加速器治疗床对VMAT剂量影响研究

Effect of accelerator treatment couch on the VMAT dose distribution

目的目前的医用直线加速器，大多配备碳素纤维治疗床。本研究通过在Monaco计划系统中构建加速器治疗床模型，探讨治疗床对旋转容积调强（volumetric modulated arc therapy,VMAT）剂量分布的影响。方法将16．5cm的固体水置于治疗床左右位置的正中，测量固体水等中心的吸收剂量Dmea。在Monaco计划系统中加入治疗床模型，计算固体水等中心的吸收剂量Dcal。调整床模型外层碳素纤维和内层填充泡沫的相对电子密度，当计算值Dcal和测量值Dmea最接近时，确认床模型的相对电子密度值。选取中国科学院合肥肿瘤医院2017-03-01-2017-08-31收治的宫颈癌患者15例，均设计VMAT全弧计划，在加和不加床模型的情况下分别进行质量保证（quality assurance，QA）剂量计算，并将计算得到的剂量体积直方图导出并保存为文本文件，用matlab读取上述文件，然后编写程序，基于剂量体积的统计分析和四分位数，评估加速器碳素纤维治疗床对VMAT剂量分布的影响。结果加入治疗床模型前后，固体水等中心的吸收剂量Dcal与Dmea的相对偏差由（2．66±1．14）％降低至（-0．07±0．20）％，差异有统计学意义，t=9．74，P〈0．05。靶区的平均剂量Dmeean由（52．88±0．82）Gy减小至（52．05±0．80）Gy，差异有统计学意义，z=-3．41，P〈0．05；体积剂量D95%由（50．71±0．25）Gy减小至（49．87±0．34）Gy，差异有统计学意义，t=19．04，P〈0．05。皮肤的剂量差异最为显著，平均剂量由（7．41±2．81）Gy增加至（9．89±3．71）Gy，差异有统计学意义，t=-9．44，P〈0．05。直肠、膀胱、股骨头、小肠的剂量体积V20、V30、V40，以及Dmax和Dmean均有所降低，差异均有统计学意义，P〈0．05。结论加速器治疗床对VMAT剂量分布的影响不应被忽略，通过Monaco计划系统正确的构建和调用治疗床模型，可以很好的模拟加速器治疗床对剂量的影响。

OBJECTIVE The current medical linear accelerators are mostly equipped with carbon fiber treatment couches. In this paper, the effect of treatment couch on the dose distribution of volumetric modulated arc therapy （VMAT） was studied by constructing accelerator couch model in the Monaco treatment planning system. METHODS The 16.5 cm solid water was placed in the middle of the left and right position of the treatment couch and the absorbed dose Dmea ofthe solid water＇s isocenter was measured. A treatment couch model was imported into the Monaco planning system to cal- culate the absorbed dose Dcal of the solid water＇s isocenter. The relative electron densities of the outer carbon fiber and the inner layer foam core of the couch model were adjusted,when the calculated value Dcal and the measured value Dmea were the closest,the relative electron densities of the couch model were confirmed. The study involved 15 cervical cancer pa- tients treated in H efei Cancer Hospital of the Chinese Academy of Sciences from March 1,2017 to August 31,2017,15 ea- ses were designed with the VMAT full arc plans,and QA dose calculations were performed with and without couch mod- el. The calculated dose volume histograms were exported and saved as text files,using MATLAB software to read the a- bove files,and then programming to evaluate the effect of accelerator couch on VMAT dose distribution pairs based on quartile and dose volume. RESULTS The relative deviation of absorbed dose between Dcal and Dmea at solid water＇s iso- center decreased from （2.66 ± 1. 14） % to （ - 0.07±0. 20）%before and after adding the treatment couch model, the difference was significant（t=9.74,P〈0.05）. The target Dmean decreased from （52.88±0.82） Gy to （52.05±0.80） Gy, the difference was significant（z= - 3.41, P〈0.05）, and the volume dose D95% decreased from （50. 71 ± 0.25） Gy to （49.87±0.34） Gy, the difference was significant （t= 19.04, P〈0.05）. The most significant difference was the dosage of skin,the mean dose increased from （7.41±2.81） Gy to （9.89±3.71） Gy,the difference was significant （t=-9.44,P〈0.05）. The dose volume V20, V30, V40, Dmax and Dmean in rectum, bladder, femoral head and small intestine all decreased, the differences were significant（P〈0.05）. CONCLUSION The effect of accelerator treatment couch on VMAT dose distri- bution should not be overlooked, and the effect can be well simulated by properly constructing and invoking the treatment couch model with the Monaco treatment planning system.