Field-size correction factors of a radiophotoluminescent glass dosimeter for small-field and intensity-modulated radiation therapy beams

Field-size correction factors of a radiophotoluminescent glass dosimeter for small-field and intensity-modulated radiation therapy beams Shimpei Hashimoto a) Department of Radiation Oncology, Tokyo Metropolitan Cancer and Infectious Diseases Center Komagome Hospital, 3-18-22 Honkomagome, Bunkyo-ku, Tokyo, Japan Yukio Fujita Department of Radiation Oncology, Tokai University School of Medicine, 143 Shimokasuya, Isehara-shi, Kanagawa, Japan Tetsurou Katayose Department of Radiation Oncology, Chiba Cancer Center, 666-2 Nitona-cho Chuo-ku, Chiba-shi, Chiba, Japan Hideyuki Mizuno Department of Radiation Measurement and Dose Assessment, National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, 4-9-1 Anagawa, Inage-ku, Chiba-shi, Chiba, Japan Hidetoshi Saitoh Graduate School of Human Health Sciences, Tokyo Metropolitan University, 7-2-10 Higashiogu, Arakawa-ku, Tokyo, Japan Katsuyuki Karasawa Department of Radiation Oncology, Tokyo Metropolitan Cancer and Infectious Diseases Center Komagome Hospital, 3-18-22 Honkomagome, Bunkyo-ku, Tokyo, Japan (Received 27 March 2017; revised 19 October 2017; accepted for publication 24 October 2017; published 5 December 2017) Purpose: We evaluated the energy responses of a radiophotoluminescent glass dosimeter (RPLD) to variations in small-field and intensity-modulated radiation therapy (IMRT) conditions using experimental measurements and Monte Carlo simulation. Methods: Several sizes of the jaw and multileaf collimator fields and various plan-class IMRT-beam measurements were performed using the RPLD and an ionization chamber. The field-size correction factor for the RPLD was determined for 6- and 10-MV x rays. This correction factor, together with the perturbation factor, was also calculated using Monte Carlo simulation with the EGSnrc/ egs_chamber user code. In addition, to evaluate the response of the RPLD to clinical-class-specific reference fields, the field-size correction factor for the clinical IMRT plan was measured. Results: The calculated field-size correction factor ranged from 1.007 to 0.981 (for 6-MV x rays) and from 1.012 to 0.990 (for 10-MV x rays) as the jaw-field size ranged from 1 9 1cm 2 to 20 9 20 cm 2. The atomic composition perturbation factor for these jaw fields decreased by 3.2% and 1.9% for the 6- and 10-MV fields, respectively. The density perturbation factor was unity for field sizes ranging from 3 9 3cm 2 to 20 9 20 cm 2, whereas that for field sizes ranging from 3 9 3cm 2 to 1 9 1cm 2 decreased by 3.2% (for 6-MV x rays) and 4.3% (for 10-MV x rays). The volume-averaging factor rapidly increased for field sizes below 1.6 9 1.6 cm 2. The results for the MLC fields were similar to those for the jaw fields. For plan-class IMRT beams, the field-size correction and perturbation factors were almost unity. The difference between the doses measured using the RPLD and ionization chamber was within 1.2% for the clinical IMRT plan at the planning-target volume (PTV) region. Conclusions: For small fields of size 1.6 9 1.6 cm 2 or less, it was clarified that the volume averaging and density perturbation were the dominant effects responsible for the variation in the RPLD response. Moreover, perturbation correction is required when measuring a field size 1.0 9 1.0 cm 2 or less. Under the IMRT conditions, the difference in the responses of the RPLD between the reference conditions and the PTV region calculated by Monte Carlo simulation did not exceed 0.8%. These results indicate that it is feasible to measure IMRT dosage using an RPLD at the PTV region. 2017 American Association of Physicists in Medicine [https://doi.org/10.1002/mp.12665] Key words: energy response, intensity-modulated radiation therap, radiophotoluminescent glass dosimeter, small field 1. INTRODUCTION The accuracy of the output dose is one of the most important factors in radiotherapy. Recently, several groups have performed dosimetry audits for intensity-modulated radiation therapy (IMRT) using a thermoluminescent dosimeter (TLD) and an alanine dosimeter. 1 4 Radiophotoluminescent glass dosimeters (RPLDs) have been used 382 Med. Phys. 45 (1), January 2018 0094-2405/2018/45(1)/382/9 2017 American Association of Physicists in Medicine 382

