容积旋转调强技术用于直肠癌时最佳机架角度设置研究

Optimal gantry angle setting in volumetric modulated arc therapy for rectal cancer

目的：探讨对直肠癌患者应用容积旋转调强（VMAT）技术时是否需要避开膀胱的问题。方法：随机选取12例直肠癌放疗患者的CT图像进行靶区及危及器官（OAR）勾画,并对同一CT图像分别设计“不避,避开30°～330°,40°～320°,50°～310°,60°～300°,70°～290°,80°～280°”7个计划,每个计划采用两个旋转弧进行照射,评估靶区及OAR的剂量分布。利用方差分析不同避开方法的结果是否存在显著性差异,若有就找出最好的避开方法,并分析其原因。结果：7个计划的靶区覆盖度均满足临床处方剂量要求,不避计划的靶区适形度和靶区均匀性均较其他设计方案最优（P=0.000,P=0.000）,而且不避计划处方剂量的95%所包绕的体积V95最多（P=0.000）,处方剂量的105%包绕的体积V105、D_（mean）、D_（max）、D5最低（P=0.000、P=0.000、P=0.000、P=0.000）,膀胱V40、V50、D_（max）最低（P=0.044、P=0.043、P=0.000）,小肠、骨髓、Body的D_（max）最低（P=0.039、P=0.000、P=0.000）。避开60°～300°的计划的膀胱V20最低（P=0.005）。角度变化对其他量的影响不显著。结论：在对直肠癌患者应用VMAT技术时,可以不用避开膀胱,用两个全弧照射。

Objective To investigate whether it is need to avoid the bladder in volumetric modulated arc therapy （VMAT） for patients with rectal cancer. Methods The CT images of 12 patients with rectal cancer were randomly selected to contour the target volume and organs at risk. Seven plans without avoiding bladder, and respectively avoiding 30°-330°, 40°-320°, 50°- 310°, 600-300°, 70°-290°, 80°-280° were designed for the same CT image. Two rotating arcs were adopted by each plan to irradiate. The dose distributions of target volumes and organs at risk were evaluated. The variance analysis method was applied to analyze whether there were the significant differences among the different avoiding methods. The optimal avoiding method was researched, and the reasons were analyzed. Results The target coverage of 7 plans met the clinical requirements. For the plan without avoiding the bladder among the 7 plans, the conformity index and homogeneity index of target volume were the optimal （P=-0.000, P=0.000）, and the planning target volume （PTV） covered by 95% prescribed dose （PTV95） was the largest （P=-0.000）, and the PTV covered by 105% prescribed dose （V105）, D D D, were minimum （P=- 0.000, P=0.000, P=0.000, P=0.000）, and the V40, V50, Dmax of bladder were the lowest （P=0.044, P=0.043, P=-0.0001, and the Dmax of small intestine, bone marrow, body were the lowest （P=0.039, P=0.000, P=0.000）. The V20, of the bladder in plan avoiding 60°-300° is lowest （P=0.005）. No significant differences were tbund in other measurements concerned with angle changes. Conelusion The plan without avoiding bladder can be applied in VMAT for patients with rectal cancers, and two rotating arcs can be adopted to irradiate.