000 0.040 ± 0.002 0.35 ± 0.04 0.39 ± 0.05 8.7 9.8 9.48 ± 0.47 55.5 ± 2.9 300, 000 0.061 ± 0.002 0.41 ± 0.06 0.47 ± 0.06 6.9 7.8 7.86 ± 0.39 46.4 ± 2.7 The shape of the distribution at 100 cm at an angle of 90° with respect to the proton beam direction was compared with the spectra at 120 cm from the previous measurements at iThemba LABS(4). Besides the differences with the experimental setup carried out in both experiments, the distributions show a similar shape, showing three distinguished regions, typically found in these type of mixed radiation fields. Only the resulting ratio of H/D calculated from the distributions were rather different because of the installation of shielding after the first measurement campaign. In Table 1 the resulting ratio of DTEPC/D and H/D values, Q values and averaged microdosimetric quantities (frequency–mean lineal energy y¯F and dose–mean lineal energy y¯D) for the different positions are summarized. On the one hand, the measurements show that H/D and DTEPC/D values are higher in the perpendicular direction in comparison with the forward direction. Comparing the spectra measured at 0°, 45° and 90° at 1 m from the isocenter we can see that there is a factor 2.2 between 90° and 0°. On the other hand, the radiation quality values and the microdosimetric quantities are of the same order of magnitude due to the small differences in the distributions at 90° and 0° directions (Figure 2d). The current results show that the ratio of H/D determined at different positions in the room has been decreased significantly after adding extra shielding in the treatment room. At 100 cm from the isocenter in the perpendicular direction the value of the H/D is 0.44 mSv Gy−1 in comparison with the value of 33 mSv Gy−1 found at 120 cm in the past(4). Different types of shielding collimators were inserted in the beamline to protect the patient and the electronic equipment(8) after 1994. In particular, a 19.5 cm thick wall of concrete was placed perpendicular to the beam axis at a distance of 355 cm from the isocenter. This wall extends laterally to a distance of 120.5 cm to the left of the beam axis and 114 cm on the right side. Furthermore, two anti-scatter collimators were added at the distances of 335 and 179 cm from the isocenter, with each collimator being 5 cm thick and made from steel. It can be noticed from Table 1 that at 90°, H/D values increase after 200 cm from the isocenter instead of decreasing and this corresponds more or less to the distance until which the shielding is placed. The overall uncertainty of the ratio of H/D is of the order of 11% which includes 5% calibration factor, 1% statistical uncertainty and 10% associated to the differences depending on which radiation quality factor is used. The uncertainty associated to the microdosimetric quantities, y¯F and y¯D, includes the statistical uncertainty and the calibration uncertainty. The overall uncertainty of the ratio DTEPC/D is of the order of 5% including the calibration and the statistical errors. Figure 3 shows the results of H/D as a function of the distance based on the Q factor defined in ICRP 60 from the isocenter both parallel and perpendicular to the proton beam axis together with the quality of the radiation Q calculated using ICRP 60 and ICRU 40 approaches. Figure 3. View largeDownload slide Ratio of H/D values based on the Q factor defined in ICRP 60 (square symbols) and average quality factor (calculated as ICRP 60 and ICRU 40) as a function of the distance perpendicular to the proton beam axis (a) and parallel to the beam (b). The lines are to guide the eye. Figure 3. View largeDownload slide Ratio of H/D values based on the Q factor defined in ICRP 60 (square symbols) and average quality factor (calculated as ICRP 60 and ICRU 40) as a function of the distance perpendicular to the proton beam axis (a) and parallel to the beam (b). The lines are to guide the eye. CONCLUSIONS In this work microdosimetric measurements of the secondary