Outcome and Late Toxicity of Breast Cancer Patients Treated with Post-Mastectomy Proton Therapy

Abstract

Post-operative whole-breast proton radiotherapy improves normal tissue sparing in comparison to photon-based approaches. Several studies have reported tolerable acute toxicity profile with limited follow up. We report an update of our institutional experience of PMRT with longer follow up. From December 2013 to February 2015, 42 patients who received mastectomy for non-metastatic breast cancer and no history of prior radiation to the ipsilateral breast or chest wall were treated with adjuvant chest wall and regional nodal proton therapy at a single proton center. 3D conformal uniform scanning with en face matching fields was used to treat all patients. Patient characteristics, early and late toxicities, and failure rates were obtained through chart review. Toxicity was assessed using CTCAE v4.0. Digital photographs were taken at baseline, weekly on treatment visits, and at each follow-up visit to document skin toxicity. The median follow-up among patients was 26 months (range 1 to 43 months). Median age of patients was 49 (range 21 to 86). Twenty-six patients (61.9%) had either implant (25 patients) or autologous reconstruction (1 patient) following mastectomy. All patients received radiation to the reconstructed chest wall and regional lymph nodes. The internal mammary chain was included in the radiation field in 33 patients (78.6%). Median dose of radiation administered was 50 RBE weighted dose Gy (RBE) (range 45 to 61.2 Gy(RBE) ). The median mean heart dose was 0.73 Gy (RBE) and median ipsilateral lung V20 was 16.1% and lung V5 was 33.7%. At 3-year follow-up, there was one local failure in the chest wall, zero regional nodal failures, and five distant failures. One patient died of metastatic disease since the completion of treatment. The 3-year rates of locoregional disease-free survival was 93.3%, metastasis-free survival was 85.0%, and overall survival was 96.9%. Fifteen patients (34.8%) developed grade 1 late hyperpigmentation, telangiectasia, or fibrosis. There was no late grade 2 skin or soft tissue toxicity. Five patients (18.5%) had reconstruction complications requiring revision/replacement but only one patient was left without successful reconstruction at the time of last followup. One patient (2%) with a history of prior contralateral chest wall radiation and underlying lung disease developed grade 3 radiation pneumonitis one year after treatment but made a full recovery with medical therapy. There was no observed cardiac toxicity with no cardiac events or cardiac deaths. Grade 1 lymphedema was observed in six patients, all of whom have had axillary lymph node dissection. With more than two years of median follow up, post-mastectomy proton radiation has excellent locoregional control rates and favorable toxicity profile.