Near-Surface Dose Avoidance in Patients with Implant-Based Reconstruction Receiving Post-Mastectomy Proton Irradiation

Abstract

Dose to the near-surface region may contribute to toxic effects and breast reconstruction complications in patients receiving post-mastectomy irradiation. Here we report the acute and chronic toxic effects in patients with implant-based reconstruction receiving post-mastectomy proton irradiation with prospective near-surface dose avoidance. We reviewed the charts of patients with breast cancer who underwent a mastectomy with implant-based reconstruction and a first course of pencil beam scanning proton irradiation at a single institution from 2017-2022. Patients were evaluated weekly on-treatment, 3- and 12-months post-completion, and then annually. Baseline demographics, treatment details, acute and chronic toxicities (CTCAE v.4), unplanned reconstructive surgery, and implant failure were recorded. Inverse probability weighting was used to generate a balanced cohort receiving post-mastectomy photon irradiation. Logistic regression was used for comparison. Sixteen patients with implant reconstruction at the time of treatment completed proton irradiation with a median follow up of 1.6 years. Nine (56%) underwent direct-to-permanent implant reconstruction and seven (44%) underwent two-stage reconstruction via tissue expander; of these, five (71%) completed permanent implant exchange prior to irradiation. Implant placement was pre-pectoral in twelve (75%). All patients received a dose of 50 Gy in 25 fractions; fifteen (94%) received a sequential boost to the mastectomy scar of a median 10 Gy. Fifteen (94%) used a near-surface contour (bound by the chest wall CTV, the skin surface, and the skin-3mm contour) for plan optimization to avoid dose to the near-surface region with a median value of the mean dose of 46.8 Gy. Acute grade > 2 radiation dermatitis was present in thirteen (81%) patients. Two (13%) experienced moist desquamation and ten (63%) experienced grade > 2 chest wall pain. Three (19%) underwent unplanned reconstructive surgery at a median time of 1.6 years post-treatment: all were capsulectomies due to capsular contracture. One (6%) patient experienced implant failure. A matched cohort of 59 patients who received photon irradiation was identified with a weighted median follow up of 2.5 years. The cohorts were similar with respect to rates of grade ≥2 radiation dermatitis, grade ≥2 chest wall pain, and unplanned reconstructive surgery. Proton irradiation resulted in statistically significant decreases in rates of moist desquamation (OR 0.01, 95% CI 0.0002-0.182) and implant failure (OR 0.01, 95% CI 0.0001-0.997). Post-mastectomy proton irradiation with prospective avoidance of the near-surface region in patients with implant-based reconstruction is feasible and well-tolerated; it results in low rates of unplanned reconstructive surgery and implant failure.