Intraoperative Supine Breast MRI for Residual Tumor Assessment after Breast-Conserving Therapy in Early-Stage Breast Cancer.

Purpose To evaluate the diagnostic performance of intraoperative supine MRI (isMRI) in identifying residual tumor burden immediately after breast-conserving therapy (BCT). Materials and Methods This single-institution prospective study (April 2012-December 2022) included 43 consecutive participants with stage 0-II breast cancer. Three participants with multicentric disease were excluded from the final analysis. Preoperative supine MRI was performed after standard preoperative prone MRI to compare tumor sizes and distances to the nipple, chest wall, and skin. After lumpectomy, the saline-filled surgical cavity was assessed for residual tumor at 3-T isMRI in the operating suite. Diagnostic accuracy of isMRI findings in identifying residual tumor at resection margins was assessed using histopathology of shave margin specimens as the reference standard. Performance metrics of isMRI and re-excision rates were analyzed at per-participant and per-margin levels. Results Forty participants (median age, 58.5 years; range, 40-76 years) with 44 breast cancers (36 unifocal and four multifocal) underwent BCT, all with a single lumpectomy site. Margin assessment using isMRI yielded accuracy, sensitivity, specificity, positive predictive value, and negative predictive value of 80%, 50%, 93%, 75%, and 81% per participant, respectively; and 93%, 52%, 97%, 65%, and 96% per margin. Second re-excision was avoided in two of six (33%) participants with true-positive isMRI readings, decreasing the final re-excision rate from 18% to 13%. Histopathology of six false-negative isMRI cases revealed residual invasive carcinomas, all smaller than 0.3 cm, or intermediate-to-high grade ductal carcinoma in situ. Conclusion Intraoperative assessment for residual tumor after BCT using isMRI demonstrated promising accuracy to guide targeted margin clearance within the same operation. Keywords: Breast, MR-Imaging, MR-Dynamic Contrast Enhanced, Oncology Supplemental material is available for this article. © RSNA, 2025.