螺旋断层放疗在全脑照射保护海马回区计划中的影响因素

Infuencing factors of hippocampus-sparing whole-brain radiotherapy with helical tomotherapy

目的:研究螺旋断层放疗(HT)在全脑照射保护海马回区(HS-WBRT)计划中各项参数对结果的影响。方法:选取8例行HS-WBRT的患者,在瓦里安Eclipse 13.5医生工作站进行靶区和危及器官的勾画,左右海马基于CT图像和MR图像融合勾画,同时外扩5 mm作为海马减量区,靶区为全脑减去海马回区均匀外扩5 mm的区域,危及器官包括海马回区、海马减量区、眼球和晶体。将勾画好的结构和图像传至HT物理师工作站进行计划设计,处方剂量25 Gy/10 F,射野宽度(FW)分别选择1.0、2.5、5.0 cm,螺距(Pitch)分别选择0.215、0.287、0.430,调制因子(MF)分别选择1.5、2.0、2.5、3.0、3.5、4.0、4.5、5.0,剂量计算网格(0.195 cm×0.195 cm),其余计划参数都保持一致,分别设计不同组合参数的计划。最后统计分析不同计划参数对靶区及危及器官的剂量分布及执行效率的影响。结果:使用不同参数制作的计划均满足临床要求。通过比较不同FW、Pitch和MF对靶区及危及器官的影响,FW和MF影响最大,Pitch无影响。从计划质量考虑,FW为1.0 cm时剂量分布最佳,FW为2.5 cm时次之,FW为5 cm时最差;从治疗效率考虑,FW为5 cm时效率最高,FW为2.5 cm时效率次之,FW为1.0 cm时最差。当FW为1.0 cm时,MF选择2.5左右最佳;当FW为2.5 cm,MF选择4左右最佳;当FW为5 cm时,MF选择4左右最佳;此时减小MF会降低计划质量,而增大MF对改善剂量分布无意义,仅会增加治疗出束时间,降低治疗效率。结论:在进行HS-WBRT的计划设计中,需根据临床要求选择合适的计划参数。当临床侧重计划质量时,可选择FW为1 cm或2.5 cm,此时MF选择对应的2.5左右或4左右最佳;当侧重治疗效率时,可选择FW为5.0 cm或2.5 cm,此时MF选择4左右最佳;当兼顾计划质量和执行效率时,可选择FW为2.5 cm,此时MF选择4左右最佳,实现计划质量和治疗效率的平衡。

Objective To study the effects of various parameters on the outcome of hippocampus-sparing whole-brain radiotherapy(HS-WBRT)with helical tomotherapy(HT).Methods Eight patients receiving HS-WBRT were enrolled in the study,and the target areas and organs-at-risk(OAR)were delineated at the Varian Eclipse 13.5 doctor’s workstation.The left and right hippocampus was delineated based on the fusion of CT images and MR images,and the external expansion of 5 mm was taken as the hippocampus reduction region.The target area was the whole brain minus hippocampal gyrus with an uniform exteranal expansion of 5 mm;and OAR included hippocampus,hippocampus reduction region,eyeballs and lens.The delineated structures and images were transmitted to the HT physicist workstation for designing plans with different parameter combinations.The prescribed dose was25 Gy/10 F.Field widths(FW)was set at 1.0,2.5,5.0 cm,respectively,Pitch at 0.215,0.287,0.430,and modulation factors(MF)at 1.5,2.0,2.5,3.0,3.5,4.0,4.5,5.0,and the dose calculation grid was 0.195 cm×0.195 cm.The other planning parameters remained consistent.Finally,the effects of various parameters on the dose distributions of target area and OARs and execution efficiency were statistically analyzed.Results Plans designed with different parameter combinations met clinical requirements.FW and MF were found to have great dosimetric effects on the target area and OAR,while Pitch had no effect on them.From the perspective of plan quality,the dose distribution was the optimal when FW was 1.0 cm,followed by FW of 2.5 cm and FW of5.0 cm;and from the perspective of treatment efficiency,the efficiency was highest when FW was 5.0 cm,followed by FW of2.5 cm and FW of 1.0 cm.The best choice of MF was about 2.5 for FW of 1.0,and about 4.0 for FW of 2.5 cm and 5.0 cm.Reducing MF would reduce plan quality,while increasing MF was meaningless for improving the dose distribution,and it would only increase treatment delivery time and reduce treatment efficiency.Conclusion During HS-WBRT planning,appropriate planning parameters needed to be selected according to clinical requirements.When focusing on plan quality,FW should be selected at 1.0 cm or 2.5 cm,and at this time,the MF should be about 2.5 or 4.0.When focusing on treatment efficiency,FW at5.0 cm or 2.5 cm and MF about 4.0 was the optimal choice.When considering both plan quality and execution efficiency,FW at2.5 cm and MF about 4.0 can balance plan quality and treatment efficiency.