Abstract PS2-01-22: Changes in reoperation rate after publication of guidelines defined positive margins for breast conserving surgery: Collaboration study of Japanese Breast Cancer Society

Abstract Background: In 2014, the SSO-ASTRO consensus guidelines redefined a positive margin requiring re-excision after breast-conserving surgery (BCS) as “tumor on ink.” The adoption of this standard has been reported internationally to significantly reduce the rate of surgical re-excision for positive margins. In Japan, this definition was explicitly incorporated into the 2018 Japanese Breast Cancer Society clinical practice guidelines. To clarify changes in re-excision rates before and after guideline publication in Japan, we conducted a multicenter collaborative study. Method 1: A nationwide questionnaire survey was distributed to representatives of 521 Japanese Breast Cancer Society-accredited institutions to assess current practices related to BCS, including imaging, surgical and pathological assessment, rates of positive margins, and re-excision. Method 2: Multicenter cohort study was performed using databases from seven participating institutions, examining trends in positive margin and re-excision rates among patients undergoing BCS from 2008 to 2022. Result 1: Questionnaire Survey The questionnaire survey collected responses from 262 out of 521 Japanese Breast Cancer Society-accredited institutions, representing a response rate of 55.7%. Most institutions (60%) performed between 100 and 300 breast surgeries annually, and the majority (64%) had one or two breast surgeons. A full-time pathologist was present in 89% of the institutions, and preoperative MRI was utilized in 97% of cases for disease extent assessment. Notably, the proportion of institutions reporting a positive margin rate of 0-10% increased from 47% to 63% around 2018, reflecting a shift in clinical practice following the introduction of new guidelines. However, only 18% of institutions noted a decrease in re-excision rates after a positive margin post-2018, while 77% reported no change. Despite this, the proportion of institutions with a re-excision rate of 0-10% has increased slightly from 45% to 49% since 2018, indicating that more facilities have decreased their reoperation rates. Result 2: Multicenter Cohort Survey The multicenter cohort survey analyzed data from 18,688 cases of breast-conserving surgery performed between 2008 and 2022 across seven participating institutions. Over this period, the positive margin rate showed a consistent decline, decreasing from 10.7% in 2008-2014 to 8.8% in 2015-2018, and further to 6.9% in 2019-2022. Similarly, the re-excision rate dropped from 7.6% to 6.6% and then to 4.9% across the same intervals. Multivariate analysis identified several significant predictors of re-excision: younger patient age, diagnosis of invasive lobular carcinoma, larger tumor size, and presence of lymph node metastasis. Additionally, surgeries performed before 2014 (OR 1.34, 95% CI 1.34-1.88, P<0.001) and before 2018 (OR 1.33, 95% CI 1.10-1.60, P=0.003) were associated with a higher likelihood of re-excision compared to those performed after 2019, underscoring the impact of guideline implementation on surgical practice. Conclusions: Our findings suggest considerable diversity in the approach to BCS margin assessment among Japanese breast surgeons. Following the publication of both international and domestic guidelines regarding positive margins, a significant reduction in re-excision rates after BCS was observed in Japan. This trend was gradual, corresponding to the 2014 and 2018 consensus statements, indicating that formal guideline publication has contributed to reducing re-excision rates after breast-conserving surgery. Citation Format: T. Sakai, M. Ishitobi, Y. Sagara, A. Yoshida, Y. Takahashi, T. Tsukioki, K. Takada, Y. Ono, Y. Kimura, T. Osako. Changes in reoperation rate after publication of guidelines defined positive margins for breast conserving surgery: Collaboration study of Japanese Breast Cancer Society [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 PS2-01-22.