Supine Magnetic Resonance Imaging in Breast-Conserving Surgery Planning: A Systematic Review of Margin and Re-excision Outcomes.

Achieving negative margins at the first operation is a central goal of breast-conserving surgery (BCS), yet positive margins and re-excision remain common. Conventional preoperative magnetic resonance imaging (MRI) is acquired in the prone position, a position the breast does not assume during surgery, and the resulting prone-to-supine deformation may degrade image transfer to the operative field. Supine MRI (SMRI) has been proposed to overcome this limitation. This systematic review evaluated its effect on positive surgical margin and re-excision rates in BCS. Four databases were searched from inception to May 2026, with backward citation tracking. Absolute rates were summarized narratively by transfer technique; a post-hoc exploratory relative-risk meta-analysis was restricted to studies with a comparator. Thirteen studies (1718 patients) were included. Positive margin rates with SMRI ranged from 0% to 25.9%; heterogeneity in margin definitions precluded pooling of absolute rates. Across the 5 comparative studies, SMRI was associated with a significantly lower risk of a positive margin (relative risk [RR] 0.57 [95% confidence interval (CI), 0.36-0.88]) and a nonsignificant reduction in re-excision (RR 0.64 [95% CI, 0.40-1.04]), without detectable heterogeneity. The evidence base was limited: a single randomized trial and 11 of 12 nonrandomized studies at serious risk of bias, with baseline confounding that generally disfavored the intervention. SMRI is a biologically coherent and promising approach that may reduce positive margins, but current evidence is preliminary. Randomized comparison against contemporary localization techniques, using harmonized margin definitions, is required.