Postmastectomy Intensity Modulated Proton Therapy: 5-Year Oncologic and Patient- Reported Outcomes

Purpose To report oncologic, physician-assessed, and patient-reported outcomes (PROs) for a group of women homogeneously treated with modern, skin-sparing multifield optimized pencil-beam scanning proton (intensity modulated proton therapy [IMPT]) postmastectomy radiation therapy (PMRT).Methods and Materials We reviewed consecutive patients who received unilateral, curative-intent, conventionally fractionated IMPT PMRT between 2015 and 2019. Strict constraints were applied to limit the dose to the skin and other organs at risk. Five-year oncologic outcomes were analyzed. Patient-reported outcomes were evaluated as part of a prospective registry at baseline, completion of PMRT, and 3 and 12 months after PMRT.Results A total of 127 patients were included. One hundred nine (86%) received chemotherapy, among whom 82 (65%) received neoadjuvant chemotherapy. The median follow-up was 4.1 years. Five-year locoregional control was 98.4% (95% CI, 93.6-99.6), and overall survival was 87.9% (95% CI, 78.7-96.5). Acute grade 2 and 3 dermatitis was seen in 45% and 4% of patients, respectively. Three patients (2%) experienced acute grade 3 infection, all of whom had breast reconstruction. Three late grade 3 adverse events occurred: morphea (n = 1), infection (n = 1), and seroma (n = 1). There were no cardiac or pulmonary adverse events. Among the 73 patients at risk for PMRT-associated reconstruction complications, 7 (10%) experienced reconstruction failure. Ninety-five patients (75%) enrolled in the prospective PRO registry. The only metrics to increase by >1 point were skin color (mean change: 5) and itchiness (2) at treatment completion and tightness/pulling/stretching (2) and skin color (2) at 12 months. There was no significant change in the following PROs: bleeding/leaking fluid, blistering, telangiectasia, lifting, arm extension, or bending/straightening the arm.Conclusions With strict dose constraints to skin and organs at risk, postmastectomy IMPT was associated with excellent oncologic outcomes and PROs. Rates of skin, chest wall, and reconstruction complications compared favorably to previous proton and photon series. Postmastectomy IMPT warrants further investigation in a multi-institutional setting with careful attention to planning techniques.