radiation from the passive scattered PT room of iThemba LABS were performed using a TEPC. These measurements were carried out after the installation of additional shielding in the treatment room because high dose equivalent values were encountered in 1994. The present results show how the ratio of H/D values measured at the iThemba facility have decreased after adding extra shielding by a factor of 75 in the treatment room, giving values of the order of 0.44 mSv Gy−1 at 100 cm from the isocenter. These results measured are now of the same order of magnitude as other passive proton facilities worldwide. ACKNOWLEDGEMENTS We would like to thank the Physics Advisory Committee (PAC) of NRF iThemba LABS for support and beam time allocation. FUNDING This work was supported by the Belgian Nuclear Research Centre (SCK•CEN), the Belgian Hadron Therapy Consortium (BHTC) and the NRF iThemba LABS in South Africa. 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In: Proceedings of SAIP2013: the 58th annual conference of the South African Institute of Physics. SA Institute of Physics 2013 . ISBN: 978-0-620-62819-8. 9 FWT ( 2011 ). Environmental Radiation Monitor With 5 Inches Tissue Equivalent Proportional Counter (LET-SW5). Operations and Repair Manual. Far West Technology, Inc. Available on http://www.fwt.com. 10 Schrewe , U. J. , Brede , H. J. , Pihet , P. and Menzel , H. G. On the calibration of tissue-equivalent proportional counters with built-in alpha particle sources . Radiat. Prot. Dosim. 23 ( 1–4 ), 249 – 252 ( 1988 ). Google Scholar CrossRef Search ADS 11 Moro , D. , Chiriotti , S. , Conte , V. , Colautti , P. and Grosswendt , B. Lineal energy calibration of a spherical TEPC . Radiat. Prot. Dosim. 166 ( 1–4 ), 233 – 237 ( 2015 ). Google Scholar CrossRef Search ADS 12 International Commission on Radiation Units and Measurements . Microdosimetry. ICRU Report 36 ( 1983 ). 13 International Commission on Radiological Protection . Recommendations of the ICRP on Radiological Protection. ICRP Publication 60 . Ann. ICRP 21 ( 1–3 ), 1 – 201 ( 1991 ). 14 International Commission on Radiation Units and Measurements . The radiation quality factor in radiation protection. ICRU Report 40 ( 1986 ). © 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

MICRODOSIMETRIC MEASUREMENT OF SECONDARY RADIATION IN THE PASSIVE SCATTERED PROTON THERAPY ROOM OF iTHEMBA LABS USING A TISSUE-EQUIVALENT PROPORTIONAL COUNTER

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Abstract

Abstract Measurements of the dose equivalent at different distances from the isocenter of the proton therapy center at iThemba LABS were previously performed with a tissue-equivalent proportional counter (TEPC). These measurements showed that the scattered radiation levels were one or two orders of magnitude higher in comparison to other passive scattering delivery systems. In order to reduce these radiation levels, additional shielding was installed shortly after the measurements were done. Therefore, the aim of this work is to quantify and assess the reduction of the secondary doses delivered in the proton therapy room at iThemba LABS after the installation of the additional shielding. This has been performed by measuring microdosimetric spectra with a TEPC at 11 locations around the isocenter when a clinical modulated beam of 200 MeV proton was impinging onto a water phantom placed at the isocenter. INTRODUCTION In the last decades proton therapy (PT) has become a popular technique worldwide for the treatment of cancer because it maximizes the tumor absorbed dose while decreasing the absorbed dose to the surrounding healthy tissues(1). However, some researchers are concerned that the quantification and understanding of the undesired radiation to these healthy tissues (out-of-field doses) and associated risk of inducing secondary tumors is not sufficiently complete to justify the use of PT, especially for treating pediatric tumors(2). This concern is due to the secondary