Shen Linyi, Li Mengyang, Li Guiyuan, Chen Xinyuan, Xu Shouping, Dai Jianrong, Tian Yuan
National Cancer Center/National Clinical Research Center for Cancer/Cancer Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China.
J Appl Clin Med Phys. 2025 Jun;26(6):e70050. doi: 10.1002/acm2.70050. Epub 2025 Feb 19.
To study the performance of MapCHECK 3 (MC3) in measuring universal wedge fields and propose a dose rate correction strategy to improve MC3 measurement accuracy.
Universal wedge fields with different wedge angles and field sizes were measured at different depths using MC3. Considering the more prominent dose rate dependence of type 4 diodes equipped by MC3, a program was developed to automatically correct the measurements based on the instantaneous dose rate (IDR) correction curve. Central axis (CAX) doses and off-axis doses along the wedge direction, with and without the correction, were compared with those measured by an ion chamber under the same condition. Measurements using MC3 with and without correction were also compared with the planned doses calculated by the treatment planning system (TPS).
If MC3 was used for universal wedge field measurement with the dose calibration factor (DCF) derived from a reference open field, an error of up to -2.4% would be introduced into the CAX dose. Other factors (field size and measurement depth) would also affect the accuracy of measurement when they differed from the absolute dose calibration and the maximum error was up to -2.9%. While greater errors were observed in the off-axis doses at the heel side of the wedge compared to the toe side due to the greater effective thickness of the wedge inserted into the beam. After dose rate correction, the deviations in the CAX dose were reduced to within ± 1.5%. The average gamma pass rate was also improved to over 99.5%.
Because of the more prominent dose rate dependence of type 4 diodes, MC3 is not suitable for universal wedge field measurement using the methodology for open field measurement. The correction strategy proposed in this study is convenient and can improve the accuracy of universal wedge field measurement using MC3.
研究MapCHECK 3（MC3）在测量通用楔形野方面的性能，并提出一种剂量率校正策略以提高MC3测量精度。
使用MC3在不同深度测量具有不同楔形角和射野大小的通用楔形野。考虑到MC3配备的4型二极管对剂量率的依赖性更为显著，开发了一个程序，根据瞬时剂量率（IDR）校正曲线自动校正测量值。将校正前后沿楔形方向的中心轴（CAX）剂量和离轴剂量与相同条件下离子室测量的剂量进行比较。还将使用有或无校正的MC3测量结果与治疗计划系统（TPS）计算的计划剂量进行比较。
如果使用从参考开放野得出的剂量校准因子（DCF），用MC3测量通用楔形野时，CAX剂量会引入高达-2.4%的误差。当射野大小和测量深度与绝对剂量校准不同时，其他因素也会影响测量精度，最大误差高达-2.9%。由于插入射束的楔形物有效厚度更大，与趾侧相比，在楔形物足跟侧的离轴剂量中观察到更大的误差。剂量率校正后，CAX剂量的偏差降低到±1.5%以内。平均γ通过率也提高到99.5%以上。
由于4型二极管对剂量率的依赖性更为显著，MC3不适用于采用开放野测量方法来测量通用楔形野。本研究提出的校正策略方便易行，可提高使用MC3测量通用楔形野的精度。
