磁共振引导放疗中射野外电子流效应的初步研究

A preliminary study on the out-of-field in-air electron streaming effect in MRI guided radiotherapy

目的研究1.5 T磁共振引导放疗中的电子流效应(ESE)及其对射野外剂量的影响。方法首先,利用Monaco系统研究了均匀射野(5 cm×5 cm)在规则几何模体外入射端和出射端的空气中ESE。其次,回顾性地选取在常规加速器上接受过放疗的1例喉癌病例和1例乳腺癌病例,重新用Unity机器模型在Monaco系统中进行剂量计算,研究磁场下ESE对射野外皮肤剂量的影响。结果在磁场作用下,规则几何模体的入射端和出射端都观察到ESE效应,但出射端ESE明显强于入射端。在喉癌病例中,ESE现象不明显,对射野外皮肤剂量影响很小。在乳腺癌病例中,射野外ESE会延伸到下颌附近,导致下颌皮肤剂量升高,1 cm^(3)体积对应剂量D_(1 cm^(3))=454.6 cGy。添加1 cm虚拟防护膜后,下颌皮肤剂量D_(1 cm^(3))降低到113.6 cGy,几乎与无磁场情况下相当(D_(1 cm^(3))=92.5 cGy)。结论磁场下的ESE会改变射野外的剂量分布,导致局部剂量升高。在喉癌病例中ESE不明显,但对于乳腺癌病例,ESE可延伸到下颌位置,外加防护膜对ESE具有较好的屏蔽效果。

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 D_(1 cm^(3))454.6 cGy.After the application of 1 cm bolus,the corresponding skin dose of the chin D_(1 cm^(3)) reduced to as low as 113.6 cGy,which is almost equivalent to that in the absence of a magnetic field(D_(1 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.