particles, mainly neutrons, created during a PT treatment that contribute to the out-of-field doses and can potentially increase the risk of secondary cancers(3). Secondary neutrons are created as part of the interactions of the proton beam with the several components of the beamline (collimators, range modulator, nozzle, etc.) and inside the patient. Moreover, the neutron radiation field will be strongly dependent on whether the proton beam is delivered using a passive scattering or active scanning modality. On top of that, the resulting neutron fluence and neutron absorbed dose can vary largely between facilities. Therefore, assessment of neutron doses in each individual clinical PT facility is needed and requires special attention for radiation protection as part of prevention of secondary tumor induction. For instance, the secondary neutron absorbed doses as well as dose equivalent data produced in the passive beam delivery system of the iThemba LABS facility with a 200 MeV energy beam were measured in 1994 by means of a tissue-equivalent proportional counter (TEPC). They used a TEPC with an internal diameter of 1.27 cm (Model LET-SW1/2) manufactured by Far West Technology (FWT) and results were presented at the eighth Neutron Dosimetry Symposium in 1995(4). That study showed that the dose equivalent per therapeutic proton absorbed dose as given to the target (H/D) was in the order of 33 mSv Gy−1 at 120 cm from the isocenter at an angle of 90° with respect to the proton beam direction. Such radiation levels were high in comparison to other proton passive scattering facilities with similar irradiation set-ups which measured values in the range of 0.2–2.0 mSy Gy−1 at 100 cm from the isocenter (at 90°)(5–7). In order to reduce the neutron scattered radiation dose additional shielding was installed at iThemba proton treatment room afterwards, but experimental measurements to assess the neutron field could not be repeated at that time. Therefore, the aim of this paper is to quantify and assess the secondary radiation field during a clinical modulated 200 MeV proton beam impinging onto a water phantom at the iThemba LABS facility after the installation of the additional radiation shielding. Microdosimetric measurements were performed at different distances from the water phantom and at different angles (0°, 45° and 90°) with a TEPC to perform a full characterization of the stray radiation inside the room of the proton passive scattering system of iThemba LABS facility. MATERIALS AND METHODS The PT facility The measurements were carried out in the PT vault at the iThemba LABS facility in South Africa. Protons are accelerated to 200 MeV by a K-200 separated-sector cyclotron before being transported to the PT vault. The beam is fixed horizontally and it is laterally spread and flattened using a double scattering system. A 30 × 30 × 30 cm3 water phantom was positioned on the beam axis, with the upstream surface at 10 cm of the isocenter. It was irradiated with a beam degraded to a range of 100 mm in water, 30 mm diameter collimator aperture and a modulator wheel were used to generate a spread out Bragg peak (SOBP) width of 31 mm. More details of the beam delivery system can be found elsewhere(8). Tissue-equivalent proportional counter A commercial TEPC (LET SW5) built by FWT(9) was used in this work. It consists of a 2.13 mm thick-spherical shell made of tissue-equivalent plastic (A-150) with an internal diameter of 12.55 cm. It is filled with propane-based tissue-equivalent gas at a low gas pressure of 899 Pa to simulate 2.0 μm diameter sphere of unit density tissue. The counter was operated at 700 V to have enough gas gain and contains a built-in 244Cm source that emits alpha particles with an average energy of 5.8 MeV that is used for calibration purposes. The output signal of the TEPC is connected to a preamplifier (ORTEC Model 142PC) that converts the charge signal into a voltage, which is then sent to a digital acquisition system CAEN