A preliminary study on the out-of-field in-air electron streaming effect in MRI guided radiotherapy
10.3760/cma.j.issn.0254-5098.2021.09.002
- VernacularTitle:磁共振引导放疗中射野外电子流效应的初步研究
- Author:
Hongdong LIU
1
;
Shouliang DING
;
Lu YANG
;
Bin WANG
;
Yongbao LI
;
Xiaoyan HUANG
Author Information
1. 中山大学肿瘤防治中心放疗科，华南肿瘤学国家重点实验室，肿瘤医学协同创新中心，广州 510060
- Keywords:
MR-linac;
MRgRT;
Electron streaming effect;
Out-of-field dose
- From:
Chinese Journal of Radiological Medicine and Protection
2021;41(9):647-652
- CountryChina
- Language:Chinese
-
Abstract:
Objective:To investigate the impacts of electron streaming effect (ESE) on out-of-field dose distribution in 1.5 T MRI-guided radiotherapy.Methods:Firstly, the Monaco v5.40.1 (Elekta AB, Stockholm, Sweden) treatment planning system (TPS) was implemented to investigate the ESE in a square field (5 cm × 5 cm) at the entry and exit sides of a special homogeneous water phantom. Afterward, a retrospective investigation was conducted into one laryngeal cancer case and one breast cancer case who had been treated on a conventional linear particle accelerator (linac). Then doses were recalculated in the Monaco system using a Unity machine model. Meanwhile, the out-of-field skin dose enhancement induced by ESE was investigated.Results:ESE-induced dose variations were observed at both the entry and exit sides of the phantom surface in the presence of a magnetic field, with the ESE on the exit side notably stronger than that on the entry side. For the laryngeal cancer case, the ESE was not notable and had insignificant impacts on the out-of-field skin dose. In contrast, ESE-induced in-air high-dose region outside the body stretched to the chin area for the breast cancer case. This led to the skin dose escalation of the chin at D1 cm 3 454.6 cGy. After the application of 1 cm bolus, the corresponding skin dose of the chin D1 cm 3 reduced to as low as 113.6 cGy, which is almost equivalent to that in the absence of a magnetic field ( D1 cm 3=92.5 cGy). Conclusions:The ESE in a magnetic field can alter out-of-field dose and lead to local dose enhancement along the electron path. Although the ESE had insignificant impacts on the out-of-field dose of the laryngeal cancer case, it reached the chin area of the breast cancer case. ESE can be effectively shielded by adding protective bolus.
