Five-Fraction Prone Accelerated Partial Breast Irradiation: Long-Term Oncologic, Dosimetric, and Cosmetic Outcome

Abstract Purpose/Objective(s) Randomized data support accelerated partial breast irradiation (APBI) for early-stage breast cancer with variable techniques and cosmesis outcomes. We have treated patients with 5-fraction prone external beam APBI for over a decade and herein report acute and late outcomes. Materials/Methods Patients receiving APBI 600cGy x 5 between 2010-2019 were included. APBI was primarily delivered prone, with opposed tangents targeting the tumor bed expanded by 1.5cm, (cropped 6mm from skin). Ipsilateral breast was constrained to V50% 6 months follow-up (FU). Results Of 345 patients meeting criteria, 14 were excluded due to APBI given for ipsilateral breast tumor recurrence (IBTR; n=3), palliation (n=9), and incomplete RT course (n=2). Of the 331 remaining, median age was 70, 7.2% had ductal carcinoma in situ, 94.3% were treated prone, with 32% treated every other day and 68% on consecutive days. Mean heart dose was 23.8cGy for left-sided and 12.7cGy for right-sided cancers. Ipsilateral lung V30% was 0.4%. At 5-year median FU, there were 7 (2.1%) IBTR, 9 (2.7%) contralateral recurrences, and 1 (0.3%) distant metastasis. Five-year local-recurrence-free, disease-free, and overall survival was 99.5%, 96.7%, and 98.1%, respectively. When comparing patients with IBTR vs. without, a higher proportion did not receive endocrine therapy (71.4% vs. 26.2%, p = 0.018). Rates of acute grade 1-2 dermatitis, fatigue and pain were 35.4%, 21.8% and 9.4%, respectively, with no grade 3 toxicity. The rate of good-excellent physician- and patient-rated cosmesis (n=199, median FU 2.8 years) was 92.5% and 89.4%, respectively. Patients experienced low rates of telangiectasia, fibrosis, and retraction/atrophy. Conclusion We report excellent dosimetric, oncologic, cosmetic, and late toxicity outcomes for patients treated with five-fraction APBI. To our knowledge this is the largest series of women treated with prone APBI.