Department of Radiation Oncology, Seoul National University Hospital, Seoul, Korea (Republic of).
Biomedical Research Institute, Seoul National University Hospital, Seoul, Korea (Republic of).
Strahlenther Onkol. 2018 Jan;194(1):50-59. doi: 10.1007/s00066-017-1212-z. Epub 2017 Sep 15.
To investigate and to prevent irradiation outside the treatment field caused by an electron stream in the air generated by the magnetic field during magnetic resonance image-guided accelerated partial breast irradiation (APBI).
In all, 20 patients who received APBI with a magnetic resonance image-guided radiation therapy (MR-IGRT) system were prospectively studied. The prescription dose was 38.5 Gy in 10 fractions of 3.85 Gy and delivered with a tri-cobalt system (the ViewRay system). For each patient, primary plans were delivered for the first five fractions and modified plans with different gantry angles from those of the primary plan (in-treatment plans) were delivered for the remaining five fractions to reduce the skin dose. A 1 cm thick bolus was placed in front of the patient's jaw, ipsilateral shoulder, and arm to shield them from the electron stream. Radiochromic EBT3 films were attached to the front (towards the breast) and back (towards the head) of the bolus during treatment. Correlations between the measured values and the tumor locations, treatment times, and tumor sizes were investigated.
For a single fraction delivery, the average areas of the measured isodoses of 14% (0.54 Gy), 12% (0.46 Gy), and 10% (0.39 Gy) at the front of the boluses were as large as 3, 10.4, and 21.4 cm, respectively, whereas no significant dose could be measured at the back of the boluses. Statistically significant but weak correlations were observed between the measured values and the treatment times.
During radiotherapy for breast cancer with an MR-IGRT system, the patient must be shielded from electron streams in the air generated by the interaction of the magnetic field with the beams of the three-cobalt treatment unit to avoid unwanted irradiation of the skin outside the treatment field.
研究并预防在磁共振影像引导加速部分乳房照射（APBI）过程中，磁场与三钴治疗单元射线相互作用产生的电子流在空气中导致的照射野外照射。
共前瞻性研究了 20 例接受磁共振影像引导放疗（MR-IGRT）系统 APBI 的患者。处方剂量为 38.5Gy，10 次分割，每次 3.85Gy，采用三钴系统（ViewRay 系统）实施治疗。对于每位患者，前 5 次分割实施主计划，对于后 5 次分割，采用与主计划不同的机架角度（治疗中计划）修改计划，以减少皮肤剂量。在患者的下巴、同侧肩部和手臂前侧放置 1cm 厚的补偿物，以屏蔽电子流。在治疗过程中，将 EBT3 光致变色胶片贴在补偿物的前侧（朝向乳房）和后侧（朝向头部）。研究了测量值与肿瘤位置、治疗时间和肿瘤大小之间的相关性。
单次分割治疗时，补偿物前侧测量的 14%（0.54Gy）、12%（0.46Gy）和 10%（0.39Gy）等剂量线的平均面积分别为 3cm、10.4cm 和 21.4cm，而补偿物后侧未测量到明显剂量。测量值与治疗时间之间存在显著但较弱的相关性。
在采用 MR-IGRT 系统进行乳腺癌放射治疗时，患者必须屏蔽来自磁场与三钴治疗单元射线相互作用产生的电子流，以避免治疗野外皮肤受到不必要的照射。
