A GEANT4 Monte Carlo simulation study for the computation of output factors and out-of-field dosimetry in breast IOERT.

The goal of breast Intraoperative Electron Radiation Therapy (IOERT) is to deliver a uniform single fraction dose of 10-25 Gy to a surgically exposed volume of tissue at risk (tumor bed) while minimizing exposure to surrounding healthy tissue, with the patient under anesthesia. Currently, all the necessary information for the geometric and physical treatment arrangement is collected during the commissioning phase, when a preliminary dosimetric characterization of the accelerator is performed for clinical use in accordance with international protocols. During this process, output factors (OFs)-correction factors that account for dose differences relative to the reference condition-are measured. Therefore, since treatment plans rely exclusively on the OF and experimental measurements can be affected by large errors (on the order of 30%), a tuned Monte Carlo model provides a valuable tool for treatment validation. It is particularly valuable for evaluating quantities that are difficult to measure directly, such as dose variations resulting from shielding disk misalignment (to protect underlying healthy tissue) or out-of-field exposure to organs at risk in complex clinical scenarios (e.g., patients with implantable electronic devices or during pregnancy).