Changes in re-excision rates after publication of guidelines defining positive margins for breast-conserving surgery: a collaborative study of the Japanese breast cancer society.

BACKGROUND: The 2014 SSO-ASTRO consensus guidelines redefined a positive margin requiring re-excision after breast-conserving surgery (BCS) as “tumor on ink,” with several studies internationally demonstrating decreased re-excision rates thereafter. In Japan, this definition has been explicitly incorporated into the 2018 Japanese Breast Cancer Society (JBCS) guidelines; however, its impact on national clinical practice remains unclear. METHODS: We conducted a nationwide questionnaire survey of 521 JBCS-accredited institutions regarding BCS practices, imaging, surgery, margin assessment, and re-excision rates before and after 2018. In parallel, a multicenter retrospective cohort study from seven institutions analyzed positive margins and re-excision trends between 2008 and 2022. RESULTS: Overall, 262 institutions (response rate 55.7%) participated in the survey. Institutions reporting a positive margin rate of 0–10% increased from 47 to 63% in 2018. Institutions with a re-excision rate of 0–10% increased from 45 to 49%. In a cohort study of 18,688 BCS cases, positive margin rates fell from 11.2% (2008–14) to10.1% (2015–18) and 7.9% (2019–22), and re-excision rates from 7.6% to 6.6% and 4.9%. Multivariate analysis identified younger age, invasive lobular carcinoma, larger tumor size, lymph node positivity, and earlier surgical period as significant predictors of re-excision. CONCLUSIONS: This nationwide study demonstrates that the implementation of international and domestic guidelines has contributed to a significant yet gradual reduction in re-excision rates following BCS in Japan. These improvements reflect Japan's already-low positive margin and re-excision rates, which have been maintained through rigorous comprehensive margin assessment methodologies.