Young Lori A, Yang Fei, Cao Ning, Meyer Juergen
University of Washington.
J Appl Clin Med Phys. 2016 Nov 8;17(6):149-162. doi: 10.1120/jacmp.v17i6.6343.
During volume-modulated arc therapies (VMAT), dosimetric errors are introduced by multiple open dynamic leaf gaps that are present in fixed diaphragm linear accelerators. The purpose of this work was to develop a methodology for adjusting the rounded leaf end modeling parameters to improve out-of-field dose agreement in SmartArc VMAT treatment plans delivered by fixed jaw linacs where leaf gap dose is not negligible. Leaf gap doses were measured for an Elekta beam modulator linac with 0.4 cm micro-multileaf collimators (MLC) using an A16 micro-ionization chamber, a MatriXX ion chamber detector array, and Kodak EDR2 film dosimetry in a solid water phantom. The MLC offset and rounded end tip radius were adjusted in the Pinnacle treatment planning system (TPS) to iteratively arrive at the optimal configuration for 6 MV and 10 MV photon energies. Improvements in gamma index with a 3%/3 mm acceptance criteria and an inclusion threshold of 5% of maximum dose were measured, analyzed, and validated using an ArcCHECK diode detector array for field sizes ranging from 1.6 to 14 cm square field arcs and Task Group (TG) 119 VMAT test cases. The best results were achieved for a rounded leaf tip radius of 13 cm with a 0.1 cm MLC offset. With the optimized MLC model, measured gamma indices ranged between 99.9% and 91.7% for square field arcs with sizes between 3.6 cm and 1.6 cm, with a maximum improvement of 42.7% for the 1.6 cm square field size. Gamma indices improved up to 2.8% in TG-119 VMAT treatment plans. Imaging and Radiation Oncology Core (IROC) credentialing of a VMAT plan with the head and neck phantom passed with a gamma index of 100%. Fine-tune adjustments to MLC rounded leaf ends may improve patient-specific QA pass rates and provide more accurate predictions of dose deposition to avoidance structures.
在容积调强弧形治疗（VMAT）期间，固定准直器直线加速器中存在的多个开放动态叶片间隙会引入剂量误差。本研究的目的是开发一种方法，用于调整圆形叶片末端建模参数，以改善由固定机头直线加速器执行的SmartArc VMAT治疗计划中的射野外剂量一致性，其中叶片间隙剂量不可忽略。使用A16微型电离室、MatriXX电离室探测器阵列和柯达EDR2胶片剂量测定法，在固体水模体中测量了配备0.4 cm微型多叶准直器（MLC）的医科达束流调制直线加速器的叶片间隙剂量。在Pinnacle治疗计划系统（TPS）中调整MLC偏移和圆形末端半径，以迭代得出6 MV和10 MV光子能量的最佳配置。使用ArcCHECK二极管探测器阵列，针对面积从1.6到14 cm方形野弧形以及任务组（TG）119 VMAT测试病例，测量、分析并验证了接受标准为3%/3 mm且包含阈值为最大剂量5%时伽马指数的改善情况。对于圆形叶片尖端半径为13 cm且MLC偏移为0.1 cm的情况，取得了最佳结果。采用优化的MLC模型，对于尺寸在3.6 cm至1.6 cm之间的方形野弧形，测量的伽马指数在99.9%至91.7%之间，对于1.6 cm方形野尺寸，最大改善率为42.7%。在TG - 119 VMAT治疗计划中，伽马指数提高了2.8%。使用头部和颈部模体的VMAT计划在影像与放射肿瘤学核心（IROC）认证中，伽马指数为100%通过。对MLC圆形叶片末端进行微调可能会提高特定患者的质量保证通过率，并更准确地预测对避让结构的剂量沉积。
