Optimal Surgical Margin After Breast-Conserving Surgery in Pure Ductal Carcinoma in Situ: Is a 1 mm Margin Sufficient? A Retrospective Single-Center Cohort Study

Background/Objectives: The optimal surgical margin width after breast-conserving surgery (BCS) for pure ductal carcinoma in situ (DCIS) remains controversial; although current guidelines consider a surgical margin of ≥2 mm sufficient, the clinical safety of narrower margins is unclear. This study aimed to evaluate the association between surgical margin width and ipsilateral breast tumor recurrence (IBTR), with a focus on the 1 mm threshold. Materials and Methods: In this retrospective single-center cohort study, 107 patients with pure DCIS treated with BCS followed by adjuvant radiotherapy (RT) between 1 January 2009 and 1 January 2025 were analyzed. Final surgical margins were categorized as <1 mm, 1–2 mm, and ≥2 mm. The primary endpoint was IBTR. Kaplan–Meier analysis was performed. Results: The median age of the study population was 52 years (IQR: 46–61). High-grade DCIS was present in 48 patients (44.9%), comedo necrosis in 68 (63.6%), and estrogen receptor positivity in 87 (81.3%). Overall, 10 patients (9.3%) underwent re-excision for margin widening. Final surgical margin widths were <1 mm in 36 patients (34%), 1–2 mm in 18 (17%), and ≥2 mm in 53 (49%). IBTR occurred in seven patients (6.5%) during a median follow-up of 48 months (range, 12–217 months), with a median time to recurrence of 33 months. Kaplan–Meier analysis showed no significant difference in recurrence-free survival according to a 2 mm margin threshold, whereas margins < 1 mm were associated with significantly worse outcomes (p = 0.002). Conclusions: Margins < 1 mm were associated with increased IBTR risk, whereas margins < 2 mm did not appear to confer uniform risk. These findings suggest that margin widths between 1 and 2 mm may represent a heterogeneous group, and clinical decision-making in this range should be individualized. However, further studies are needed to validate these outcomes.