383 Hashimoto et al.: Field-size correction factors of RGD for small-field 383 for the in vivo dosimetry 5,6 and postal dosimetry audits of megavoltage radiotherapy photon beams 7,8 because of their small fading effects, precise readings owing to repeatable readout, and ease of handling. 9 Therefore, because of the above properties, RPLDs can be used in patient-specific measurements, in vivo dosimetry, and postal dosimetry audits for IMRT. The absorbed dose to water measured in the nonreference field D f nr was defined as the following equation, which is the modified equation of Alfonso et al.: 10 D f nr ¼ M f nr N D;w;Q0 k Q;Q0 ;Q ; (1) where M f nr is the reading of the dosimeter in the nonreference field f nr. N D;w;Q0 is the calibration coefficient in terms of the absorbed dose to water for an ionization chamber at a reference beam quality Q 0 (usually 60 Co). N D;w;Q0 is measured at the standards laboratory for a reference field of size 10 9 10 cm 2. k Q;Q0 is the beam-quality correction factor, which corrects the differences between the reference beam quality Q 0 at the standards laboratory and the beam quality Q of the reference field f ref. ;Q is a field-size correction factor that corrects the differences between the conditions of field size of f ref and f nr. Several studies reported the energy and field-size dependence for RPLD because of changes in the photonenergy spectrum. 7,9,11 13 Mizuno et al. reported a correction method for the energy and field-size dependence of postaldose audits for reference and nonreference conditions (e.g., several field-size and wedge-beam conditions) of conventional linac photon beams. 13 IMRT comprises a smaller field, together with on- and off-axis multileaf collimator (MLC) fields. Therefore, the responses of the RPLD to the IMRT beams may vary. Moreover, in small-photon-field measurements, detector response is largely determined by the mass densities of detector materials, whose influence on the small-to-large-field ratio of detector response is greater than that of atomic number. 14,15 In addition, a nonuniform dose distribution over the detection volume leads to a volume-averaging effect in a small field. The mass density of the RPLD is 2.61 g/cm 3, and the active volume is 1.5 mm in diameter and 6.0 mm in length. Therefore, the RPLD may be affected by density perturbation and volume averaging in the IMRT measurements. Although Azangwe et al. evaluated the RPLD response for field sizes ranging from 1.8 9 1.8 cm 2 to 10 9 10 cm 2 field, 16 a considerably smaller field and an IMRT beam have not been previously evaluated. Therefore, to evaluate the reliability of the RPLD for the small-field and IMRT-beam measurements, we evaluated the energy response and perturbation factor of the RPLD for the small-field, MLC field, and IMRT conditions using experimental measurements and Monte Carlo simulation. 2. MATERIAL AND METHODS 2.A. Measurement of field-size correction factor To evaluate the RPLD response to variations in the small field and IMRT beam, a field-size correction factor was determined. An RPLD (GD-302M, Asahi Techno Glass, Shizuoka, Japan) and an ionization chamber (A1SL, Standard Imaging, Middleton, WI, USA) were positioned at the center of a 30 9 30 9 20 cm 3 water-equivalent phantom (WE211, Kyotokagaku, Kyoto, Japan), and the RPLD was oriented perpendicular to the beam axis (Fig. 1). The source-to-detector distance was 100 cm. Irradiated-field conditions included jaw fields, MLC fields, off-axis MLC fields (Fig. 2), simple- IMRT fields, and two plan-class-specific reference fields. The jaw fields were defined using the X Y collimator jaws alone. The collimator jaws defined square fields with side lengths of 1, 1.6, 2, 3, 4, 5, 10, and 20 cm. In the clinical FIG. 1. Diagram of the measurement setup and detector orientation. [Color figure can be viewed at wileyonlinelibrary.com]