DT5780 that shapes the signal using a trapezoid filter to determine the pulse amplitude. The output signal of the preamplifier was amplified using two different electronic gains 1 and 8 simultaneously after the filtering of the digitizer and the resulting two spectra were joined into a single distribution by scaling the gain factor. The resulting microdosimetric spectrum covered a dynamic range of lineal energies between 1 and 2000 keV μm−1 necessary to measure in a mixed neutron-gamma field. Experimental setup The TEPC was placed in air at angles of 0°, 45°, 90° from the longitudinal direction of the proton beam and at different distances from 100 to 300 cm in the perpendicular direction and from 100 to 200 cm in the axis direction of the beam as can be seen in Figure 1. Figure 1. View largeDownload slide Schematic layout of the experimental set up (top view). The small blue circles are the positions where the TEPC was placed. Figure 1. View largeDownload slide Schematic layout of the experimental set up (top view). The small blue circles are the positions where the TEPC was placed. The center of the TEPC was placed at the same height of the isocenter using a laboratory support. The beam intensity was lowered by a factor 100 relative to the values used in clinical practice in order to minimize pulse pile-up and dead time effects measured with the TEPC. Thus, using an absorbed dose rate of about 0.026 Gy min−1, no distortions were found in the measured pulse-height spectrum at the closest distance of 100 cm from the phantom. A total absorbed dose of 4 Gy was given to the center of the SOBP for each measurement location in order to get a statistical uncertainty of ≤1% on the measured absorbed dose with the counter. Data acquisition and data processing The ionization pulse-height spectrum collected with the TEPC is first calibrated in terms of lineal energy y using the internal alpha source. Due to the high uncertainties (~10%) related with the alpha calibration(10), the proton edge position was checked and eventually recalibrated in order to be at 136 keV μm−1 for a 2.0 μm simulated site size cavity(11). The pulse-height distributions were rebinned into 50 intervals per decade of lineal energy using an in-house MATLAB script based on the equations of appendix A of ICRU 36(12) to obtain the so-called microdosimetric spectrum. The absorbed dose DTEPC, quality factor Q¯ and dose equivalent H were determined from the frequency lineal energy distributions f(y) and dose lineal energy distributions d(y) using the following relations: DTEPC=C⋅y¯F⋅NTot=0.204⋅y¯F⋅NTotdg2 (1) D is in Gy. Where y¯F (keV μm−1) is the frequency–mean lineal energy, C is a factor depending on the geometry of the detector and Ntot is the total number of counts. For a spherical detector, like the one used in this study, C = 0.204 dg−2 being dg the diameter of the detector in μm units. In this study, two different approaches were used to calculate the quality factor q(y) based on ICRP 60(13) and ICRU 40(14) recommendations. The average quality factor Q¯ was calculated as follows: Q¯=∫q(y)⋅d(y)dy∫q(y)dy (2)q(y) is the quality factor as a function of y. Then, the dose equivalent is then obtained using equations (1) and (2): H=DTEPC⋅Q¯ (3) RESULTS AND DISCUSSION Figure 2a–d shows the single-event distributions measured with the TEPC at different positions around the water phantom. Three different contributions can be distinguished in the microdosimetric spectra, (i) events between ~1 and ~10 keV μm−1 are due mainly to gamma radiation and high energy neutrons that produce high energy recoil protons which have low LET values contributing to events up to ~10 keV μm−1, (ii) contribution from low energy neutrons that generate secondary low energy protons which have larger LET values up to the maximum stopping power of protons, the so-called proton edge at 136 keV μm−1 for a cavity of 2 μm(11). Events between ~10 and ~100 keV μm−1 correspond mainly to neutrons of several tens