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Abstract P3-12-16: Five year outcomes of a prospective study of proton radiotherapy for breast cancer regional nodal irradiation

Purpose: Through improved nodal coverage and decreased dose to the heart and lung, proton therapy (PT) may improve the therapeutic ratio for treatment of breast cancer requiring regional nodal irradiation (RNI). The purpose of this study is to report 5 year disease control and toxicity. Methods: From May 2012 to February 2014, 18 women (stage IIA-IIIB) prospectively enrolled on a pilot study. Median age was 52 years (range, 42-73), with equal division between breast-conserving therapy (BCT) and mastectomy and right and left-sided cancers. Median number of positive nodes among the 16 node-positive patients was 2 (range, 1-14). Five patients had ≥ 10 nodes positive on axillary dissection (N3a). Treatment targets (CTVs for breast/chest wall, supraclavicular, axillary, internal mammary nodes (IMNs)) and organs at risk were delineated on CT scans. Double scatter PT alone was used for 10 patients (9 post-mastectomy, 1 after BCT) and combined proton-photon in 8 (all BCT). Toxicity was prospectively recorded using CTCAE v4.0. Results: Median follow-up was 4 years (range, 0.3 – 6). 5 year overall survival was 82% and locoregional control was 100%. 5 year distant metastases-free survival was 82%. No grade 4+ toxicity developed. Four patients developed grade 3 cellulitis, which was the only grade 3 toxicity. One patient had a reconstructive failure associated with a post-surgical cellulitis. A right-sided patient was diagnosed with CHF within 2 months of completion of PT, following diagnosis of a pulmonary embolus 1 month prior. She had an elevated BNP (1101) prior to PT following adriamycin-based chemotherapy. Mean heart dose was 0 Gy and cardiac V5 0%. Atrial fibrillation developed in two patients. One patient developed grade 2 pneumonitis (she received concurrent chemotherapy). One patient, with a history of rib fractures prior to PT, developed an ipsilateral rib fracture 7 months after PT. Conclusion: In a population of women with locally advanced breast cancer, PT for RNI has proven feasible after either mastectomy (with or without reconstruction) or BCT with excellent locoregional control. PT allows for highly conformal radiation delivery without compromise of target coverage or excess exposure of normal tissue. Documentation of the ultimate benefits of PT in this population, freedom from radiation induced cardiac disease and breast cancer recurrence will require prospective study of a larger numbers of patients. Citation Format: Bradley JA, Dagan R, Liang X, Ho MW, Rutenberg M, Mailhot R, Morris C, Mendenhall NP. Five year outcomes of a prospective study of proton radiotherapy for breast cancer regional nodal irradiation [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-12-16.