Validation of Monte Carlo simulations with measurement data for fetal dose assessment in proton therapy for breast cancer

<i>Objective</i>. This study aims to validate a Monte Carlo model for fetal dose estimation in the complex secondary field of pencil beam scanning (PBS) proton therapy for breast cancer, one of the most common cancers occurring during pregnancy.<i>Approach</i>. A TOPAS/GEANT4 Monte Carlo simulation environment based on an IBA ProteusOne beam model was developed, reflecting the experimental setup of a breast irradiation using a pregnant anthropomorphic phantom. Experimental doses were acquired with thermoluminescent dosimeters for protons and gammas, and bubble detectors (BDs) for neutrons. Simulated doses were scored at the same positions using three hadronic physics models: BIC_HP, BIC_AllHP, and BERT_HP. Experimental doses were corrected for detector energy response using simulation-derived energy spectra.<i>Main results</i>. Agreement between simulation and measurement varied depending on hadronic model, scoring volume size, and correcting for BD energy response. Two physics models conservatively estimated fetal neutron doses within the combined measurement and simulation uncertainties, with BIC_AllHP showing the closest agreement. Combined proton and gamma doses were accurately reproduced for all models for inserts 2-6, but were underestimated for insert 1, likely due to dose gradients and modeling limitations near the treatment field. The total simulated fetal dose equivalent at the fundus height was 5.17 mSv. This value is substantially lower than doses reported for photon-based therapies, remains well below the 100 mSv threshold for deterministic effects, and is within range of the public 1 mSv dose limit.<i>Significance</i>. The results demonstrate that, within the tested experimental framework, the TOPAS/GEANT4 Monte Carlo model is suitable for fetal dose estimation in PBS proton therapy for breast cancer. In this setting, calculated fetal doses were substantially lower than those reported for photon-based radiotherapy. The validated framework provides a practical basis for treatment planning optimization and risk assessment and can be extended to other clinical scenarios following similar validation.