Physical contributors to dose in patients with dual-port temporary tissue expanders treated post-mastectomy with 10 MV x-rays: a Monte Carlo study.

Objective. To evaluate how radiation interactions, influenced by a dual-port temporary tissue expander (TTE), impact dosimetry in post-mastectomy radiotherapy (PMRT) with 10 MV x-rays.Approach. The individual dose contributions from the radiation interaction processes within the patient, specifically the photoelectric effect, pair production, bremsstrahlung, and neutrons, were evaluated in a PMRT treatment involving the dual-port AlloX2 TTE using Monte Carlo simulations. The plan setup was two 10 MV tangential half-beam-blocked fields (40 Gy in fifteen fractions). Individual contributions of the different physical processes were computed using a dedicated physics list that allows to activate/deactivate each process. The yield of photoneutrons produced in TTE neodymium ports (ρ = 7.4 g/cm³) and their impact on equivalent neutron dose were computed using previously validated physics modules. The effect of the presence of the TTE was estimated by comparing results in plans with and without the TTE.Results. The presence of the TTE reduced the dose to the breast skin distal to the ports up to 19.3% of the prescribed dose. The contribution of the photoelectric effect and bremsstrahlung was confined to the metallic ports, accounting for 9% and 1% of the total dose. Pair production accounted for 20% of the dose deposited within the ports and contributed 2.2 Gy and 0.9 Gy to the maximum dose to the lung and heart, respectively. We found that no photoneutron was produced in the TTE, not having an effect on the equivalent neutron dose to the patient.Significance. This work extended the current knowledge on the impact of TTE on dose distributions, including neutron contamination, in PMRT treatments.