Correspondence Comment on “Monte Carlo evaluations of the absorbed dose and quality dependence of Al2O3 in radiotherapy photon beams” [Med. Phys. 36(10), 4421–4424 (2009)] (Received 18 September 2014; accepted for publication 11 January 2015; published 1 May 2015) [http://dx.doi.org/10.1118/1.4914139]
We read with great interest the work by Chen et al.,1 who performed Monte Carlo calculations on the dose response of Al2O3:C optically stimulated luminescent dosimeters (OSLD) in the buildup region. They evaluated the ratio of dose to water versus dose to the OSLD material at the same location ( f md) and found that this ratio changed with depth in the buildup region. For example (as highlighted in their Fig. 3), in a 6 MV beam, the authors calculated a 6% difference in detector response (relative to water at the same location) at 0.5 cm depth compared to 1.5 cm depth. This sizeable difference would need to be accounted for if using this dosimeter in the buildup region. This effect was attributed to changes in the energy spectrum: because Al2O3 is not water equivalent, the authors assert that energy-dependent corrections are required for OSLD-based measurements done in the buildup region. They also suggest that the high density of OSLD can displace the effective point of measurement downstream from the center of the detector. This work has serious implications for clinical applications of OSLD as it is a common practice to place the dosimeter on the patient surface or elsewhere in the buildup region (e.g., under bolus). Both of these possible effects deserve consideration. (1) Energy effects: Subsequent to the work of Chen et al., detailed evaluations have been conducted on the energy dependence of OSLD.2 That more recent work observed differences of up to 3% in detector response because of changes in the energy spectrum. These variations were associated with large changes in measurement location (e.g., off-axis position) or the treatment field (e.g., field size or depth). However, between 0.5 cm depth and d max on the central axis in water, there is almost no change in the photon energy spectrum (Fig. 1). Based on cavity theory calculations,2,3 the difference between these energy spectra would manifest as less than a 0.2% change in the detector response. This is a substantially smaller effect than the 6% observed by Chen et al. While the contamination and secondary electron spectra are also changing in this region, this has relatively little impact on dosimeter response because the stopping power changes little with energy.3 (2) Buildup effects: If OSLD is placed in the buildup region, its high-density displaces the effective point of measurement downstream of the detector center on 2648
Med. Phys. 42 (5), May 2015
the PDD curve. Chen et al. assumed a 1 mm thick OSLD with a density of 3.95 g/cm3 (nearly 4 mm total water equivalent thickness), which would provide an effective point of measurement nearly 2 mm behind the front of the OSLD crystal. The dose at this location was compared to the same calculation made in water, where the effective point of measurement would correspond to the center of the 1 mm-thick volume of water, i.e., 0.5 mm behind the front of it. Effectively, the OSLD is measuring the dose 1.5 mm deeper than the measurement location in water. The authors report that the OSLD dose at 5 mm depth would be 6% higher than in water; this is virtually identical to the increase in dose that would be seen by moving 1.5 mm further up the PDD in water—from 5 to 6.5 mm.4,5 That is, the entire effect observed by Chen et al. is well described by effective point of measurement displacement. This is also consistent with no depth dependent effects being observed at depths greater than d max because beyond d max, the dose is relatively insensitive to a shift of 1.5 mm. The correction factors proposed by Chen et al. appear to be almost entirely due to buildup effects and not spectral changes. However, buildup effects are certainly real. If one
F. 1. Photon energy spectra on the central axis of a 10 × 10 cm 6 MV x-ray field in water at depths of 0.5 cm at 1.6 cm (d max).
0094-2405/2015/42(5)/2648/2/$30.00
© 2015 Am. Assoc. Phys. Med.
