利用蒙特卡罗模拟评估空气间隙对点扫描质子治疗的剂量影响

Evaluating the effects of air gap on dose of spot scanning proton therapy using Monte Carlo simulation

目的:利用蒙特卡罗模拟探究空气间隙对点扫描质子治疗的剂量影响。方法:利用通用蒙特卡罗程序Geant4平台构建使用射程移位器的治疗头末端的点扫描质子束流模型,并进行验证。模拟计算不同能量、不同射程移位器、不同束斑尺寸、不同束斑数目在不同空气间隙条件下的质子束流在水模体中的剂量沉积,并通过获得的积分深度剂量生成剂量修正因子对剂量的差异进行比较。结果:不同空气间隙会造成剂量损失,随空气间隙增大而增大,随水模体中深度增加而减小。对于能量更高的射束和使用水等效厚度更薄的射程移位器,剂量损失越大。束斑尺寸改变和束斑数目增加较少时造成的剂量损失与同条件下单一束流无显著差别。结论:当使用射程移位器、肿瘤位置较浅、空气间隙较大时,建议建立剂量修正因子数据库应用于治疗计划系统对剂量进行修正。

Objective To explore the effects of air gap on the dose of spot scanning proton therapy by Monte Carlo simulation.Methods Monte Carlo program Geant4 platform was used to establish aspot scanning proton model at the end of beam nozzle when using range shifter,and the model was verified.In the cases of phantom beams with different energies,range shifters,spot sizes,spot numbers under different air gaps,the dose deposition in the water model was calculated,and the dose difference was analyzed using dose correction factor generated by integrated depth dose.Results Different air gaps resulted in dose loss,and the dose loss increased as the increase of air gap and decreased as increasing depth in the water phantom.For higher beam energy and the range shifter with a thinner water equivalent thickness,the dose loss was greater.The dose loss caused by the change of spot size and the small increase of spot number did not differ significantly with a single beam in the same case.Conclusion When the range shifter is used,the tumor is in the superficial,and the air gap is large,it is recommended to establish a dose correction factor database and apply it to the treatment planning system to correct the dose.