of MeV. Above the proton edge, the (iii) contribution to the absorbed dose is due mainly to heavy recoil ions (He, N, C and O) produced by neutrons with an energy high enough to produce fissions. The relative contribution of each type of radiation depends on the position and can be observed in these graphs. Figure 2. View largeDownload slide (a) Microdosimetric spectra at different distances on the forward direction (b) on the perpendicular direction to the beam and (c) at 45°. (d) Microdosimetric spectra measured at 0°, 45° and 90° at 1 m from the isocenter. Figure 2. View largeDownload slide (a) Microdosimetric spectra at different distances on the forward direction (b) on the perpendicular direction to the beam and (c) at 45°. (d) Microdosimetric spectra measured at 0°, 45° and 90° at 1 m from the isocenter. In the forward direction (Figure 2a) the quality of the radiation does not change from 100 until 250 cm. In Figure 2b the distributions measured on the perpendicular direction to the beam are compared. It can be observed that at 300 cm from the target there is a higher contribution from low LET radiation and a lower contribution of low energy neutrons but the heavy recoil contribution from high energy neutrons remains almost unchanged at different distances. Figure 2c shows the distributions at 45° from the longitudinal direction of the proton beam. Here the contribution of low neutron energies increases from distance 100 to 200 cm increasing the quality of the radiation (Table 1). Microdosimetric spectra measured at 0°, 45° and 90° at 100 cm from the isocenter (Figure 2d) show that the low energy neutron component is slightly higher at 90° with respect to the forward radiation and this can also be seen on the averaged values indicated in Table 1. Table 1. DTEPC/D is the absorbed dose measured with the TEPC divided by absorbed dose measured with the ionization chamber at the isocentre. HICRP60/D and HICRU40/D values are obtained with the quality factors defined in ICRP 60 and ICRU 40, respectively. Q factor obtained with recommendations from ICRP 60 and ICRU 40 and averaged microdosimetric quantities determined at different distances from the phantom. ϕ (deg) Distance XY (cm, cm) DTEPC/D (mGy Gy−1) HICRP 60/D (mSv Gy−1) HICRU 40/D (mSv Gy−1) QICRP60 QICRU 40 y¯F (keV μm−1) y¯D (keV μm−1) 0 000, 100 0.021 ± 0.001 0.20 ± 0.02 0.22 ± 0.02 9.5 10.6 10.32 ± 0.52 59.8 ± 3.2 000, 150 0.019 ± 0.001 0.19 ± 0.02 0.21 ± 0.02 9.6 10.6 10.49 ± 0.53 57.5 ± 3.1 000, 200 0.018 ± 0.001 0.17 ± 0.02 0.19 ± 0.02 9.6 10.6 10.54 ± 0.53 55.6 ± 3.0 45 100, 100 0.036 ± 0.002 0.30 ± 0.04 0.33 ± 0.04 8.4 9.4 9.10 ± 0.46 51.1 ± 2.7 150, 150 0.026 ± 0.001 0.23 ± 0.03 0.26 ± 0.03 9.0 10.0 9.80 ± 0.49 53.0 ± 2.8 200, 200 0.022 ± 0.001 0.20 ± 0.02 0.22 ± 0.03 9.1 10.1 9.90 ± 0.50 54.1 ± 2.9 90 100, 000 0.047 ± 0.002 0.44 ± 0.05 0.48 ± 0.06 9.2 10.2 10.11 ± 0.51 57.0 ± 2.9 150, 000 0.039 ± 0.002 0.37 ± 0.04 0.41 ± 0.05 9.4 10.5 10.42 ± 0.52 57.5 ± 3.0 200, 000 0.035 ± 0.002 0.33 ± 0.04 0.37 ± 0.04 9.5 10.5 10.52 ± 0.53 58.1 ± 3.0 250, 000 0.040 ± 0.002 0.35 ± 0.04 0.39 ± 0.05 8.7 9.8 9.48 ± 0.47 55.5 ± 2.9 300, 000 0.061 ± 0.002 0.41 ± 0.06 0.47 ± 0.06 6.9 7.8 7.86 ± 0.39 46.4 ± 2.7 ϕ (deg) Distance XY (cm, cm) DTEPC/D (mGy Gy−1) HICRP 60/D (mSv Gy−1) HICRU 40/D (mSv Gy−1) QICRP60 QICRU 40 y¯F (keV μm−1) y¯D (keV μm−1) 0 000, 100 0.021 ± 0.001 0.20 ± 0.02 0.22 ± 0.02 9.5 10.6 10.32 ± 0.52 59.8 ± 3.2 000, 150 0.019 ± 0.001 0.19 ± 0.02 0.21 ± 0.02 9.6 10.6 10.49 ± 0.53 57.5 ± 3.1 000, 200 0.018 ± 0.001 0.17 ± 0.02 0.19 ± 0.02 9.6 10.6 10.54 ± 0.53 55.6 ± 3.0 45 100, 100 0.036 ± 0.002 0.30 ± 0.04 0.33 ± 0.04 8.4 9.4 9.10 ± 0.46 51.1 ± 2.7 150, 150 0.026 ± 0.001 0.23 ± 0.03 0.26 ± 0.03 9.0 10.0 9.80 ± 0.49 53.0 ± 2.8 200, 200 0.022 ± 0.001 0.20 ± 0.02 0.22 ± 0.03 9.1 10.1 9.90 ± 0.50 54.1 ± 2.9 90 100, 000 0.047 ± 0.002 0.44 ± 0.05 0.48 ± 0.06 9.2 10.2 10.11 ± 0.51 57.0 ± 2.9 150, 000 0.039 ± 0.002 0.37 ± 0.04 0.41 ± 0.05 9.4 10.5 10.42 ± 0.52 57.5 ± 3.0 200, 