2648
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Kry, Olch, and Mohan: Comment on Chen et al.
was using a 1 mm-thick solid Al2O3 crystal, as considered by Chen et al., their results would be appropriate. However, 1-mm thick is not a realistic description of the most common commercial OSLD: nanoDots from Landauer, Inc. (Glenwood, IL). While the entire nanoDot detector is 2 mm thick, there is only 0.2 mm of Al2O3:C crystal and it has a density of only 1.41 g/cm3,6 with the rest being plastic casing (1.06 mm, density 1.03 g/cm3), polyester binding materials (0.1 mm), and air.7,8 The effective point of measurement of the nanoDot OSLD is 0.8 mm from the front surface,8 which is notably less than the effective point of measurement of approximately 2 mm used by Chen et al. Consequently, the correction factors proposed by Chen et al. are not appropriate for nanoDot OSLD. In clinical practice, consideration should be given to the effective point of measurement of the specific dosimeter being used. NanoDot OSLDs have an effective point of measurement that is 0.8 mm from the front surface, and accounting for this distance provides accurate dose measurements without consideration for energy spectral effects.8
2649 3S. B. Scarboro and S. F. Kry, “Characterisation of energy response of Al2O3:
C optically stimulated luminescent dosimeters (OSLDs) using cavity theory,” Radiat. Prot. Dosim. 153, 23–31 (2013). 4S. J. Becker, R. R. Patel, and T. R. Mackie, “Increased skin dose with the use of a custom mattress for prone breast radiotherapy,” Med. Dosim. 32, 196–199 (2007). 5J. A. Purdy, “Buildup/surface dose and exit dose measurements for a 6-MV linear accelerator,” Med. Phys. 13, 259–262 (1986). 6J. 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 measrements at depth in clinical megavoltage beams,” Med. Phys. 41(6), 061712 (1pp.) (2014). 7L. 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). 8A. H. Zhuang and A. J. Olch, “Validation of OSLD and a treatment planning system for surface dose determination in IMRT treatments,” Med. Phys. 41, 081720 (8pp.) (2014).
Stephen F. Krya) Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, Texas 77030
Arthur Olch a)[email protected];
Telephone: 713-745-8939. 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(10), 4421–4424 (2009). 2S. B. Scarboro, D. S. Followill, J. R. Kerns, R. A. White, and S. F. Kry, “Energy response of optically stimulated luminescent dosimeters for nonreference measurement locations in a 6 MV photon beam,” Phys. Med. Biol. 57(9), 2505–2515 (2012).
Children’s Hospital of Los Angeles, Los Angeles, California 90027
1S.
Medical Physics, Vol. 42, No. 5, May 2015
Radhe Mohan Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, Texas 77030
We read with great interest the work by Chen et al.,1 who performed Monte Carlo calculations on the dose response of Al2O3:C optically stimulated luminescent dosimeters (OSLD) in the buildup region. They evaluated the ratio of dose to water versus dose to the OSLD material at the same location ( f md) and found that this ratio changed with depth in the buildup region. For example (as highlighted in their Fig. 3), in a 6 MV beam, the authors calculated a 6% difference in detector response (relative to water at the same location) at 0.5 cm depth compared to 1.5 cm depth. This sizeable difference would need to be accounted for if using this dosimeter in the buildup region. This effect was attributed to changes in the energy spectrum: because Al2O3 is not water equivalent, the authors assert that energy-dependent corrections are required for OSLD-based measurements done in the buildup region. They also suggest that the high density of OSLD can displace the effective point of measurement downstream from the center of the detector. This work has serious implications for clinical applications of OSLD as it is a common practice to place the dosimeter on the patient surface or elsewhere in the buildup region (e.g., under bolus). Both of these possible effects deserve consideration. (1) Energy effects: Subsequent to the work of Chen et al., detailed evaluations have been conducted on the energy dependence of OSLD.2 That more recent work observed differences of up to 3% in detector response because of changes in the energy spectrum. These variations were associated with large changes in measurement location (e.g., off-axis position) or the treatment field (e.g., field size or depth). However, between 0.5 cm depth and d max on the central axis in water, there is almost no change in the photon energy spectrum (Fig. 1). Based on cavity theory calculations,2,3 the difference between these energy spectra would manifest as less than a 0.2% change in the detector response. This is a substantially smaller effect than the 6% observed by Chen et al. While the contamination and secondary electron spectra are also changing in this region, this has relatively little impact on dosimeter response because the stopping power changes little with energy.3 (2) Buildup effects: If OSLD is placed in the buildup region, its high-density displaces the effective point of measurement downstream of the detector center on 2648
