PURPOSE: Multicatheter interstitial brachytherapy (MIB) is an established technique of partial breast irradiation (PBI). However, postoperative catheter implant is an invasive, inconvenient, and skillful procedure. In this study, local control and cosmesis of perioperative interstitial brachytherapy (PIB) by intraoperative catheter implant were evaluated by comparing with those of whole breast irradiation (WBI) following breast-conserving surgery (BCS). METHODS: Between October 2007 and August 2019, consequent patients who underwent either PIB or WBI following BCS were included. In general, additional indications for PIB to WBI included age ≥ 40 years, tumor ≤ 3 cm, and pN0 or pNmi. WBI was initiated with a total dose of 50 Gy in 25 fractions, whereas PBI was delivered immediately following BCS at 32 Gy in eight fractions. Local recurrence (LR) was the primary endpoint, and subjective and objective cosmetic outcomes at 5 years using the Harvard Cosmesis Scale and BCCT.core software, respectively, were the secondary endpoints. RESULTS: During the 10-year follow-up, the crude rate of LR was 3.8% (95% confidence interval [CI] 2.3-5.4) in 577 patients receiving PIB and 3.3% (95% CI 1.1-5.6) in 241 patients receiving WBI (P = 0.73). The 5- and 10-year LR-free survival rates in the PBI and WBI cohorts were 97.9% versus 97.9% and 95.4% versus 96.8%, respectively (P = 0.64). Multivariate analysis selected age <
50 years as an independent risk factor for LR. Excellent or good cosmesis in the PBI and WBI cohorts assessed by subjective and objective measures was 89.5% versus 84.5% (P = 0.26) and 83.7% versus 68.1% (P <
0.005), respectively. CONCLUSIONS: Although this study was based on a retrospective chart review in a single institution, the largest series of data with a long follow-up suggested that acceptable local tumor control and cosmesis were achieved following PIB compared with WBI.