Tomic Nada, Quintero Chrystian, Whiting Bruce R, Aldelaijan Saad, Bekerat Hamed, Liang LiHeng, DeBlois François, Seuntjens Jan, Devic Slobodan
Medical Physics Unit, McGill University, Montréal, Québec H3G 1A4, Canada and Department of Radiation Oncology, SMBD Jewish General Hospital, McGill University, Montréal, Québec H3T 1E2, Canada.
Department of Radiology, University of Pittsburgh, Pittsburgh, Pennsylvania 15213.
Med Phys. 2014 Jun;41(6):062105. doi: 10.1118/1.4876295.
The authors investigated the energy response of XR-QA2 GafChromic™ film over a broad energy range used in diagnostic radiology examinations. The authors also made an assessment of the most suitable functions for both reference and relative dose measurements.
Pieces of XR-QA2 film were irradiated to nine different values of air kerma in air, following reference calibration of a number of beam qualities ranging in HVLs from 0.16 to 8.25 mm Al, which corresponds to effective energy range from 12.7 keV to 56.3 keV. For each beam quality, the authors tested three functional forms (rational, linear exponential, and power) to assess the most suitable function by fitting the delivered air kerma in air as a function of film response in terms of reflectance change. The authors also introduced and tested a new parameter χ = netΔR·e(m netΔR) that linearizes the inherently nonlinear response of the film.
The authors have found that in the energy range investigated, the response of the XR-QA2 based radiochromic film dosimetry system ranges from 0.222 to 0.420 in terms of netΔR at K(air)(air) = 8 cGy. For beam qualities commonly used in CT scanners (4.03-8.25 mm Al), the variation in film response (netΔR at K(air)(air) = 8 cGy) amounts to ± 5%, while variation in K(air)(air) amounts to ± 14%.
Results of our investigation revealed that the use of XR-QA2 GafChromic™ film is accompanied by a rather pronounced energy dependent response for beam qualities used for x-ray based diagnostic imaging purposes. The authors also found that the most appropriate function for the reference radiochromic film dosimetry would be the power function, while for the relative dosimetry one may use the exponential response function that can be easily linearized.
作者研究了XR-QA2 GafChromic™ 胶片在诊断放射学检查所使用的宽能量范围内的能量响应。作者还对用于参考剂量测量和相对剂量测量的最合适函数进行了评估。
在对多种半值层范围从0.16至8.25毫米铝（对应有效能量范围从12.7千电子伏至56.3千电子伏）的射束质进行参考校准之后，将XR-QA2胶片块照射至空气中空气比释动能的九个不同值。对于每种射束质，作者测试了三种函数形式（有理函数、线性指数函数和幂函数），通过将空气中传递的空气比释动能拟合为反射率变化方面的胶片响应函数，来评估最合适的函数。作者还引入并测试了一个新参数χ =净ΔR·e(m净ΔR)，该参数可使胶片固有的非线性响应线性化。
作者发现，在所研究的能量范围内，基于XR-QA2的放射变色胶片剂量测定系统的响应在空气比释动能K(air)(空气)= 8厘戈瑞时，净ΔR范围为0.222至0.420。对于CT扫描仪常用的射束质（4.03 - 8.25毫米铝），胶片响应的变化（空气比释动能K(air)(空气)= 8厘戈瑞时的净ΔR）达±5%，而空气比释动能K(air)(空气)的变化达±14%。
我们的研究结果表明，对于基于X射线的诊断成像目的所使用的射束质，使用XR-QA2 GafChromic™ 胶片会伴随着相当明显的能量依赖性响应。作者还发现，参考放射变色胶片剂量测定的最合适函数是幂函数，而对于相对剂量测定，可以使用易于线性化的指数响应函数。
