Abstract PS5-05-22: A Multicenter Retrospective Cohrt Study on Long-Term Outcomes after Partial Mastectomy in Japan

Abstract Background: With recent advances in breast cancer treatment, changes in local recurrence and survival following breast-conserving surgery (BCS) are also anticipated over time. Techniques for BCS vary across countries, and anatomical differences among ethnic groups may influence patterns of local recurrence risk. Objective of this study: To investigate the recent rates of local recurrence and prognosis following BCS in Japan using multi-institutional real-world data, with a particular focus on identifying trends and risk factors for local recurrence through comparison with historical data. Methods: This multicenter retrospective cohort study included 8,897 breast cancer patients who underwent BCS between January 2014 and December 2018 at 25 participating institutions across Japan. Data on patient demographics, surgical details, pathological factors including margin status, adjuvant therapy, and outcomes were collected. The primary endpoint was local recurrence rate. Secondary endpoints included distant recurrence rate, overall mortality, and breast cancer-specific mortality. Local recurrence rate (LRR), disease-free survival (DFS) and overall survival (OS) were estimated by Kaplan-Meier analysis; Results: Median follow-up was 6.6 years. In the overall population (N=8,897), Clinical stages at diagnosis were Stage 0 (n=918:10.3%), Stage I (n=4,539:51.0%), Stage II (n=2,566: 28.8%), and Stage III (n=360:4.0%). 5 year LRR, DFS and OS were 2.0%(1.7-2.3: 95%Cl), 94.1%(93.6-94.6: 95%Cl) and 97.8%(97.4-98.1: 95%Cl), respectively. 10 year LRR, DFS and OS were 4.6%(3.6-5.5: 95%Cl), 86.2%(84.1-88.3: 95%Cl) and 94.1%(93.1-95.1: 95%Cl), respectively. Potential risk factors for local recurrence are younger age of onset, tumor size, high Ki-67 , LV invasion, lack of adjuvant therapy (Table 1). Adjuvant systemic treatment was suggested to contribute to a reduced risk of local recurrence in all subtypes. Conclusions: Our recent data suggest that outcomes of BCS in Japan have improved compared to the past. As a growing number of new systemic therapies continue to be introduced each year, it is crucial to incorporate contemporary data—rather than relying solely on historical studies—when considering optimal surgical strategies. Citation Format: G. Kutomi, K. Kida, Y. Kajiura, K. Narui, K. Kataoka, M. Taguri, K. Harada, H. Shima, R. Nakamura, T. Ohnishi, T. Nakayama, M. Nagahashi, T. Sankai, K. Orimoto, Y. Horimoto, T. Ishikawa. A Multicenter Retrospective Cohrt Study on Long-Term Outcomes after Partial Mastectomy in Japan 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 PS5-05-22.