384 Hashimoto et al.: Field-size correction factors of RGD for small-field 384 FIG. 2. Diagram of the multileaf collimator (MLC) position of the 2 9 10 cm 2 field. The cross indicates the isocenter, whereas the circle indicates the field center. 2-cm off-axis and 4-cm off-axis indicate that the distance between the isocenter and the field center is 2 and 4 cm, respectively. [Color figure can be viewed at wileyonlinelibrary.com] situation, an IMRT beamlet is formed using the MLC, and the slit shape is often used for the IMRT beamlet. Therefore, the MLC field was investigated. The MLC fields used MLC alone, with the collimator jaws set to 10 9 10 cm 2. The MLC was used to define 1, 2, 4, 6, and 8 9 10 cm 2 fields. A simple-imrt field (s-imrt) was created to duplicate the simulated field-five uniform beamlets of a 2 9 10 cm 2 field. The two plan-class-specific reference fields used the AAPM TG119 prostate (PR) and head-and-neck (HN) plans 17 with seven and nine fields, respectively. In these plans, the measurement points (isocenter) and the center of the planning-target volume (PTV) were aligned. Under each condition, irradiation was performed five times and one RPLD was used with each irradiation. In addition, a specially made plug which is made of WE211 was employed to hold the RPLD. Irradiation was performed using a Clinac 21EX for 6-MV x ray and TrueBeam (Varian Medical Systems, Palo Alto, CA, USA) for 10-MV x ray. The RPL of the RPLD was read five times using an RPL reader (FDG-1000, Asahi Techno Glass), and the outputs were averaged. The field-size correction factor ;Q can be expressed as follows: ;Q ¼ Dfnr D f ref =M f nr w;q ref =M f ref Q ref ¼ D f nr =RPL f nr D f ref w;q ref =RPL f ; (2) ref Q ref where RPL is a radiophotoluminescence reading. The absorbed dose to water was determined using the ionization chamber according to the Japanese standard dosimetry code, 18 which is similar to IAEA TRS-398. 19 The reference condition was 10 cm depth in water and 10 9 10-cm 2 jaw field. While measuring a small field, the response of the A1SL chamber varies from the reference condition. Therefore, the field factor ;Q for the A1SL chamber was calculated for a small field using Monte Carlo simulation. This factor was used to correct the measured absorbed dose to water for the A1SL chamber. ;Q for the A1SL chamber is expressed as follows: ;Q ¼ Dfnr = D f nr ch; D f ref w;q = Df ref ch;q ; (3) where D ch is the absorbed dose to air in an ion chamber in the reference and nonreference fields. Simulations were performed using the egs_chamber user code for the EGSnrc Monte Carlo code system. 20 The A1SL chamber was modeled using blueprint specifications obtained from the manufacturer (Standard Imaging, Inc., Middleton, WI, USA). The calculation parameters are describedinsection2.b. To validate the accuracy of A1SL modeling, the determined correction factor was compared with that obtained by Kamio et al. 21 In their investigation, the collimator jaws defined square fields with side lengths of 0.5, 0.75, 1, 1.5, and 4 cm, and the volume of water in which the dose is absorbed is the sensitive volume of the detector filled with water. In this validation, the calculation setting was the same as their setting. Figure 3 shows the field-size correction factor for the A1SL chamber as a function of field size. The average difference between our result and that of Kamio et al. is 0.4% (i.e., good agreement was obtained). For the 0.5 9 0.5 cm 2 field, the difference is 1.4%. The electron-collecting region is not defined in the blueprint of an A1SL chamber. The field-size correction factor calculated via the Monte Carlo method depends on how the collecting region is defined in a considerably small field. Therefore, in a 0.5 9 0.5 cm 2 field, the difference in the field-size correction factor is greater than other fields. 2.B. Monte Carlo simulation To evaluate the cause of response variation of the RPLD to the small field and IMRT beam, the field-size correction FIG. 3. Variation in the calculated field-size correction factors as a function of the jaw-field size. [Color figure can be viewed at wileyonlinelibrary.com]

385 Hashimoto et al.: Field-size correction factors of RGD for small-field 385 factor ;Q, density perturbation factor (P q), atomic composition perturbation factor (P med ), and volume-averaging factor (P vol ) were calculated using the EGSnrc code system. 