Out-of-field doses from pediatric craniospinal irradiations using 3D-CRT, IMRT, helical tomotherapy and electron-based therapy

Medulloblastoma treatment involves irradiation of the entire central nervous system, i.e craniospinal irradiation (CSI). This is associated with significant exposure of large volumes of healthy tissuewith a growing concern regarding treatment associated side effects. The current study compares out-of-field organ doses in children receiving CSI with three-dimensional conformalradiotherapy (3D-CRT), intensity modulated radiotherapy (IMRT), helical tomotherapy (HT) and an electron-based technique, including as well radiation doses resulting from imaging performed duringtreatment. An extensive phantom study is performed, using an anthropomorphic phantom corresponding to a 5-year old child, in which organ absorbed doses are measured using thermoluminescent detectors (TLDs).Additionally the study evaluates and explores tools for calculating out-of-field patient doses using the treatment planning system (TPS) and analytical models.In our study, 3D-CRT resulted in very high doses to a limited number of organs while it was able to spare organs such as the lungs and breast when compared to IMRT and HT. Both IMRT and HT spread the doseover more organs and were able to spare heart, thyroid, bladder, uterus and testes when compared to 3D-CRT. The electron-based technique considerably decreased the out-of-field doses in deep seated organs but cannotavoid nearby out-of-field organs such as lungs, ribs, adrenals, kidneys and uterus. Daily imaging dose is small compared to the treatment dose burden. TPS error for out-of-field doses was most pronounced for organsfurther away from the target nevertheless no systematic underestimation was observed for any of the studied TPS systems. Finally analytical modeling was most optimal for 3D-CRT although the number of organs thatcan be modeled was limited.To conclude none of the techniques studied was able to spare doses in all organs. Nevertheless the electron based technique showed most promising for out-of-field organ dose reduction during CSI when compared tophoton techniques.