Objective The aim of this study was to determine the impact of margin width and boost radiotherapy on the local recurrence risk of pure ductal carcinoma in situ (DCIS).

Methods and analysis This is a prospectively registered systematic review and meta-analysis reporting relative risk (RR), OR and HR margin width outcomes. Eligible studies included prospective and retrospective case series with defining margin widths and 48 months of minimum follow-up. All patients (100%) received adjuvant whole breast radiotherapy (WBRT).

Results A total of 40 265 patients with pure DCIS in 31 studies were included. ORs and RR were calculated from 15 studies in 12 519 patients, and HRs were calculated from 12 studies in 12 946 patients. Local recurrence was significantly greater with narrower ‘close’ margins; 0.1–1 mm versus >1 mm in RR (2.88, 95% CI 1.86 to 3.90; p<0.05), OR (4.82, 95% CI 2.45 to 9.48; p<0.05) and HR analysis (1.34, 95% CI 1.01 to 1.67; p<0.05). Compared with margins >2 mm, significantly greater local recurrence was observed in margins 0.1–2 mm in RR (1.72, 95% CI 1.09 to 2.35; p<0.05) and OR (4.43, 95% CI 3.02 to 6.50; p<0.05). Comparing 0.1–1 mm versus >1 mm and 0.1–2 mm versus >2 mm, differences in local recurrence were not statistically significant, once adjusted for boost radiotherapy.

Conclusions In pure DCIS with WBRT, the local recurrence risk reduces as margin width increases up to 2 mm. The strength of the recommendation for a minimum clear margin of 2 mm is limited by a lack of data comparing 1.1–2 mm with >2 mm. The association between recurrence and close margins is not significant following boost radiotherapy, suggesting a possible alternative to re-excision in patients with close margins <2 mm.

Systematic review registration CRD42022308524.