22 25 Each perturbation factor was determined by a ratio of doses scored in two different simulation geometries. Each scored dose was calculated using the egs_chamber code. 20,24 As with previous studies, 14,26,27 a series of cavity doses is defined as follows: (1) D RPLD : absorbed dose in the sensitive volume of the RPLD. (2) D WERPLD : absorbed dose in the sensitive volume of the WERPLD, which is an artificial material that has all the dosimetric (and atomic) properties of normal RPLD, except that its density is equal to that of water. (3) D w,vol : absorbed dose in the sensitive volume of the RPLD, replaced with water. (4) D w,point : absorbed dose in a 0.5 9 0.5 9 0.5 mm 3 voxel of water placed at the centroid of the active volume of the RPLD, representing the absorbed dose at a point in water at the location of measurement. The series of scoring volumes and each perturbation factor are illustrated in Fig. 4. ;Q was determined as follows: ;Q ¼ Dfnr = D f nr RPLD; D f ref w;q = Df ref RPLD;Q ¼ P q P med P vol P q P med P vol fnr fref Q : (4) The calculation-field conditions were the same as the measurement condition. These factors were calculated with a statistical uncertainty of 0.1%. The energy threshold and cutoff energy were set to AE = ECUT = 0.521 MeV and AP = PCUT = 0.01 MeV, respectively. For the Monte Carlo calculations using a photon-beam source, radiation transport in the accelerator was modeled using the BEAMnrc Monte Carlo code. 28,29 The accelerator geometry and materials were obtained from the manufacturer s data for the Clinac 21EX and TrueBeam. Simulation parameters were optimized to reproduce the actual dose distribution, and reproducibility was confirmed by the good agreement between the measured and simulated dose distributions at the first step of the Monte Carlo procedure. 30 2.C. Dose measurement using an RPLD and an ionization chamber for clinical-class-specific reference fields To evaluate the response of the RPLD to clinical-classspecific reference fields, the field-size correction factor for the clinical IMRT plan was determined. To study the representative clinical IMRT plan, 24 cases were selected from the clinical case databases of the Tokyo Metropolitan Komagome Hospital (TMKH) and the Chiba Cancer Center (CCC). The selected cases comprised 12 PR and 12 HN cancer patients. All plans used 2.0 Gy per fraction. The RPLD and A1SL ionization chamber were alternately positioned at the center of the 30 9 30 9 20 cm 3 WE211 phantom and irradiated at the same coordinates and under the same conditions. The measurement point was set to high-dose and low-dose gradient regions. Measurements at the TMKH were performed using Siemens ONCOR and a Varian Clinac 21EX linear accelerator and those at the CCC were performed using a Varian TrueBeam linear accelerator. Five RPLDs were used for each planned measurement. For reference, five RPLDs were irradiated to a dose of 2.0 Gy at each machine and beam energy, each under the same experimental conditions using a reference field of size 10 9 10 cm 2. The field-size correction factor was obtained using Eq. (2). 3. RESULTS AND DISCUSSION 3.A. Field-size correction factors for jaw fields Figure 5 shows the measured and calculated ;Q for several jaw fields. The differences between the measured and calculated ;Q values were within 0.8%. This result is in good agreement with the calculated ;Q and the data from other studies. 7,12,13 To analyze the field-size dependence of various correction factors, we evaluated the field response in three subgroups: large fields (20 9 20 cm 2 3 9 3cm 2 ), small fields (3 9 3cm 2 1.6 9 1.6 cm 2 ), and very small fields (1.6 9 1.6 cm 2 1 9 1cm 2 ). For large fields, the calculated ;Q values increased by 2.6% and 1.2% with decreasing field sizes for the 6- and 10-MV x rays, respectively. Figure 6 shows P med, P q,andp vol, as calculated using Monte Carlo simulations for several jaw-field conditions. The FIG. 4. Illustration of the series of cavity doses simulated to calculate the perturbation factors. [Color figure can be viewed at wileyonlinelibrary.com]

386 Hashimoto et al.: Field-size correction factors of RGD for small-field 386 FIG. 5. Variation in the measured and Monte Carlo-calculated field-size correction factors as a function of the jaw-field size for (a) the 6- and (b) 10-MV x rays. [Color figure can be viewed at wileyonlinelibrary.com] P med value in the large fields increased by 2.9% and 1.6% with decreasing field sizes for the 6- and 10-MV x rays, respectively. These results show that the variation in P med was the largest contributor to the variation in ;Q for the large fields. For small fields, the calculated ;Q decreased by 0.9% and 0.4% with decreasing field size for the 6- and 10-MV x rays, respectively. In this field range, no variation in P med was observed, P q decreased by 1.0% and 2.0%, and P vol was unity and increased by 1.4% for the 6- and 10-MV x rays, respectively, when the field size decreased. In addition, P q had the dominant influence on the variation in ;Q. Scott et al. reported that in the 0.45 9 0.45-cm 2 6-MV photon field, the P q value was 0.90 and 0.96 for diamond density water (q = 3.5 g/cm 3 ) and silicon density water (q = 2.3 g/cm 3 ), respectively. 