000 0.035 ± 0.002 0.33 ± 0.04 0.37 ± 0.04 9.5 10.5 10.52 ± 0.53 58.1 ± 3.0 250, 000 0.040 ± 0.002 0.35 ± 0.04 0.39 ± 0.05 8.7 9.8 9.48 ± 0.47 55.5 ± 2.9 300, 000 0.061 ± 0.002 0.41 ± 0.06 0.47 ± 0.06 6.9 7.8 7.86 ± 0.39 46.4 ± 2.7 Table 1. DTEPC/D is the absorbed dose measured with the TEPC divided by absorbed dose measured with the ionization chamber at the isocentre. HICRP60/D and HICRU40/D values are obtained with the quality factors defined in ICRP 60 and ICRU 40, respectively. Q factor obtained with recommendations from ICRP 60 and ICRU 40 and averaged microdosimetric quantities determined at different distances from the phantom. ϕ (deg) Distance XY (cm, cm) DTEPC/D (mGy Gy−1) HICRP 60/D (mSv Gy−1) HICRU 40/D (mSv Gy−1) QICRP60 QICRU 40 y¯F (keV μm−1) y¯D (keV μm−1) 0 000, 100 0.021 ± 0.001 0.20 ± 0.02 0.22 ± 0.02 9.5 10.6 10.32 ± 0.52 59.8 ± 3.2 000, 150 0.019 ± 0.001 0.19 ± 0.02 0.21 ± 0.02 9.6 10.6 10.49 ± 0.53 57.5 ± 3.1 000, 200 0.018 ± 0.001 0.17 ± 0.02 0.19 ± 0.02 9.6 10.6 10.54 ± 0.53 55.6 ± 3.0 45 100, 100 0.036 ± 0.002 0.30 ± 0.04 0.33 ± 0.04 8.4 9.4 9.10 ± 0.46 51.1 ± 2.7 150, 150 0.026 ± 0.001 0.23 ± 0.03 0.26 ± 0.03 9.0 10.0 9.80 ± 0.49 53.0 ± 2.8 200, 200 0.022 ± 0.001 0.20 ± 0.02 0.22 ± 0.03 9.1 10.1 9.90 ± 0.50 54.1 ± 2.9 90 100, 000 0.047 ± 0.002 0.44 ± 0.05 0.48 ± 0.06 9.2 10.2 10.11 ± 0.51 57.0 ± 2.9 150, 000 0.039 ± 0.002 0.37 ± 0.04 0.41 ± 0.05 9.4 10.5 10.42 ± 0.52 57.5 ± 3.0 200, 000 0.035 ± 0.002 0.33 ± 0.04 0.37 ± 0.04 9.5 10.5 10.52 ± 0.53 58.1 ± 3.0 250, 000 0.040 ± 0.002 0.35 ± 0.04 0.39 ± 0.05 8.7 9.8 9.48 ± 0.47 55.5 ± 2.9 300, 000 0.061 ± 0.002 0.41 ± 0.06 0.47 ± 0.06 6.9 7.8 7.86 ± 0.39 46.4 ± 2.7 ϕ (deg) Distance XY (cm, cm) DTEPC/D (mGy Gy−1) HICRP 60/D (mSv Gy−1) HICRU 40/D (mSv Gy−1) QICRP60 QICRU 40 y¯F (keV μm−1) y¯D (keV μm−1) 0 000, 100 0.021 ± 0.001 0.20 ± 0.02 0.22 ± 0.02 9.5 10.6 10.32 ± 0.52 59.8 ± 3.2 000, 150 0.019 ± 0.001 0.19 ± 0.02 0.21 ± 0.02 9.6 10.6 10.49 ± 0.53 57.5 ± 3.1 000, 200 0.018 ± 0.001 0.17 ± 0.02 0.19 ± 0.02 9.6 10.6 10.54 ± 0.53 55.6 ± 3.0 45 100, 100 0.036 ± 0.002 0.30 ± 0.04 0.33 ± 0.04 8.4 9.4 9.10 ± 0.46 51.1 ± 2.7 150, 150 0.026 ± 0.001 0.23 ± 0.03 0.26 ± 0.03 9.0 10.0 9.80 ± 0.49 53.0 ± 2.8 200, 200 0.022 ± 0.001 0.20 ± 0.02 0.22 ± 0.03 9.1 10.1 9.90 ± 0.50 54.1 ± 2.9 90 100, 000 0.047 ± 0.002 0.44 ± 0.05 0.48 ± 0.06 9.2 10.2 10.11 ± 0.51 57.0 ± 2.9 150, 000 0.039 ± 0.002 0.37 ± 0.04 0.41 ± 0.05 9.4 10.5 10.42 ± 0.52 57.5 ± 3.0 200, 000 0.035 ± 0.002 0.33 ± 0.04 0.37 ± 0.04 9.5 10.5 10.52 ± 0.53 58.1 ± 3.0 250, 000 0.040 ± 0.002 0.35 ± 0.04 0.39 ± 0.05 8.7 9.8 9.48 ± 0.47 55.5 ± 2.9 300, 000 0.061 ± 0.002 0.41 ± 0.06 0.47 ± 0.06 6.9 7.8 7.86 ± 0.39 46.4 ± 2.7 The shape of the distribution at 100 cm at an angle of 90° with respect to the proton beam direction was compared with the spectra at 120 cm from the previous measurements at iThemba LABS(4). Besides the differences with the experimental setup carried out in both experiments, the distributions show a similar shape, showing three distinguished regions, typically found in these type of mixed radiation fields. Only the resulting ratio of H/D calculated from the distributions were rather different because of the installation of shielding after the first measurement campaign. In Table 1 the resulting ratio of DTEPC/D and H/D values, Q values and averaged microdosimetric quantities (frequency–mean lineal energy y¯F and dose–mean lineal energy y¯D) for the different positions are summarized. On the one hand, the measurements show that H/D and DTEPC/D values are higher in the perpendicular direction in comparison with the forward direction. Comparing the spectra measured at 0°, 45° and 90° at 1 m from the isocenter we can see that there is a factor 2.2 between 90° and 0°. On the other hand, the radiation quality values and the microdosimetric quantities are of the same order of magnitude due to the small differences in the distributions at 90° and 0° directions (Figure 2d). The current results show that the