Correspondence Response to “Comment on ‘Monte Carlo evaluations of the absorbed dose and quality dependence of Al2O3 in radiotherapy photon beams’ ” [Med. Phys. 36, 4421–4424 (2009)] (Received 23 March 2015; accepted for publication 23 March 2015; published 1 May 2015) [http://dx.doi.org/10.1118/1.4916692]
Recently Kry et al. presented a comment on our paper, Chen et al.1 focusing on the dose response of Al2O3:C optically stimulated luminescent dosimeters (OSLDs) in the buildup region of radiotherapy photon beams. They proposed that the correction factors in the paper of Chen et al.1 were not appropriate for nanoDot OSLD. The proposal by Sky et al. is right, as the OSLD modeled in the paper of Chen et al.1 is quite different from the OSLD nanoDot. Kry et al. also alleged that the variations of the absorbed dose-ratio factor in the buildup region were due to the effective point of measurement (EPOM) displacement which could be reckoned by the radiological depth rule of thumb. These issues will be discussed below. In radiation therapy applications, different sizes and thicknesses of Al2O3:C OSLD have been studied.2 In the paper of Chen et al.,1 the OSLD was modeled as an Al2O3:C disc of a diameter of 0.4 cm and a thickness of 0.1 cm with a density of 3.97 g/cm3. As described by Lehmann et al.3 and Dunn et al.4 the sensitive element in the commercial OSLD is a 0.5 cm diameter by 0.02 cm thick disc of Al2O3:C, covered by thin polyester films and enclosed in a black light-tight casing with a density of 1.03 g/cm3.
In Monte Carlo (MC) simulations, the carbon dopant of Al2O3:C can be neglected, as the addition of carbon into Al2O3 does not change the MC-determined response by more than 1%.5 Because Al2O3 is not tissue equivalent, the presence of the detector perturbs the radiation field. The volume and the density (mass) of the detector are also critical in determining the degree of radiation-fluence perturbation. The absorbed dose-ratio factor f md = Dwater/DAl2O3, which gives the relation between the absorbed dose to the detector and the absorbed dose to water at the point of measurement in the absence of the detector,6–11 depends on the material, the volume and the density of the detector and the energy and the field size of the radiation beams, as well as the position of the detector in water. In radiotherapy photon beams, at the depth beyond the buildup region, transient charged particle equilibrium (CPE) is obtained. Burlin6 general cavity theory can be used to calculate the f md.7–9 Cavity theory calculations by Zhu et al.10 showed that the variation of f md as a function of detector thickness can be as large as 8.3%. MC simulations by Wang et al.11 showed differences in f md as large as 5%. The dependence of f md on detector thickness cannot be neglected even beyond the buildup region.
F. 1. f md for a 0.6 × 0.02 cm and a 0.4 × 0.1 cm disc Al2O3:C detector at various depths of water in a 6 MV beam based upon the spectrum reported by Mohan et al. (Ref. 13). Error bars correspond to 1 standard deviation estimates of the calculation uncertainty. 2650
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F. 2. Comparisons of Rf md = f md (d)/ f md (1.5 cm) and Ef md = Dwat(d)/Dwat (d + 0.15 cm) at different depths for the 1 mm thick OSLD detector investigated by Chen et al. (Ref. 1).
In the buildup region of the radiotherapy photon beams, CPE does not exist. In this high dose-gradient region, the dependence of f md on detector thickness may become large. Experimental verification by Snir et al.12 showed that the nanoDot detector response in the buildup region was different from that predicted by the model in the paper of Chen et al.1 These differences were attributed to differences in the thickness, size, and structure of the detectors. To illustrate these effects, we have performed additional MC simulations (Fig. 1) of the f md factors for a thinner detector (with a diameter of 0.6 cm and a thickness of 0.02 cm) and the thicker OSLD prototype studied in our earlier paper.1 In these EGSnrc simulations, AE, AP, ECUT, and PCUT are all set to 1 keV. The phantom and the beam radius are the same as that in the paper of Chen et al.1 The scoring volume size used to calculate the absorbed dose to water is the same as that of the thinner detector. This volume size is sufficient to evaluate Dwater at the point of measurement.14,15 It is obvious that the f md for these two Al2O3:C