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PURPOSE/OBJECTIVE To evaluate feasibility and early clinical outcomes of a single fraction of multi-catheter interstitial high-dose rate brachytherapy for accelerated partial breast irradiation (APBI) in the elderly. MATERIAL/METHODS From November 2012 to September 2014, 26 patients (≥70) with early breast cancer were enrolled in a prospective phase II trial (NCT01727011). After lumpectomy, intra-operative catheter implant was performed for post-operative APBI (single fraction 16 Gy). Surveillance was achieved at 1, 3 and 6 months after APBI, then twice a year. Acute toxicity was investigated. Early cosmetic outcome was analyzed (patient, radiation oncologist, 2 observers). Local and regional relapse-free survival, cancer specific survival and overall survival were analyzed. RESULTS Median age was 77 years [69-89]. Median CTV was 41 cc [22-95]. Acute toxicity was observed in 18 pts (70%) with a total of 44 events: G1: 75.7%; G2: 22.8%; G3: 4.5%. Breast fibrosis (31.8%), puncture site inflammation (13.6%) and skin hyperpigmentation (11.4%) were the most frequent side effects. Cosmetic evaluation at 6 months was excellent/good in 88%, 92%, 85% and 88% for patient, radiation oncologist, observer #1 and #2 respectively. With a median follow-up of 37.2 months [35.6-42.3], side effects were G1: 4 pts (15%) and G2: 1 pt (4%). Three-year Local and regional relapse-free survival, cancer specific survival and overall survival rates were 100%, 100%, 100% and 95.2% respectively. CONCLUSIONS For elderly early breast cancer, a post-operative multi-catheter interstitial high-dose rate brachytherapy single dose (16 Gy) appears feasible. Acute toxicity is acceptable as well as early cosmetic outcome. Oncologic outcome seems encouraging and allows going forward with new clinical trials focusing on single fraction APBI.