ratio of H/D determined at different positions in the room has been decreased significantly after adding extra shielding in the treatment room. At 100 cm from the isocenter in the perpendicular direction the value of the H/D is 0.44 mSv Gy−1 in comparison with the value of 33 mSv Gy−1 found at 120 cm in the past(4). Different types of shielding collimators were inserted in the beamline to protect the patient and the electronic equipment(8) after 1994. In particular, a 19.5 cm thick wall of concrete was placed perpendicular to the beam axis at a distance of 355 cm from the isocenter. This wall extends laterally to a distance of 120.5 cm to the left of the beam axis and 114 cm on the right side. Furthermore, two anti-scatter collimators were added at the distances of 335 and 179 cm from the isocenter, with each collimator being 5 cm thick and made from steel. It can be noticed from Table 1 that at 90°, H/D values increase after 200 cm from the isocenter instead of decreasing and this corresponds more or less to the distance until which the shielding is placed. The overall uncertainty of the ratio of H/D is of the order of 11% which includes 5% calibration factor, 1% statistical uncertainty and 10% associated to the differences depending on which radiation quality factor is used. The uncertainty associated to the microdosimetric quantities, y¯F and y¯D, includes the statistical uncertainty and the calibration uncertainty. The overall uncertainty of the ratio DTEPC/D is of the order of 5% including the calibration and the statistical errors. Figure 3 shows the results of H/D as a function of the distance based on the Q factor defined in ICRP 60 from the isocenter both parallel and perpendicular to the proton beam axis together with the quality of the radiation Q calculated using ICRP 60 and ICRU 40 approaches. Figure 3. View largeDownload slide Ratio of H/D values based on the Q factor defined in ICRP 60 (square symbols) and average quality factor (calculated as ICRP 60 and ICRU 40) as a function of the distance perpendicular to the proton beam axis (a) and parallel to the beam (b). The lines are to guide the eye. Figure 3. View largeDownload slide Ratio of H/D values based on the Q factor defined in ICRP 60 (square symbols) and average quality factor (calculated as ICRP 60 and ICRU 40) as a function of the distance perpendicular to the proton beam axis (a) and parallel to the beam (b). The lines are to guide the eye. CONCLUSIONS In this work microdosimetric measurements of the secondary radiation from the passive scattered PT room of iThemba LABS were performed using a TEPC. These measurements were carried out after the installation of additional shielding in the treatment room because high dose equivalent values were encountered in 1994. The present results show how the ratio of H/D values measured at the iThemba facility have decreased after adding extra shielding by a factor of 75 in the treatment room, giving values of the order of 0.44 mSv Gy−1 at 100 cm from the isocenter. These results measured are now of the same order of magnitude as other passive proton facilities worldwide. ACKNOWLEDGEMENTS We would like to thank the Physics Advisory Committee (PAC) of NRF iThemba LABS for support and beam time allocation. FUNDING This work was supported by the Belgian Nuclear Research Centre (SCK•CEN), the Belgian Hadron Therapy Consortium (BHTC) and the NRF iThemba LABS in South Africa. 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Recommendations of the ICRP on Radiological Protection. ICRP Publication 60 . Ann. ICRP 21 ( 1–3 ), 1 – 201 ( 1991 ). 14 International Commission on Radiation Units and Measurements . The radiation quality factor in radiation protection. ICRU Report 40 ( 1986 ). © 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)

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Radiation Protection DosimetryOxford University Press

Published: Apr 13, 2018

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