14 For jaw fields of size 2 9 2cm 2 or less, the lateral charged particle equilibrium (LCPE) was not established for the 6- and 10-MV x rays. Under the condition without LCPE, the contribution of the absorbed dose by the primary beam directed at the cavity in the dense material FIG. 6. Variation in the Monte Carlo-calculated perturbation factor as a function of the jaw-field size for (a) the 6- and (b) 10-MV x rays. [Color figure can be viewed at wileyonlinelibrary.com] increased; therefore, the dense-detector response increased and P q for this detector decreased. 15 The density of RPLD was 2.61 g/cm 3, and RPLD was affected by the density perturbations in the smaller field. For jaw fields of size 2 9 2cm 2 or less, the P q value for the RPLD decreased. The calculated ;Q values for very small fields increased by 0.7% and 1.4% with decreasing field sizes for the 6- and 10-MV x rays, respectively. The variation in P med was unity, the P q values were decreased by 2.2% and 2.3%, and the P vol values were increased by 2.7% and 3.7% for the 6- and 10- MV x rays for field size 1.0 9 1.0 cm 2. Azangwe et al. reported that the P vol value for the RPLD in the 0.9 9 0.9 cm 2 field was 1.03 for the 6-MV x ray. 16 In this field range, P vol had the dominant influence on the variation in ;Q. However, P vol increased and P q decreased with decreasing field size. Therefore, P vol and P q canceled each other. Overall, these results show that the volume-averaging and density perturbation effects were dominant in terms of the variation in the RPLD response in a small-photon field. Moreover, for fields of size 1 9 1cm 2 or less, perturbation corrections are required for measurements using an RPLD.

387 Hashimoto et al.: Field-size correction factors of RGD for small-field 387 3.B. Field-size correction factor for an MLC field and an IMRT beam Figure 7 shows the measured and calculated ;Q values for MLC fields. The ;Q value was within 1.0% of unity for all MLC field sizes. The differences between the measured and calculated ;Q values were within 0.5%. For MLC fields with sizes ranging from 8 9 10 cm 2 to 2 9 10 cm 2, the calculated ;Q value was increased by 0.4% and 0.2% for the 6- and 10-MV x rays, respectively. For 1 9 10 cm 2 MLC field, ;Q decreased by 0.5% and 0.9% for the 6- and 10-MV x rays, respectively, from its value for a 2 9 10 cm 2 MLC field. Figure 8 shows P med and P q for the MLC fields. P med in the MLC field with size ranging from 8 9 10 cm 2 to 1 9 10 cm 2 increased by 1.1% and 0.8% for the 6- and 10- MV x rays, respectively. P q was unity over the 4 9 10 cm 2 MLC field and decreased by 0.8% and 1.6% for the 6- and 10-MV x rays, respectively, from its value for a 1 9 10 cm 2 MLC field. These results were similar to those of the jaw field. For MLC fields with sizes ranging from 8 9 10 cm 2 to 2 9 10 cm 2, P med had the dominant influence on the variation in k f clin;f ref Q clin ;Q and P q had an influence on the variation in ;Q for MLC fields with size 2 9 10 cm2 or less. However, in these MLC fields, the variation in ;Q was within 1.0% for all field sizes, which can be ignored. Figure 9 shows the measured and calculated ;Q values, and Fig. 10 shows P med and P q for the off-axis MLC, simple- IMRT, and plan-class-specific reference fields. The differences between the measured and calculated ;Q values were within 1.5%. The calculated ;Q values in the 2- and 4-cm off-axis MLC field were 1.007 and 0.969 for 6 MV and 1.015 and 0.967 for 10 MV, respectively. These values are considerably large, and correction is required. The P med values for the 2- and 4-cm off-axis MLC fields were 0.993 and 0.957 for 6 MV and 0.985 and 0.961 for 10 MV, respectively. Scarboro et al. reported that for 10 9 10-cm 2 6-MV photon field, the mean photon energy was 0.31 and 0.28 MeV, and the fieldsize correction factor for the optically stimulated luminescent dosimeter was 0.85 and 0.81 at 15- and 20-cm off-axis distance, respectively. 31 For out-of-field measurement, the number of low-energy scattered photons increased and photon FIG. 7. Variation in the measured and Monte Carlo-calculated field-size correction factor as a function of the X side of the MLC field size for (a) the 6- and (b) 10-MV x rays. The Y-collimator jaws were set to 10 cm for all MLC fields. [Color figure can be viewed at wileyonlinelibrary.com] FIG. 8. Variation in the Monte Carlo-calculated perturbation factor as a function of the X side of the MLC field size for (a) the 6- and (b) 10-MV x rays. [Color figure can be viewed at wileyonlinelibrary.com]

