3~6个脑转移灶海马保护立体定向放射外科计划的剂量学分析

Dosimetry for Stereotactic Radiosurgery Plans with Hippocampal-Sparing for 3-6 Brain Metastases

目的重新优化使用加速器立体定向放射外科(SRS)治疗3~6个脑转移灶的计划,评估减少海马剂量的可行性及靶区和危及器官剂量学的变化。方法收集2020年10月至2023年2月医院收治疗的23例单分次、单等中心、多靶区(3~6个脑转移瘤)SRS患者的数据进行剂量分析。定位CT和核磁共振图像融合,勾画靶区及海马区(勾画参照RTOG 0933),采用Eclipse15.0治疗计划系统设计TrueBeam加速器6MV非均整光子线两组计划:5个非共面容积旋转调强(VMAT)计划和保护海马的5个非共面VMAT重新优化计划。各向异性解析算法,处方剂量至少覆盖99%的计划靶体积(PTV),脑干最大剂量<12 Gy,视神经及视交叉最大剂量<8 Gy,限制大脑低剂量。海马剂量限制:RTOG 0933剂量限制(处方30 Gy/10 f时海马剂量限制条件D100%≤9 Gy,最大剂量≤16 Gy)海马接受剂量转换为单分次生物有效剂量,使用α/β=2(D100%≤4.21 Gy,最大剂量≤6.65 Gy)。分析两种计划的靶区和危及器官剂量分布差别。结果23个病例中有12例重新优化计划,8例实现了海马保留SRS。原计划对比重新优化的计划,海马最大剂量降低,处方99%PTV覆盖率、适形指数(CI)、跌落指数(GI)、脑组织(V_(4)、V_(8)、V_(12)、V_(16)、平均剂量)及机器跳数(MU)未见明显变化。结论与非海马保护计划相比,海马保护SRS计划并未降低靶区CI和GI指数,增加低剂量正常脑组织体积及增加治疗MU数,提示多发脑转移接受SRS治疗患者都应考虑到海马保护。

Objective Evaluate the feasibility of re-optimizing stereotactic radiosurgery(SRS)treatment plans to hippocampal-sparing and dosimetry of target and organ-at-risk for patients with 3-6 brain metastases.Methods Collected data from 23 patients with single-fraction,single isocenter,multi-target(3 to 6 brain metastases)SRS admitted to the hospital for dose analysis form October 2020 to February 2023.Target and hippocampal volumes(outlined according to RTOG 0933 guidelines)were delineated using fused simulation CT and MRI images.Two treatment plans were designed using the Eclipse 15.0 treatment planning system for the TrueBeam accelerator with 6MV flattening filter-free photon beams:5 non-coplanar beam volumetric modulated arc therapy(VMAT)plans and re-optimized 5 non-coplanar beam VMAT plans that hippocampal-sparing.Using the anisotropic analytic algorithm,the prescription dose should cover at least 99%of planning target volume(PTV),with maximum doses to the brainstem<12 Gy and to the optic nerves and chiasm<8 Gy,while limiting low doses to the brain.The hippocampal dose was limited according to the RTOG 0933 dose constraints(D100%≤9 Gy,max dose≤16 Gy for a prescription dose of 30 Gy in 10 fractions).The hippocampal dose was converted to a single-fraction biologically effective dose,usingα/β=2(D100%≤4.21 Gy,max dose≤6.65 Gy).The differences in target and organ-at-risk(OAR)dose distributions between the two plans were analyzed.Results Out of 23 cases,12 were re-optimized and 8 achieved hippocampal-sparing SRS.Compared with the re-optimized plan,the original plan showed a significant decrease in max dose in the hippocampus,with no significant changes observed in the 99%PTV coverage,conformity index(CI),gradient index(GI),brain tissue(V_(4),V_(8),V_(12),V_(16),mean)and machine unit(MU).Conclusion Compared to non-hippocampal sparing plans,hippocampal sparing SRS plans did not decrease target CI and GI index,increase volume of low-dose normal brain tissue,or increase treatment MU time.This suggests that hippocampal sparing should be considered for multiple brain metastases receiving SRS treatment.