ESTRO 2024 - Abstract Book

S4608

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTR0 2024

In the water phantom, both the LET and Q eff had similar depth dependencies with a rapid increase around the Bragg Peak and reaching the largest values beyond. Assessed at a voxel level, LET and Q eff showed a close to linear relationship (r 2 >0.99). In the pediatric brain dose plan, spatial distributions of both LET d and Q eff,d showed that the highest values were located distal of the target (Fig. 1 a and b). The median LET d inside the 50Gy (RBE=1.1) isodose was 2.8 keV/µm in the brainstem and 3.1 keV/µm in the chiasm compared to 2.3 keV/µm in the CTV. The same tendency was observed for median Q eff,d inside the 50Gy RBE=1.1 isodose which was 37.3 in the brainstem and 39.4 in the chiasm compared to 29.7 in the CTV. In the distribution of the dose differences between the models (Fig. 1 (c-f)) similar patterns were observed, where higher dose differences were observed distal of the target compared to within the CTV. The median modelled RBE values within the 50Gy (RBE=1.1) isodose (Fig. 2) were consistently higher in the brainstem and chiasm than in the CTV, across all models. The absolute RBE values were seen to be highly dependent on the specific model used and less on whether based on LET d or Q eff,d . Both RBE models with linear relations predicted RBE>1.1 in OARs (Fig. 2), whereas models with non linear relationships predicted RBE<1.1. When compared to the McNamara et al model, there was greater consistency with the linear models than the non-linear ones.