多叶准直器透射对调强放疗剂量计算准确性影响观察

Dosimetric effect of multi - leaf collimator transmission on intensity - modulated radiation therapy

目的探讨多叶准直器透射及其对调强放疗剂量计算准确性的影响。方法使用电离室分别测量瓦里安Trilogy直线加速器多叶准直器（MLC）叶片平均透射和叶片剂量学问距（DLG），并使用电子射野影像装置测量MLC叶片内透射和叶片问透射。通过10例肿瘤调强放疗患者资料评价计划系统使用实际测量结果模拟MLC透射计算剂量准确性。结果6、10MVX射线MLC叶片平均透射值分别为1．6％、1．8％，透射值随射野增大而增加、随深度增加而增加、离轴位置稍低于中心轴；叶片内透射值分别为0．8％～1．2％、1．1％～1．6％；叶片间透射值分别为1．3％～1．9％、1．6％～2．5％。计划系统使用实际测量结果模拟MLC的透射值具有很好一致性。3％3mmy通过率2例鼻咽癌患者分别为93．6％和94．5％，其余8例患者均〉95％。结论调强放疗中MLC透射直接影响靶区剂量准确性，MLC叶片透射与测量深度、射野大小和离轴位置有关。

Objective To investigate the dosimetric effect of multi-leaf collimator （MLC） transmission on intensity-modulated radiation therapy （IMRT）. Methods MLC transmission through the leaves and rounded ends were measured with ion chamber for Varian Trilogy linear accelerator with the X-ray of 6 MV and 10 MV. The intraleaf and interleaf transmission were also measured with the electronic portal imaging device of aS1000. 10 tumor patients treated with IMRT were used to evaluate the MLC parameters modeling in the Eclipse treatment planning system. Results The average transmissions of Millennium MLC were 1.6% and 1.8% for 6 MV and 10 MV X-rays. The transmission increased with the field size and depth of measurement. The transmissions at off axis position were a little lower than those on the central axis. The intraleaf transmissions measured with aS1000 were 0.8%--1.2% and 1.1%--1.6%, the interleaf transmission were 1.3%--1.9% and 1.6% --2. 5% for 6 MV and 10 MV X-rays respectively. Modeling with the measured MLC parameters, the Eclipse treatment planning system could calculate the dose distribution accurately. The y pass rate at 3% 3 mm was above 95%, except for two patients with nasopharyngeal cancer with the pass rate of 93.6% and 94. 5%. Conclusions The transmissions through the MLC leaves and the leaf ends contribute to the dose throughout the target significantly for IMRT. MLC transmission varied with the field size, depth of measurement and off axis position.