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

Abstract

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.

Keywords: Monte Carlo; photoneutrons; post-mastectomy radiotherapy; temporary tissue expanders.

Figure 1.

Contribution to the total dose in water from the photoelectric, pair production, and scattering computed by TOPAS and derived from theoretical mass attenuation coefficients (Grodstein, 1957) as a function of the x-ray energy.

Figure 2.

(a) Dose distribution in the treatment plan considering the metallic ports. (b) Geometry of AlloX2 ports (Ramos-Méndez et al 2023). Profiles calculated along the lines shown in (a) for the plans with (w ports) and without (w/o ports) ports at the skin on the (c) left and (d) right sides of the patient breast, and within the TTE both in (e) the beam direction and (f) perpendicular to the beam. In all panels, the right y-axis corresponds to the relative dose difference in these two scenarios, defined as 100 × (w port - wo port)/w port (%). The position and shadow of the ports in the profiles are represented by a yellow shaded region.

Figure 3.

(a) Dose volume histogram of relevant patient structures in the plans with (w ports) and without (w/o ports) metallic ports. (b) Relative difference in the mean and maximum (D_2cc) dose to relevant patient structures between the plans with and without (reference scenario) metallic ports.

Figure 4.

Dose distributions produced by (a) pair production, (b) photoelectric, and (c) Bremsstrahlung processes. The metallic ports are displayed in brown color, the TTE balloon in yellow, the breast skin in blue, the bolus in green, and the chest wall tissue in purple. Doses correspond to doses per fraction (40 Gy in 15 fraction treatment).