容积旋转调强放疗中射野等中心选择对宫颈癌剂量学的影响

The dosimetry effect of the different positions of isocenter for volumetric modulated arc therapy treatment of cervical cancer

目的:采用容积旋转调强技术设计放疗计划,选择不同的射野等中心进行优化,讨论射野等中心的改变对宫颈癌容积旋转调强放疗剂量学的影响。方法:随机选取宫颈癌术后患者15例,在同一套CT图像上设计不同的VMAT计划,根据宫颈癌病例靶区几何形状特点,选择不同等中心位置制定四组不同计划,分别为:计划等中心为靶区几何中心,称为Center计划;计划等中心为靶区几何中心y方向上移9 cm,称为Y计划,计划等中心为靶区几何中心x方向左移7 cm,称为X计划,计划靶区几何中心y方向上移9 cm再向x方向左移7 cm,称为X-Y计划。比较分析四种方案的靶区及危及器官的剂量学参数。结果:Center和Y计划能够满足临床靶区及危及器官限量。相较于Center计划,Y、X、X-Y计划靶区覆盖率分别降低了0.36%、15.48%和6.52%;Y计划的适形度指数(CI)、均匀性指数(HI)、脊髓的D_(max)、膀胱的V_(40)、直肠的V_(40)均优于X计划和X-Y计划,差异具有统计学意义(P<0.05)。结论:宫颈癌术后病例的VMAT计划设计中,使用不同等中心设计计划,与等中心位置在几何中心相比,向y轴方向偏移,对靶区和危及器官的影响较小,变化范围在临床可接受范围之内;而等中心位置向x轴方向偏移,对靶区和危及器官影响较大。

Objective:To quantify the impact of the different positions of isocenter for volumetric modulated arc therapy treatment of postoperative cervical cancer cervical cancer.Methods:Fifteen patients receiving postoperative radiotherapy for cervical cancer were selected.According to different position of isocenter,four VMAT plans were disigned on the same CT image,including Center plan,Y plan,X plan and X-Y plan.The position of isocenter had been distributed in 4 positions:symmetry center of the target(Center plan),9 cm up in the Y direction of the symmetry center(Y plan),7 cm to the left in the X direction of the symmetry center(X plan),9 cm up in the Y direction and 7 cm to the left in the X direction of the symmetry center(X-Y plan).The conformity index and homogeneity index of target areas,dosimetric differences of organs-at-risk and monitor units were compared.Results:The center plan and Y plan were satisfied with the clinical requirements.Compared with the center plan,the target coverage of the Y,X and X-Y plans decreased by 0.36%,15.48%and 6.52%,respectively.Compared with X plan and X-Y plan,conformity index(CI),uniformity index(HI),the D_(max) of spinal cord,the V_(40) of bladder,the V_(40) of the rectem were better in Y plan,with statistical significance(P<0.05).Conclusion:In the design of the VMAT plan for postoperative cases of cervical cancer,the isocenter position is offset from the y-axis,which is suitable for clinical application.However,the isocenter position is offset in the x-axis direction,which is not suitable for clinical application because the OAR is at higher dose.