Factors associated with discordance between pathological and radiological tumor size and risk of re-excision in breast-conserving surgery: a post-hoc analysis within a randomized trial.

Accurate preoperative radiological assessment is essential in breast-conserving surgery (BCS) to achieve clear margins while minimizing unnecessary healthy tissue excision. Discrepancies between radiological and pathological tumor size may contribute to re-excision. This study evaluated factors associated with radiology-pathology discordance and their impact on re-excision. This post hoc analysis of the MagTotal randomized trial included patients with non-palpable breast cancer undergoing BCS with lesion localization and sentinel lymph node dissection. The Pathology-to-Radiology Ratio (PRR) quantified discordance between size on preoperative imaging and specimen pathology. Primary outcomes were predictors of PRR and re-excision. A multivariable model was used to construct a nomogram for predicting PRR. Among 414 patients (median age 65 years; median tumor size 10.3 mm), MRI was performed in 24.4%. Median PRR was 1.25, suggesting systematic underestimation. MRI consistently overestimated tumor size in most subtypes but provided more accurate estimates for invasive lobular carcinoma (ILC), compared with conventional imaging. Independent predictors of discordance included ILC, high grade, and small tumor size. Eleven patients (2.7%) required re-excision, which was associated with tumor biology and mismatches between imaging and pathology but not with MRI use. Re-excision in non-palpable breast cancer is primarily driven by tumor biology and radiology-pathology mismatch. Although MRI improved size assessment in ILC, it did not reduce re-excision rates. Integrating imaging with biological factors, supported by predictive tools such as a nomogram, may refine surgical planning in BCS.