Abstract PS2-01-07: A Multicenter Retrospective Observational Study on the Safety and Timing of Radiotherapy After Oncoplastic Breast-Conserving Surgery

Abstract Background: Whole-breast irradiation after breast-conserving surgery (BCS) is a standard treatment for breast cancer. Boost radiation is recommended in cases with positive surgical margins, and recently, its application has expanded to younger patients with negative margins to reduce local recurrence. At our center and affiliated institutions in Mie Prefecture, oncoplastic breast-conserving surgery (OPBCS) has been actively implemented over the long term, aiming to achieve both oncological safety and favorable cosmetic outcomes. OPBCS includes volume displacement techniques using intra-breast tissue and volume replacement techniques using autologous tissue, which may obscure the tumor bed and margins due to extensive tissue rearrangement, raising concerns regarding radiotherapy targeting accuracy. Methods: This multicenter retrospective observational study included 4215 patients who underwent primary breast cancer surgery from January 2017 to December 2023. The study aimed to evaluate the safety and treatment outcomes of postoperative radiotherapy in patients who received OPBCS. The primary endpoint was ipsilateral breast tumor recurrence (IBTR), while secondary endpoints were overall survival (OS), event-free survival (EFS), the interval from surgery to initiation of radiotherapy, and the incidence of radiotherapy-related complications. Results: The median observation period was 55 months in the OPBCS group and 54 months in the non-OPBCS group. Patient characteristics revealed a higher rate of re-excision in the OPBCS group. IBTR showed no significant difference between the groups (p = 1.0; OR, 0.97; 95% CI, 0.16-4.59). OS was significantly better in the OPBCS group (p = 0.0284; OR, 0.22; 95% CI, 0.025-0.95), while EFS was comparable between the groups. The median interval from surgery to radiotherapy initiation was longer in the OPBCS group (48 days) than in the non-OPBCS group (40 days) (p < 0.001); however, no significant difference was observed for delays beyond 16 weeks. Multivariate analysis identified OPBCS as an independent factor associated with delayed initiation of radiotherapy. The OPBCS group also exhibited a significantly higher incidence of postoperative complications (p < 0.001; OR, 2.24; 95% CI, 1.52-3.31), which was considered a major contributor to the delay. Notably, the incidence of radiotherapy-related complications, excluding grade 1 dermatitis, did not differ between groups (p = 1.0). Discussion: Postoperative complications likely contributed to delayed radiotherapy in the OPBCS group. Surgeons may also intentionally delay radiotherapy initiation to preserve vascular supply in rearranged tissue. However, with a median delay of only 48 days (∼7 weeks), and no difference in IBTR rates, OPBCS does not appear to negatively impact oncologic outcomes and should not be considered a barrier to its broader adoption. Citation Format: R. Yamakado, A. Noro, R. Ito, N. Imai, M. Shibusawa, M. Kimoto, M. Yoshikawa, K. Nakamura, E. Hatakawa, S. Watanabe, M. Yoshida, T. Mitsui, E. Matsumoto, C. Mizumoto, N. Hanamura, T. Nishimine, Y. Kashikura, M. Yamashita, R. KoJima, Y. Nomoto, T. Ogawa, K. Kawaguchi. A Multicenter Retrospective Observational Study on the Safety and Timing of Radiotherapy After Oncoplastic Breast-Conserving Surgery [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-01-07.