Monte Carlo study of out‐of‐field exposure in carbon‐ion radiotherapy: Organ doses in pediatric brain tumor treatment

摘要

Purpose To estimate out‐of‐field doses during carbon‐ion radiotherapy (CIRT) for pediatric cerebellar ependymoma. Methods Given that the out‐of‐field dose of CIRT depends on beam parameters, we set them for treatment of typical pediatric cerebellar ependymoma based on a previous study. The out‐of‐field dose during CIRT for pediatric cerebellar ependymoma was then estimated using the Particle and Heavy‐Ion Transport code System with Monte Carlo simulations and a computational phantom developed at the University of Florida. From the simulation results, out‐of‐field doses at dose equivalents of passive beam and active scanning beam CIRT were calculated and compared to the secondary neutron‐equivalent dose of passive beam CIRT and proton therapy. Results The out‐of‐field dose equivalent decreases from 1.45?mSv/Gy (relative biological effectiveness — RBE) at the thyroid to 0.06?mSv/Gy (RBE) at the bladder, verifying decay as the distance from the treatment target increases. The out‐of‐field neutron‐equivalent dose in organs per prescribed dose for passive beam CIRT is lower than that for passive beam proton therapy. Moreover, the out‐of‐field organ dose equivalent per prescribed dose for the active scanning beam CIRT is lower than that for the passive beam CIRT. Conclusions Active scanning beam CIRT is promising for pediatric cerebellar ependymoma regarding out‐of‐field exposure, outperforming the comparison radiotherapy modalities.