Med. Phys. 42 (5), May 2015
the PDD curve. Chen et al. assumed a 1 mm thick OSLD with a density of 3.95 g/cm3 (nearly 4 mm total water equivalent thickness), which would provide an effective point of measurement nearly 2 mm behind the front of the OSLD crystal. The dose at this location was compared to the same calculation made in water, where the effective point of measurement would correspond to the center of the 1 mm-thick volume of water, i.e., 0.5 mm behind the front of it. Effectively, the OSLD is measuring the dose 1.5 mm deeper than the measurement location in water. The authors report that the OSLD dose at 5 mm depth would be 6% higher than in water; this is virtually identical to the increase in dose that would be seen by moving 1.5 mm further up the PDD in water—from 5 to 6.5 mm.4,5 That is, the entire effect observed by Chen et al. is well described by effective point of measurement displacement. This is also consistent with no depth dependent effects being observed at depths greater than d max because beyond d max, the dose is relatively insensitive to a shift of 1.5 mm. The correction factors proposed by Chen et al. appear to be almost entirely due to buildup effects and not spectral changes. However, buildup effects are certainly real. If one
F. 1. Photon energy spectra on the central axis of a 10 × 10 cm 6 MV x-ray field in water at depths of 0.5 cm at 1.6 cm (d max).
0094-2405/2015/42(5)/2648/2/$30.00
© 2015 Am. Assoc. Phys. Med.
2648
2649
Kry, Olch, and Mohan: Comment on Chen et al.
was using a 1 mm-thick solid Al2O3 crystal, as considered by Chen et al., their results would be appropriate. However, 1-mm thick is not a realistic description of the most common commercial OSLD: nanoDots from Landauer, Inc. (Glenwood, IL). While the entire nanoDot detector is 2 mm thick, there is only 0.2 mm of Al2O3:C crystal and it has a density of only 1.41 g/cm3,6 with the rest being plastic casing (1.06 mm, density 1.03 g/cm3), polyester binding materials (0.1 mm), and air.7,8 The effective point of measurement of the nanoDot OSLD is 0.8 mm from the front surface,8 which is notably less than the effective point of measurement of approximately 2 mm used by Chen et al. Consequently, the correction factors proposed by Chen et al. are not appropriate for nanoDot OSLD. In clinical practice, consideration should be given to the effective point of measurement of the specific dosimeter being used. NanoDot OSLDs have an effective point of measurement that is 0.8 mm from the front surface, and accounting for this distance provides accurate dose measurements without consideration for energy spectral effects.8
2649 3S. B. Scarboro and S. F. Kry, “Characterisation of energy response of Al2O3:
C optically stimulated luminescent dosimeters (OSLDs) using cavity theory,” Radiat. Prot. Dosim. 153, 23–31 (2013). 4S. J. Becker, R. R. Patel, and T. R. Mackie, “Increased skin dose with the use of a custom mattress for prone breast radiotherapy,” Med. Dosim. 32, 196–199 (2007). 5J. A. Purdy, “Buildup/surface dose and exit dose measurements for a 6-MV linear accelerator,” Med. Phys. 13, 259–262 (1986). 6J. 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 measrements at depth in clinical megavoltage beams,” Med. Phys. 41(6), 061712 (1pp.) (2014). 7L. 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). 8A. H. Zhuang and A. J. Olch, “Validation of OSLD and a treatment planning system for surface dose determination in IMRT treatments,” Med. Phys. 41, 081720 (8pp.) (2014).
Stephen F. Krya) Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, Texas 77030
Arthur Olch a)[email protected];
Telephone: 713-745-8939. 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(10), 4421–4424 (2009). 2S. B. Scarboro, D. S. Followill, J. R. Kerns, R. A. White, and S. F. Kry, “Energy response of optically stimulated luminescent dosimeters for nonreference measurement locations in a 6 MV photon beam,” Phys. Med. Biol. 57(9), 2505–2515 (2012).
Children’s Hospital of Los Angeles, Los Angeles, California 90027
1S.
Medical Physics, Vol. 42, No. 5, May 2015
Radhe Mohan Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, Texas 77030
