调强放疗计划系统的剂量学特性测试方法

Dosimetric commissioning for treatment planning system of intensity modulated radiation therapy module

目的建立治疗计划系统调强放疗(IMRT)功能的剂量学特性测试方法,以期确定系统计算IMRT照射野或计划的剂量准确性。方法多叶准直器(MLC)调强是IMRT的主要实现方式,针对MLC特点设计一组测试例,测试这些特点对IMRT射野剂量分布的影响。与传统铅门相比, MLC设计的特点和带来的影响包括:(1)MLC叶片末端可能采用弧形端面设计,引起叶片末端效应; (2)MLC叶片侧面采用凸凹槽咬合设计,引起凸凹槽效应;(3)MLC叶片厚度通常比铅门簿,因此对原射线的衰减有差别;(4)IMRT照射野多个子野叠加照射形成的剂量分布将体现前述三个特点的综合效果。结果所有测试例的绝对剂量误差都较小,只有1个>2%。除了T1b和在10 cm深度时的T2外,其他测试例在3%/3 mm时的通过率都>95%。T1b和在10 cm深度时的T2位置处的计算剂量都偏高。除了对MLC凸凹槽的模拟稍差之外,经调整后的机器模型可计算得到相对准确的剂量分布,模型的各个参数基本已是最优值。结论实验结果显示该方法可准确反映MLC的设计特点对剂量的独立影响和综合影响,设计的测试方法适合于计划系统IMRT功能的验收测试。

Objective To develop a set of methods for testing dosimetric characteristics of treatment planning system with intensity modulated radiation therapy（IMRT） module. Methods Muhileaf collimator （MLC） based IMRT is the predominant technique being used presently. According to the designing characteristics of MLC, its influence includs ： （ 1 ） Curved designing may be adopted for the MLC face, which can lead to complicated scattering effect; （2）The side of MLC usually have ＂tongue-and-groove＂ feature which can change the dose distribution near the side; （3） Compared to the thicker jaw, MLC usually have smaller attenuation effect for primary photon; （4） The IMRT field composed of several segments will exhibit the composite effects of the former three characteristics. Results The absolute dose error of the normalization point was relatively low and only one case had dose error 〉 2%. The pass rate at 3% / 3 mm was above 95%, except for cases Tlb and T2 at depth of 10 cm with the calculated dose being higher than the measured dose. Except for the relatively poor performance in the simulation of the MLC tongue and groove effect, the adjusted machine model, with the optimized model parameters having been achieved, could be used to calculate field dose accurately. Conclusions The methods developed here can reveal the independent and composite dosimetric effects of MLC. These methods can be used in the commissioning test for treatment planning system of the IMRT module.