detectors are different. The f md for the thinner detector is larger than that for the thicker detector at the same depth. But the variation of the f md for the thinner detector is smaller than that for the thicker one. While the depth changes from 0.5 to 1.5 cm, the f md for the thinner detector varies from 1.143 ± 0.008 to 1.166 ± 0.008, an increase of only about 2%. Kry et al. suggested that the entire effect observed by Chen et al.1 is well described by EPOM displacement. The 1 mm thick OSLD detector effectively measures the dose 0.15 cm behind the detector center location in water. If we define the relative absorbed dose ratio factor as Rf md = f md (d)/f md (1.5 cm), Fig. 2 shows the comparisons of the Rf md and the Ef md = Dwat(d)/Dwat (d + 0.15 cm) at different depths. It can be seen that at the depth beyond 0.5 cm, the Ef md and the Medical Physics, Vol. 42, No. 5, May 2015
Rf md are almost the same. This agrees with the estimate of Kry et al., which was based solely on an evaluation of EPOM change due to the active detector density. At the depths of 0.05, 0.1, 0.3, and 0.5 cm, the difference between the Rf md and the Ef md is 13%, 11%, 3%, and 2%, respectively. While we can agree with Kry et al. that EPOM displacement is the main cause of f md variations in the buildup region, it is not the sole cause. Estimating EPOM displacement from radiological detector thickness may not be a sufficiently accurate rule of thumb for quantitative dosimetry applications. a)Electronic
mail: [email protected] to whom correspondence should be addressed. Electronic mail: [email protected] 1S. W. Chen, X. T. Wang, L. X. Chen, Q. Tang, and X. W. Liu, “Monte Carlo evaluations of the absorbed dose and quality dependence of Al2O3 in radiotherapy photon beams,” Med. Phys. 36, 4421–4424 (2009). 2E. G. Yukihara and S. W. S. McKeever, “Optically stimulated luminescence (OSL) dosimetry in medicine,” Phys. Med. Biol. 53, R351–R379 (2008). 3J. Lehmann, L. Dunn, J. E. Lye, J. W. Kenny, A. D. C. Alves, A. Cole, A. Asena, T. Kron, and I. M. Williams, “Angular dependence of the response of the nanoDot OSLD system for measurements at depth in clinical megavoltage beams,” Med. Phys. 41, 061712 (9pp.) (2014). 4L. Dunn, J. Lye, J. Kenny, J. Lehmann, I. William, and T. Kron, “Commissioning of optically stimulated luminescence dosimeters for use in radiotherapy,” Radiat. Meas. 51-52, 31–39 (2013). 5P. Mobit, E. Agyingi, and G. Sandison, “Comparison of the energy-response factor of LiF and Al2O3 in radiotherapy beams,” Radiat. Prot. Dosim. 119, 497–499 (2006). 6T. E. Burlin, “A general theory of cavity ionization,” Br. J. Radiol. 39, 727–734 (1966). 7P. N. Mobit, A. E. Nahum, and P. Maylesy, “An EGS4 Monte Carlo examination of general cavity theory,” Phys. Med. Biol. 42, 1319–1334 (1997). 8A. S. Beddar, T. R. Mackie, and F. H. Attix, “Water-equivalent plastic scintillation detectors for high-energy beam dosimetry: I. Physical characteristics and theoretical considerations,” Phys. Med. Biol. 37, 1883–1900 (1992).
b)Author
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S. Chen and X. Liu: Response to Comment on Monte Carlo evaluations
9O. T. Ogunleye, F. H. Attix, and B. R. Paliwal, “Comparison of Burlin cavity
theory with LiF TLD measurements for cobalt-60 gamma rays,” Phys. Med. Biol. 25, 203–213 (1980). 10J. H. Zhu, S. W. Chen, L. X. Chen, and X. W. Liu, “Evaluations of absorbed dose ratio factor of Al2O3 dosemeter in radiotherapy photon beams using cavity theory,” Radiat. Prot. Dosim. 152, 393–399 (2012). 11X. T. Wang, J. H. Zhu, S. W. Chen, Q. Tang, and X. W. Liu, “Monte-Carlo simulations of Al2O3 dosimetry in uniform MV photon beams: Influence of field and detector size,” Radiat. Meas. 47, 501–503 (2012). 12J. A. Snir, J. Van Dyk, and S. Yartsev, “Comment on: “Monte Carlo evaluations of the absorbed dose and quality dependence of Al2O3 in radiotherapy photon beams [Med. Phys. 36, 4421–4424 (2009)]”,” Med. Phys. 37, 3009–3010 (2010). 13R. Mohan, C. Chui, and L. Lidofsky, “Energy and angular distributions of photons from medical linear accelerators,” Med. Phys. 12, 592–597 (1985).
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14I. Kawrakow, “On the effective point of measurement in megavoltage photon
beams,” Med. Phys. 33, 1829–1839 (2006). 15F. Tessier and I. Kawrakow, “Effective point of measurement of thimble ion
chambers in megavoltage photon beams,” Med. Phys. 37, 96–107 (2010).
