Abstract P3-07-07: 10-Year Local Recurrence Rates Following Selective Omission of Re-excision for Patients with Ductal Carcinoma in Situ and Margins

Abstract Background: Current guidelines recommend ≥2mm margins for ductal carcinoma in situ (DCIS) with or without microinvasion to optimize local control following breast conserving surgery (BCS). There is some evidence to suggest it is acceptable to omit re-excision for “close” margins (i.e. not on-ink but <2mm), particularly when radiation therapy (RT) is planned. This study evaluates 10-year local recurrence (LR) rates following selective omission of re-excision in patients with DCIS and margins <2mm. Methods: We conducted a retrospective population-based cohort study on patients undergoing BCS for DCIS with or without microinvasion from 2010-2014 in the province of Alberta. Final margin status was categorized as on-ink, <1mm, 1-1.9mm and ≥2mm. Given small sample size with on-ink margins, these were included with the <1mm group for analysis. As per provincial guidelines, use of RT boost was recommended for margins <2mm. The primary outcome was local recurrence of in situ or invasive disease in the ipsilateral breast. We generated Kaplan Meier curves and performed Cox proportional hazards analysis to evaluate the effect of margin status on LR. We used an interaction test to determine if the effect of margin status differed by RT use. Results: 468 patients underwent BCS for DCIS with a median age of 59 years (IQR: 50.5-66). The majority were unifocal (n=409, 87.4%) with pure DCIS without microinvasion (n=424, 90.6%), and 51.5% (n=241) had grade 3 disease. Hormone receptor testing was not routine, but among tested patients, 88.9% were estrogen receptor (ER)-positive. The re-excision rate was 23.9% (n=112). Final margin status was on-ink in 13 (2.8%) patients, <1mm in 39 (8.3%), 1-1.9mm in 37 (7.9%) and ≥2mm in 379 (81.0%). Among those 89 patients with margins <2mm, most (94.4%) had only a single margin location <2mm. In 36 (40.4%) patients, the only margin that was <2mm was in the anterior or posterior location, and thus re-excision may not have been feasible. The majority (n=390, 83.3%) had adjuvant RT, whereas adjuvant endocrine therapy (ET) use was infrequent (n=43, 9.2%). The only clinical characteristic that differed for patients with <2mm vs. ≥2mm margins was proportion of multifocality (20.2 vs. 10.8%, p=0.02). Otherwise, there was no significant differences in age, grade, pure DCIS vs. microinvasion, ER status, RT use (82.9% vs. 83.8%) or ET use (all p>0.05), including when those with only anterior/posterior margins <2mm were excluded from the <2mm group. Median follow-up was 10.9 years (IQR: 9.8-12.3). 10-year rates of LR by final margin width were as follows: 23.0% (95%CI: 13.1-38.6) for <1mm, 8.8% (95%CI: 2.9-24.9%) for 1-1.9mm and 5.9% (95%CI: 3.9-8.9%) for ≥2mm (log-rank p<0.001). In Cox proportional hazards analysis, margins <1mm were associated with significantly higher risk of LR compared to margins ≥2mm (HR 4.22, p<0.001), while margins 1-1.9mm were not (HR 1.35, p=0.62). Results were unchanged when adjusting for multifocality (adjusted HR for <1mm margins = 4.16, p<0.001; adjusted HR for 1-1.9mm margins = 1.25, p=0.72), and also when those with only anterior/posterior margins <2mm were excluded from the <2mm groups. There was no statistically significant interaction between margin status and RT use (p=0.52), though absolute LR rates were numerically higher in patients without RT: 18.9% (RT) vs. 46.4% (no RT) for <1mm, 6.7% (RT) vs. 25.0% (no RT) for 1-1.9mm and 6.0% (RT) vs. 5.5% (no RT) for ≥2mm. Conclusion: For patients undergoing BCS for DCIS with or without microinvasion, our findings support that re-excision can be selectively omitted for limited volume 1-1.9mm margins, as the 10-year LR rates are comparable to margins ≥2mm when followed by adjuvant RT. However, we advise caution against omission of re-excision for margins <1mm given significantly higher rates of LR regardless of RT use. Citation Format: Cecily Stockley, Shiva Bahmanyar, Yuan Xu, Jeffrey Cao, May Lynn Quan, Alison Laws. 10-Year Local Recurrence Rates Following Selective Omission of Re-excision for Patients with Ductal Carcinoma in Situ and Margins [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2024; 2024 Dec 10-13; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2025;31(12 Suppl):Abstract nr P3-07-07.

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