扩充型动态楔形板楔形因子的校正方法及跳数计算

摘要

目的：研究瓦里安扩充型动态楔形板楔形因子计算修正方法，比较楔形野中心点处手工和治疗计划系统计算结果相对测量结果的剂量/跳数差异。方法对于瓦里安直线加速器的6 MV、10 MV光子线，使用指形电离室测量水下10 cm处不同动态楔形野的楔形因子及射野中心点的剂量，采用治疗计划系统计算相应射野的剂量/跳数。使用加速器输出分割模型手工计算射野的楔形因子，并采用常数因子修正手工计算结果。对手工计算、治疗计划系统计算和测量结果进行比较，分析三种方法下常规二维治疗计划下动态楔形野的楔形因子和射野中心点跳数的误差。结果以测量结果为标准，校正后，手工计算的楔形因子误差明显减小。其中，6 MV光子线下，60°楔形角下对称野最大相对误差由4.2%减小到1.3%，非对称野最大相对误差由-4.7%减小到-1.8%。10 MV所有楔形野相对误差由最大-3.0%降低到1.1%。手工计算跳数与测量结果对比，对称野相应射野跳数计算相对误差在2%以内，但部分非对称野最大相对误差超过5%。比较治疗计划系统计算结果与测量结果，其最大相对误差小于1.5%。结论使用常数因子可以有效减小输出分割模型计算楔形因子的误差。对于常规二维治疗计划楔形野的跳数计算来说，校正后对称野射野中心点的计算结果符合临床治疗要求，但对于射野边缘与等中心最短距离小于4 cm的非对称野来说，需要使用相应的非对称射野处方剂量计算方法,或者采用测量方法或利用治疗计划系统计算相应的射野跳数。%Objective To study the correction of algorithm for Varian enhanced dynamic wedge(EDW) factors and compare the dose/monitor unit (MU) deviation measured at the central axis of EDW field with that obtained by manual calculation or using the treatment planning system. Methods EDW factors and dose were measured with Thimble ion chamber at 10 cm depth under the water for 6 MV and 10 MV photon on Varian linear accelerator. The corresponding calculations were done with the radiation treatment planning system. An analytic formula, namely the“MU Fraction model”, was used to calculate the EDW factor, which was corrected with a constant factor. The MU of conventional 2-D planning derived from manual calculating, treatment planning system, and actual measurements were compared. Results With the measured results as the standard, the corrected manual calculation deviation of EDW factors was significantly reduced. For photon 6 MV, the maximum deviation reduced from 4.2%to 1.3%for 60° symmetry fields was, and from-4.7%to-1.8%for asymmetric fields. For photon 10 MV, the maximum deviation for all EDW fields was reduced from-3.0% to 1.1%. Comparison of the manual calculations with the measured results showed a MU deviation for symmetric fields within 2%, and more than 5%for some asymmetric fields. The deviation between the calculations of the treatment planning and the measured results was less than 1.5%. Conclusion Constant factor correction can effectively reduce the deviation of manual calculation. For MU calculation of EDW field in conventional 2-D dimensional treatment planning, the corrected results of symmetric fields meet clinical requirements. While the minimum distance between the field edge and the central axis was less than 4 cm in asymmetric fields, the corresponding special method, measurement or the treatment planning system should be used to calculate the dose/MU.