Shaowen Chena) School of Electronic Engineering, Dongguan University of Technology, Dongguan 523808, China
Xiaowei Liub) School of Physics Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China
Recently Kry et al. presented a comment on our paper, Chen et al.1 focusing on the dose response of Al2O3:C optically stimulated luminescent dosimeters (OSLDs) in the buildup region of radiotherapy photon beams. They proposed that the correction factors in the paper of Chen et al.1 were not appropriate for nanoDot OSLD. The proposal by Sky et al. is right, as the OSLD modeled in the paper of Chen et al.1 is quite different from the OSLD nanoDot. Kry et al. also alleged that the variations of the absorbed dose-ratio factor in the buildup region were due to the effective point of measurement (EPOM) displacement which could be reckoned by the radiological depth rule of thumb. These issues will be discussed below. In radiation therapy applications, different sizes and thicknesses of Al2O3:C OSLD have been studied.2 In the paper of Chen et al.,1 the OSLD was modeled as an Al2O3:C disc of a diameter of 0.4 cm and a thickness of 0.1 cm with a density of 3.97 g/cm3. As described by Lehmann et al.3 and Dunn et al.4 the sensitive element in the commercial OSLD is a 0.5 cm diameter by 0.02 cm thick disc of Al2O3:C, covered by thin polyester films and enclosed in a black light-tight casing with a density of 1.03 g/cm3.
In Monte Carlo (MC) simulations, the carbon dopant of Al2O3:C can be neglected, as the addition of carbon into Al2O3 does not change the MC-determined response by more than 1%.5 Because Al2O3 is not tissue equivalent, the presence of the detector perturbs the radiation field. The volume and the density (mass) of the detector are also critical in determining the degree of radiation-fluence perturbation. The absorbed dose-ratio factor f md = Dwater/DAl2O3, which gives the relation between the absorbed dose to the detector and the absorbed dose to water at the point of measurement in the absence of the detector,6–11 depends on the material, the volume and the density of the detector and the energy and the field size of the radiation beams, as well as the position of the detector in water. In radiotherapy photon beams, at the depth beyond the buildup region, transient charged particle equilibrium (CPE) is obtained. Burlin6 general cavity theory can be used to calculate the f md.7–9 Cavity theory calculations by Zhu et al.10 showed that the variation of f md as a function of detector thickness can be as large as 8.3%. MC simulations by Wang et al.11 showed differences in f md as large as 5%. The dependence of f md on detector thickness cannot be neglected even beyond the buildup region.
F. 1. f md for a 0.6 × 0.02 cm and a 0.4 × 0.1 cm disc Al2O3:C detector at various depths of water in a 6 MV beam based upon the spectrum reported by Mohan et al. (Ref. 13). Error bars correspond to 1 standard deviation estimates of the calculation uncertainty. 2650
Med. Phys. 42 (5), May 2015
0094-2405/2015/42(5)/2650/3/$30.00
© 2015 Am. Assoc. Phys. Med.
2650
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S. Chen and X. Liu: Response to Comment on Monte Carlo evaluations
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F. 2. Comparisons of Rf md = f md (d)/ f md (1.5 cm) and Ef md = Dwat(d)/Dwat (d + 0.15 cm) at different depths for the 1 mm thick OSLD detector investigated by Chen et al. (Ref. 1).
In the buildup region of the radiotherapy photon beams, CPE does not exist. In this high dose-gradient region, the dependence of f md on detector thickness may become large. Experimental verification by Snir et al.12 showed that the nanoDot detector response in the buildup region was different from that predicted by the model in the paper of Chen et al.1 These differences were attributed to differences in the thickness, size, and structure of the detectors. To illustrate these effects, we have performed additional MC simulations (Fig. 1) of the f md factors for a thinner detector (with a diameter of 0.6 cm and a thickness of 0.02 cm) and the thicker OSLD prototype studied in our earlier paper.1 In these EGSnrc simulations, AE, AP, ECUT, and PCUT are all set to 1 keV. The phantom and the beam radius are the same as that in the paper of Chen et al.1 The scoring volume size used to calculate the absorbed dose to water is the same as that of the thinner detector. This volume size is sufficient to evaluate Dwater at the point of measurement.14,15 It is obvious that the f md for these two Al2O3:C detectors are different. The f md for the