磁共振加速器计划系统测量仿真人模体照射野外剂量准确性评估

Evaluation of out-of-field dose calculation accuracy of Monaco treatment planning system for a 1.5T MR-Linac

目的评估磁共振加速器(MR-Linac)Monaco计划系统(TPS)的快速蒙特卡罗模拟算法对照射野外剂量计算的准确性。方法使用光释光剂量计分别测量仿真人模体在头、胸、腹盆3个部位分别接受相同处方剂量规则野照射后,体表20个、体内5个检查点的辐射剂量。每个部位计划重复照射5次,测量结果取平均值,并与Monaco TPS的计算剂量值进行比较。拟合照射野外辐射剂量与靶区等中心距离反比函数,并描绘TPS计算误差随靶区等中心距离变化的分布特征。结果25个检查点中实测最高为离胸部靶区距离最近的甲状腺体表剂量,约为处方剂量的1.45%。不同部位照射过程中测得模体体表及体内辐射剂量在距靶区中心10~90 cm内遵循指数函数递减关系。与光释光剂量计测量数据相比,Monaco TPS在照射野外2%~5%剂量线范围内低估检查点剂量0~35%,在0~2%剂量线范围内高估检查点剂量0~270%。结论Monaco TPS快速蒙特卡罗算法计算MR-Linac照射野外剂量在5%以下剂量线范围内与实测剂量存在偏差,且随照射野外距离增加而增大。

Objective To evaluate the accuracy of the out-of-field dose calculation for a 1.5 T MR-Linac using a Monte Carlo fast dose algorithm in the Monaco treatment planning system.Methods Using optically stimulated luminescence dosimeters,25 checkpoints were measured on surface(20 checkpoints)and internal organs(5 checkpoints)of the anthropomorphic phantom for square radiation fields,with similar prescribed doses for the head,thoracic,and pelvic sites.Each measurement was repeatedly radiated five times,and the average was compared with the calculated dose values of the Monaco treatment planning system.The inverse of the function that describes the out-of-field dose with respect to the isocentric distance from the radiation field was fitted.In addition,the relationship between the dose evaluation error of the treatment planning system and the variation in the isocentric distance from the radiation field was described.Results Among the 25 checkpoints,the thyroid gland received the highest dose.However,the measured dose did not exceed1.45%of the prescribed dose.The dose,measured both on the surface and internal checkpoints of the anthropomorphic phantom for plans with different sites,followed an exponential function of dose reduction over a distance of 10-90 cm from the isocenter of the plan.Compared with the data measured by optically stimulated luminescence dosimeters,the results of the Monte Carlo dose calculation algorithm in the Monaco treatment planning system underestimated the out-offield checkpoints dose by 0-35%in an isodose area between 2%and 5%and overestimated the out-of-field checkpoint dose by 0-270%in an isodose area between 0%and 2%.Conclusion In an isodose area below 5%,the out-of-field dose evaluated through the fast Monte Carlo algorithm in the Monaco treatment planning system deviates from the measured dose,and the deviation increases as the distance from the target volume increases.