Protons Versus Photons Postmastectomy Radiation Therapy Effects on Breast Reconstruction Outcomes and Dosimetry Analysis.

[PURPOSE] To compare the impact of proton versus photon postmastectomy radiation therapy (PMRT) on implant-related complications.

[METHODS AND MATERIALS] The records of patients with breast cancer treated with mastectomy and expander and/or implant reconstruction followed by PMRT at our institution between 2011 and 2022 were reviewed. Patients were divided into 2 groups by treatment modality: proton and photon groups. All identified patients in the proton group were treated using conventional fractionation, and radiobiological effectiveness (RBE) was scaled to 1.1. Recorded complications included infection/skin necrosis requiring operative debridement, capsular contracture necessitating capsulotomy, absolute reconstruction failure implying complete loss of reconstruction, and overall reconstruction failure defined as multiple revisions leading to replacement of the implant or salvage autologous reconstruction. Subgroup analysis for patients in the proton group explored the correlation between dosimetric parameters and complications. Logistic regression and Cox proportional hazards regression models were used.

[RESULTS] A total of 203 patients with an overall median follow-up of 4.7 years were identified. Among those 203 patients, 50 patients (25%) received proton PMRT, while 153 patients (75%) received photon PMRT. The complication rates for proton versus photon therapies were infection/necrosis (20% vs 13%; OR, 1.6; P = .2), capsular contracture (30% vs 10%; OR, 3.9; P < .001), absolute reconstruction failure (16% vs 12%; OR, 1.4; P = .4), and overall reconstruction failure (56% vs 36%; OR, 2.2; P = .01). Sensitivity analyses and time-to-event models yielded similar results. The median (Dmean) for clinical target volume, implant, and skin was 50.6, 50.8, and 6.7 Gy (RBE), respectively. The median hot spot (D1cc) for clinical target volume, implant, and skin was 52.8, 52.7, and 49.8 Gy (RBE), respectively. None of these parameters were significantly correlated with complications. The 5-year local failure cumulative incidence was 0% versus 4% (P = .13) for proton and photon cohorts, respectively.

[CONCLUSIONS] Proton PMRT was associated with higher rates of implant capsular contracture and reconstruction failures than photon PMRT with comparable local control. No dosimetric parameter correlated with reconstruction complications.