螺距和调制因子在螺旋断层放疗计划优化中的作用

Treatment plan optimization affected by pitch and modulation factor in helical tomotherapy

目的:分析螺距和调制因子在螺旋断层放疗(HT)计划中的作用。方法:选择病例5例,分别是脑胶质瘤、鼻咽癌、肺癌、肝癌和直肠癌,采用不同螺距(0.287、0.430)和调制因子(1.5、2.0、2.5、3.0、3.5)组合制定HT放疗计划,射野宽度统一设置为2.5 cm,其余优化参数importance、penalty和dose calc grid分别设置为1、1、normal,共计50例计划。系统迭代300次后完成计划。计算并记录靶区适形指数、均匀性指数、危及器官剂量指标、出束时间、机架旋转周期,并分析其与螺距、调制因子的相关性。结果:不同螺距、调制因子对脑胶质瘤病例的脑干及其危及器官计划体积的Dmax指标、鼻咽癌病例的脊髓Dmax指标、肺癌病例的脊髓Dmax和全肺Dmean指标的影响均有统计学差异。此外,两者对出束时间和机架旋转周期也有显著影响。结论:螺距对机架旋转周期的影响较明显,旋转周期与螺距成正比;调制因子对出束时间和旋转周期的影响也较明显,两者与其呈正比。

Objective To analyze the effects of pitch and modulation factor（MF） in helical tomotherapy（HT） plan. Methods Five cases of astrocytomas, nasopharyngeal carcinoma, lung cancer, liver cancer, and rectal carcinoma, respectively, were selected. With a field width setting of 2.5 cm, 2 pitches（0.287 and 0.430） and 5 MF（1.5, 2.0, 2.5, 3.0, 3.5） were combined and used to design HT plans, while the optimization parameters of importance, penalty, and dose calculation grid were set to 1, 1,and normal, respectively. After 300 iterations, a total of 50 plans were designed. The conformity index, homogeneity index of target areas, dose index of organs at risk, delivery time, and the rotation period of gantry were calculated, and their correlations with pitch and MF were analyzed. Results The dose index differences in the maximum dose（Dmax） of brain stem and other planning risk volumes of astrocytomas, spinal cord- Dmaxof nasopharyngeal carcinoma, and spinal cord- Dmaxof lung cancer,total lung- Dmeanof lung cancer were statistically significant. The delivery time and rotation periods of gantry were also obviously affected by pitch and MF. Conclusion The rotation period which is significantly affected by pitch is proportional to pitch. The rotation period and delivery time which are obviously affected by MF are proportional to MF.