ESTRO 2024 - Abstract Book

S4627

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

ESTR0 2024

therapy. Nevertheless, it is crucial to highlight that proton-RT gives rise to neutron-dominated out-of-field radiation, which introduces a range of complex dosimetry challenges.

The aim of this experimental and computational work was to explore the potential benefits of a fetus’s equivalent dose at two conceptional pregnancy ages, i.e., 20 and 30 weeks after conception, undergoing breast PBS-proton-RT in comparison to the standard photon-RT treatment.

Material/Methods:

To assess fetal radiation exposure, we employed an anthropomorphic Alderson Radiation Therapy (ART) phantom modified with polymethyl methacrylate (PMMA) slices, replicating the abdominal region of a pregnant patient at 20 and 30 weeks of pregnancy. The VMAT photon plan was made in Eclipse v16.1 and delivered on a Varian TrueBeam STX, while the proton plan was generated in RayStation 12A for a Mevion S250i Hyperscan system. Fetus radiation dose was measured at 7 different locations (see figure 1). Imaging (CT, kV-kV and CBCT) and the photon plan dosimetry was carried out using a Farmer ionization chamber 30010 (PTW, Freiburg, Germany) calibrated in terms of kerma in air at 100kV, which is approximately the average x-ray photon spectra that reach the insert locations. Neutron contribution was determined using two types of bubble detectors (BD). BD for Personal Neutron Dosimetry (BD-PND) (Bubble Technology Industries, BTI, Chalk River, Canada) measures 50keV–15MeV neutrons while thermal BD (BDT) are mainly sensitive to thermal neutrons. These comprehensive methods allowed for a point-by point evaluation of fetus radiation exposure in proton and photon therapy scenarios, assuming a tissue weighting factor of 1 (whole-body irradiation). In this study, the worst-case scenario is indicated to be the top of the uterus (insert 1, figure 1). To complement the experimental data and to predict the out-of-field (neutron) dose to the fetus during PBS-PRT, we developed a TOPAS Monte Carlo (MC) model that encompassed the entire treatment room in the simulations.