Lumpectomy Margins and Local Recurrence in DCIS: Results From the NRG Oncology/NSABP B-35 Randomized Clinical Trial.

Importance: The NRG Oncology research organization and NSABP B-35 randomized clinical trial prospectively collected margin width data on postmenopausal women with hormone receptor (HR)-positive ductal carcinoma in situ (DCIS) who underwent lumpectomy, whole-breast irradiation (WBI), and randomly assigned adjuvant anastrozole or tamoxifen therapy. This permitted analysis of outcomes per margin width. Objective: To analyze the effect of margin width on ipsilateral breast tumor recurrence (IBTR). Design, Setting, and Participants: NSABP B-35 was a phase 3, double-blind, randomized clinical trial in which patients were randomized to either 5 years of tamoxifen or anastrozole. Postmenopausal women with HR-positive DCIS and tumor-free margins were eligible. Enrollment was from January 6, 2003, to June 15, 2006, in academic and community hospital members of the NSABP. Study data were analyzed from July 2024 to April 2025. Interventions: There were no specific interventions based on lumpectomy margin width. Main Outcomes and Measures: Lumpectomy margin width data were prospectively collected within 3 months of randomization. A pathology form classified margins as positive (ink on tumor), close (<1 mm), or negative (≥1 mm). For the negative margin subgroup, closest margin width was stated separately. Thus, an ancillary analysis using 1-mm and 2-mm margin width partitions was performed. Results: A total of 3104 postmenopausal women (mean [SD] age, 61 [7.8] years) were enrolled in NSABP B-35. In an ancillary analysis, 2707 patients were included in the 1-mm margin width partition group, and 2546 patients were included in the 2-mm margin width partition group. IBTR was the most common first event, occurring in 90 of 2707 patients (3.3%): 24 of 502 patients (4.8%) with a margin width less than 1 mm and 66 of 2205 patients (3.0%) with a margin width greater than or equal to 1 mm. Ten-year unadjusted cumulative incidence of IBTR events was 5.6% vs 4.0% for margins less than 1 mm vs margins greater than or equal to 1 mm (P = .04). Using 2 mm as the discriminant threshold for margin width, 39 of 879 patients (4.4%) with margins less than 2 mm and 49 of 1667 patients (2.9%) with margins greater than or equal to 2 mm experienced an IBTR first. Ten-year unadjusted cumulative incidence of IBTR events with margins less than or equal to 2 mm was 5.3% vs 3.8% for those with margins greater than 2 mm (P = .05). In models adjusting for other patient and tumor factors, margin width was not a significant predictor of IBTR risk (2-mm threshold hazard ratio, 1.33; 95% CI, 0.86-2.06). Conclusions and Relevance: Results of this ancillary analysis of the NSABP B-35 trial show that absolute differences in IBTR rates using margin width groupings of less than 1 mm or greater than or equal to 1 mm and margin width groupings of less than 2 mm or greater than or equal to 2 mm in postmenopausal women with HR-positive DCIS receiving lumpectomy, WBI, and adjuvant endocrine therapy were small. Omission of reexcision lumpectomies based on margin widths of less than 1 mm or less than 2 mm can be reconsidered in appropriate patients. Trial Registration: ClinicalTrials.gov Identifier: NCT00053898.