Liu Hongdong, Ding Shouliang, Wang Bin, Li Yongbao, Sun Ying, Huang Xiaoyan
State Key Laboratory of Oncology in South China, Collaborative Innovation Center for Cancer Medicine, Department of Radiation Oncology, Sun Yat-sen University Cancer Center, Guangzhou, China.
Front Oncol. 2020 Dec 1;10:607061. doi: 10.3389/fonc.2020.607061. eCollection 2020.
To investigate the in-air out-of-field electron streaming effect (ESE) for esophageal cancer radiotherapy in the presence of 1.5 T perpendicular magnetic field.
Ten esophageal cancer patients treated with conventional Linac were retrospectively enrolled into a cohort of this study, with the prescription of 4,400 cGy/20 fx. All cases received IMRT replanning using Elekta Unity MR-Linac specified Monaco system, denoted as primary plan. To visualize the in-air dose outside the body in Monaco system, an auxiliary structure was created by extending the external structure. For each case, another comparable plan with no magnetic field was created using the same planning parameters. The plan was also recalculated by placing a bolus upon the neck and chin area to investigate its shielding effect for ESE. Dosimetric evaluations of the out-of-field neck and chin skin area and statistical analysis for these plans were then performed.
Out-of-field ESE was also observed in esophageal cancer treatment planning under 1.5 T magnetic field, while totally absent for plans with no magnetic field. On average, the maximum dose to the neck and chin skin area of the primary plan (657.92 ± 69.07 cGy) was higher than that of plan with no magnetic field (281.78 ± 36.59 cGy, p = 0.005) and plan with bolus (398.43 ± 69.19 cGy, p = 0.007). DVH metrics D (the minimum dose to 1 cc volume) of the neck and chin skin for primary plan was 382.06 ± 44.14 cGy, which can be reduced to 212.42 ± 23.65 cGy by using the 1 cm bolus (with p = 0.005), even lower than the plan without magnetic field (214.45 ± 23.82, p = 0.005). No statistically significant difference of the neck and chin skin dose between the plan with bolus and plan with no magnetic field was observed (all with p > 0.05).
For MRI guided esophageal cancer radiotherapy, a relatively high out-of-field neck and chin skin doses will be introduced by ESE in the presence of magnetic field. It is therefore recommended to take this into account during the planning phase. Adding bolus could effectively reduce the ESE dose contributions, achieve the shielding effect almost equivalent to the scenario with no magnetic field. Further explorations of measurement verifications for the ESE dose distributions are required.
研究在1.5 T垂直磁场存在的情况下，食管癌放射治疗中的空气中野外电子流效应（ESE）。
回顾性纳入10例接受传统直线加速器治疗的食管癌患者，处方剂量为4400 cGy/20次分割。所有病例均使用Elekta Unity MR-Linac指定的Monaco系统进行调强放疗重新计划，记为初始计划。为了在Monaco系统中可视化体外空气中的剂量，通过扩展外部结构创建了一个辅助结构。对于每个病例，使用相同的计划参数创建另一个无磁场的可比计划。还通过在颈部和下巴区域放置 bolus（等效填充物）来重新计算计划，以研究其对ESE的屏蔽效果。然后对野外颈部和下巴皮肤区域进行剂量学评估，并对这些计划进行统计分析。
在1.5 T磁场下的食管癌治疗计划中也观察到了野外ESE，而无磁场的计划中则完全没有。平均而言，初始计划中颈部和下巴皮肤区域的最大剂量（657.92±69.07 cGy）高于无磁场计划（281.78±36.59 cGy，p = 0.005）和有bolus的计划（398.43±69.19 cGy，p = 0.007）。初始计划中颈部和下巴皮肤的DVH指标D（1 cc体积的最小剂量）为382.06±44.14 cGy，使用1 cm bolus可降至212.42±23.65 cGy（p = 0.005），甚至低于无磁场计划（214.45±23.82，p = 0.005）。在有bolus的计划和无磁场的计划之间，未观察到颈部和下巴皮肤剂量有统计学显著差异（所有p>0.05）。
对于MRI引导的食管癌放射治疗，在有磁场的情况下，ESE会导致相对较高的野外颈部和下巴皮肤剂量。因此，建议在计划阶段考虑这一点。添加bolus可以有效降低ESE剂量贡献，实现几乎与无磁场情况等效的屏蔽效果。需要进一步探索ESE剂量分布的测量验证。
