侧向电子失衡对肺部肿瘤放射治疗计划设计的影响

目的 :分析高能X射线通过低密度的肺组织时 ,侧向电子失衡对肺部肿瘤放射治疗计划的影响。方法 :用 6MV和 18MVX射线对一例肺癌进行三维适形治疗 (3D CRT)计划设计 ,并用Helax TMS计划系统提供的笔形束算法和筒串算法对两种能量下的布野方案相同的 3D CRT计划进行剂量计算 ,比较靶区及危及器官的剂量分布、DVH等指标。结果 :采用笔形束算法 6MV与 18MV计划的等剂量线和DVH相近 ,18MV计划的靶区剂量均匀性略优于 6MV计划 ;而当采用能进行电子侧向散射修正的筒串算法时 ,靶区的高剂量覆盖程度明显变差 ,18MV计划靶区剂量亏损更为显著 ,6MV计划高剂量覆盖靶区的程度优于 18MV计划 ;不同能量、算法下肺和脊髓的受量基本相同。结论 :对于肺部肿瘤 ,剂量计算应采用能够准确修正不均匀组织影响的算法 ,非调强放射治疗时最好使用 6MVX射线。

THE INFLUENCE OF LATERAL ELECTRON DISEQUILIBRIUM ON THE RADIATION TREATMENT PLANNING FOR LUNG CANCER IRRADIATION

Purpose To analyze the influence of lateral electron disequilibrium on the radiation treatment planning for lung cancer when using high energy X rays. Methods Three dimension conformal treatment plans for 6MV and 18MV X ray respectively for a lung cancer case have been worked out by using pencil beam algorithm and collapsed cone algorithm provided by Helax-TMS treatment planning system at the same radiation field arrangement for both energies. The indices, such as dose profile and DVH et al, in target and organs at risk (OARs) were used for comparison of different plans. Results Using pencil beam algorithm, the target dose distribution and DVH were similar for 6MV and 18MV plans. The 18MV plan was slightly better than the 6MV plan in terms of dose uniformity. However, using collapsed cone algorithm which could make account of lateral electron scattering, the target was underdosed. The change was even more pronounced for 18MV plan. The 6MV plan provided better target dose coverage than the 18MV plan. The doses for lung and spinal cord were similar for these two energies and two algorithms. Conclusions For lung cancer, dose calculation algorithm should have the ability of handling accurately the effect of the tissue density heterogeneity. It is better to use 6MV X ray than to use 18MV X ray.