388 Hashimoto et al.: Field-size correction factors of RGD for small-field 388 FIG. 9. Variation in the measured and calculated field-size correction factors for several field conditions for (a) the 6- and (b) 10-MV x rays. [Color figure can be viewed at wileyonlinelibrary.com] energy decreased with increasing distance from the field edge to the measurement position. 31 Therefore, P med decreased for off-axis MLC field. The P q values for 2- and 4-cm off-axis MLC fields were 1.014 and 1.013 for 6 MV and 1.030 and 1.007 for 10 MV, respectively. Moreover, the average path length (and consequently, the fluence) of electrons was lower at high densities; hence, the fluence decreased with increasing cavity-mass density. Consequently, the dense-detector response decreased and P q for the dense detector increased. These results show that out-of-field variation in ;Q is caused by P med and P q. In the measurement of the IMRT plan for the organ-at-risk region (low-dose region), response correction may be required for accurate measurement because there are many contributions to the off-axis field and perturbation may increase in this region. The IMRT plan differs for each patient, and the contribution of the off-axis field is dependent on each irradiation plan; thus, in the measurement of the IMRT plan for the organ-at-risk region, a correction factor is required for each plan. The respective ;Q, P med, andp q values were within 0.6% for simple-imrt and plan-class-specific reference fields. The PTV region for the IMRT plan had high- and lowdose gradient regions. This region was constructed by many small on- and off-axis fields. When a high- and low-dose gradient region comprises small on- and off-axis fields, the lack FIG. 10. Variation in the Monte Carlo-calculated perturbation factor for several field conditions for (a) the 6- and (b) 10-MV x rays. [Color figure can be viewed at wileyonlinelibrary.com] of LCPE is compensated and the effect of density perturbation becomes small. Moreover, in the low-dose gradient region, the detector volume averaging is negligible. Therefore, perturbation was very small for measurement of the IMRT plan at a PTV region using RPLD. Scarboro et al. investigated the energy response of an OSLD and a TLD for the IMRT conditions. 31,32 They reported that the calculated correction factor of the OSLD and TLD for the simple-imrt condition, which was created to duplicate the simulated uniform field-five beamlets of the 2 9 10 cm 2 field, was 1.00 and that a correction factor for the IMRT beam was unnecessary. In this study, the calculated correction factors were 0.999 and 1.000 for the 6- and 10-MV simple-imrt conditions, respectively. These results were almost the same as the TLD and OSLD results. 3.C. Clinical-class-specific reference field Figure 11 shows a histogram of ;Q for 24 IMRT plans. The ;Q value was less than the 1.2% difference for all plans. The mean ;Q value was 1.000 with a standard deviation of 0.006. This result shows that the perturbation effects of the RPLD for the IMRT plan were insignificant and that an RPLD could be used for IMRT-plan measurement.

389 Hashimoto et al.: Field-size correction factors of RGD for small-field 389 FIG. 11. Histogram of the k fnr;fref Qnr;Q values for 24 intensity-modulated radiation therapy (IMRT) plans. 4. CONCLUSIONS In this study, we evaluated the field-size corrections and perturbation of the RPLD under the small-field and IMRT conditions using experimental measurements and Monte Carlo simulation. We showed that atomic composition perturbation was the dominant effect for the variation in the RPLD response over a 2 9 2cm 2 field. Moreover, it clarifies that the volumeaveraging effect caused by high-dose gradient and the density perturbation caused by lack of LCPE were the dominant effects causing the variation in the RPLD response for a 2 9 2cm 2 field or less and that a correction factor is required for dosimetry in smaller fields. 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