thinner detector is larger than that for the thicker detector at the same depth. But the variation of the f md for the thinner detector is smaller than that for the thicker one. While the depth changes from 0.5 to 1.5 cm, the f md for the thinner detector varies from 1.143 ± 0.008 to 1.166 ± 0.008, an increase of only about 2%. Kry et al. suggested that the entire effect observed by Chen et al.1 is well described by EPOM displacement. The 1 mm thick OSLD detector effectively measures the dose 0.15 cm behind the detector center location in water. If we define the relative absorbed dose ratio factor as Rf md = f md (d)/f md (1.5 cm), Fig. 2 shows the comparisons of the Rf md and the Ef md = Dwat(d)/Dwat (d + 0.15 cm) at different depths. It can be seen that at the depth beyond 0.5 cm, the Ef md and the Medical Physics, Vol. 42, No. 5, May 2015
Rf md are almost the same. This agrees with the estimate of Kry et al., which was based solely on an evaluation of EPOM change due to the active detector density. At the depths of 0.05, 0.1, 0.3, and 0.5 cm, the difference between the Rf md and the Ef md is 13%, 11%, 3%, and 2%, respectively. While we can agree with Kry et al. that EPOM displacement is the main cause of f md variations in the buildup region, it is not the sole cause. Estimating EPOM displacement from radiological detector thickness may not be a sufficiently accurate rule of thumb for quantitative dosimetry applications. a)Electronic
mail: [email protected] to whom correspondence should be addressed. Electronic mail: [email protected] 1S. W. Chen, X. T. Wang, L. X. Chen, Q. Tang, and X. W. Liu, “Monte Carlo evaluations of the absorbed dose and quality dependence of Al2O3 in radiotherapy photon beams,” Med. Phys. 36, 4421–4424 (2009). 2E. G. Yukihara and S. W. S. McKeever, “Optically stimulated luminescence (OSL) dosimetry in medicine,” Phys. Med. Biol. 53, R351–R379 (2008). 3J. Lehmann, L. Dunn, J. E. Lye, J. W. Kenny, A. D. C. Alves, A. Cole, A. Asena, T. Kron, and I. M. Williams, “Angular dependence of the response of the nanoDot OSLD system for measurements at depth in clinical megavoltage beams,” Med. Phys. 41, 061712 (9pp.) (2014). 4L. Dunn, J. Lye, J. Kenny, J. Lehmann, I. William, and T. Kron, “Commissioning of optically stimulated luminescence dosimeters for use in radiotherapy,” Radiat. Meas. 51-52, 31–39 (2013). 5P. Mobit, E. Agyingi, and G. Sandison, “Comparison of the energy-response factor of LiF and Al2O3 in radiotherapy beams,” Radiat. Prot. Dosim. 119, 497–499 (2006). 6T. E. Burlin, “A general theory of cavity ionization,” Br. J. Radiol. 39, 727–734 (1966). 7P. N. Mobit, A. E. Nahum, and P. Maylesy, “An EGS4 Monte Carlo examination of general cavity theory,” Phys. Med. Biol. 42, 1319–1334 (1997). 8A. S. Beddar, T. R. Mackie, and F. H. Attix, “Water-equivalent plastic scintillation detectors for high-energy beam dosimetry: I. Physical characteristics and theoretical considerations,” Phys. Med. Biol. 37, 1883–1900 (1992).
b)Author
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S. Chen and X. Liu: Response to Comment on Monte Carlo evaluations
9O. T. Ogunleye, F. H. Attix, and B. R. Paliwal, “Comparison of Burlin cavity
theory with LiF TLD measurements for cobalt-60 gamma rays,” Phys. Med. Biol. 25, 203–213 (1980). 10J. H. Zhu, S. W. Chen, L. X. Chen, and X. W. Liu, “Evaluations of absorbed dose ratio factor of Al2O3 dosemeter in radiotherapy photon beams using cavity theory,” Radiat. Prot. Dosim. 152, 393–399 (2012). 11X. T. Wang, J. H. Zhu, S. W. Chen, Q. Tang, and X. W. Liu, “Monte-Carlo simulations of Al2O3 dosimetry in uniform MV photon beams: Influence of field and detector size,” Radiat. Meas. 47, 501–503 (2012). 12J. A. Snir, J. Van Dyk, and S. Yartsev, “Comment on: “Monte Carlo evaluations of the absorbed dose and quality dependence of Al2O3 in radiotherapy photon beams [Med. Phys. 36, 4421–4424 (2009)]”,” Med. Phys. 37, 3009–3010 (2010). 13R. Mohan, C. Chui, and L. Lidofsky, “Energy and angular distributions of photons from medical linear accelerators,” Med. Phys. 12, 592–597 (1985).
Medical Physics, Vol. 42, No. 5, May 2015
2652
14I. Kawrakow, “On the effective point of measurement in megavoltage photon
beams,” Med. Phys. 33, 1829–1839 (2006). 15F. Tessier and I. Kawrakow, “Effective point of measurement of thimble ion
chambers in megavoltage photon beams,” Med. Phys. 37, 96–107 (2010).
Shaowen Chena) School of Electronic Engineering, Dongguan University of Technology, Dongguan 523808, China
Xiaowei Liub) School of Physics Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China
