The combination of the error correction methods of GAFCHROMIC EBT3 film

PurposeThe aim of this study was to combine a set of methods for use of radiochromic film dosimetry, including calibration, correction for lateral effects and a proposed triple-channel analysis. These methods can be applied to GAFCHROMIC EBT3 film dosimetry for radiation field analysis and verification of IMRT plans.MethodsA single-film exposure was used to achieve dose calibration, and the accuracy was verified based on comparisons with the square-field calibration method. Before performing the dose analysis, the lateral effects on pixel values were corrected. The position dependence of the lateral effect was fitted by a parabolic function, and the curvature factors of different dose levels were obtained using a quadratic formula. After lateral effect correction, a triple-channel analysis was used to reduce disturbances and convert scanned images from films into dose maps. The dose profiles of open fields were measured using EBT3 films and compared with the data obtained using an ionization chamber. Eighteen IMRT plans with different field sizes were measured and verified with EBT3 films, applying our methods, and compared to TPS dose maps, to check correct implementation of film dosimetry proposed here.ResultsThe uncertainty of lateral effects can be reduced to ±1 cGy. Compared with the results of Micke A et al., the residual disturbances of the proposed triple-channel method at 48, 176 and 415 cGy are 5.3%, 20.9% and 31.4% smaller, respectively. Compared with the ionization chamber results, the difference in the off-axis ratio and percentage depth dose are within 1% and 2%, respectively. For the application of IMRT verification, there were no difference between two triple-channel methods. Compared with only corrected by triple-channel method, the IMRT results of the combined method (include lateral effect correction and our present triple-channel method) show a 2% improvement for large IMRT fields with the criteria 3%/3 mm.【研究目的】本研究旨在整合一套适用于放射变色胶片剂量测定法（radiochromic film dosimetry）的方法，涵盖校准、侧向效应校正以及本文提出的三通道分析技术。上述方法可应用于GAFCHROMIC EBT3胶片剂量测定，用于辐射野分析与调强放射治疗（Intensity-Modulated Radiation Therapy, IMRT）计划验证。 【研究方法】本研究采用单胶片曝光法完成剂量校准，并通过与方形野校准方法的对比验证校准精度。在开展剂量分析前，先对像素值受侧向效应的影响进行校正：采用抛物线函数拟合侧向效应的位置依赖性，并通过二次公式获取不同剂量水平下的曲率因子。完成侧向效应校正后，采用三通道分析技术抑制干扰，将胶片扫描图像转换为剂量分布图像。本研究使用GAFCHROMIC EBT3胶片测量了常规射野的剂量分布曲线，并与电离室（ionization chamber）测得的数据进行对比。此外，本研究应用所提出的方法，使用GAFCHROMIC EBT3胶片对18例不同射野尺寸的调强放疗计划进行测量验证，并与治疗计划系统（Treatment Planning System, TPS）的剂量分布图像进行对比，以验证本文所提出的胶片剂量测定方法的有效性。 【研究结果】侧向效应带来的不确定度可降低至±1 cGy。与Micke A等人的研究结果相比，本文提出的三通道分析方法在48 cGy、176 cGy及415 cGy剂量下的残留干扰分别降低了5.3%、20.9%与31.4%。与电离室测量结果相比，离轴比与百分深度剂量的差异分别控制在1%与2%以内。在调强放疗计划验证应用中，两种三通道分析方法无显著差异。相较于仅采用三通道校正的方案，结合侧向效应校正与本文三通道分析技术的联合方法，在采用3%/3 mm评估标准的大射野调强放疗计划验证中，结果提升了2%。