Interim Breast MRI for Predicting Pathologic Complete Response After Neoadjuvant Chemotherapy in Early Breast Cancer.

Objective: Accurate prediction of pathological complete response (pCR) to neoadjuvant chemotherapy (NAC) is essential for treatment decision-making in breast cancer. The value of integrated prediction models combining interim magnetic resonance imaging (MRI) findings with clinicopathological factors remains uncertain. This study aimed to compare interim and post-NAC MRI in predicting pCR and to assess the performance of prediction models integrating MRI and clinicopathological variables. Materials and Methods: We retrospectively analyzed 249 patients with early-stage breast cancer who underwent MRI before, during (interim), and after NAC. Clinicopathological variables were selected via stepwise regression based on the minimum Akaike information criterion. Four predictive models were developed: Model A used clinicopathological variables alone; Model B added interim MRI; Model C added post-NAC MRI; and Model D incorporated both interim and post-NAC MRI. Model performance was assessed using receiver operating characteristic curve analysis, calibration plots, and decision curve analysis. Results: Sixty-two (25%) patients achieved pCR. Independent predictors in Model A included hormone receptor status, human epidermal growth factor receptor 2 status, and clinical tumor size. The areas under the curves were 0.721 (Model A), 0.819 (Model B), 0.847 (Model C), and 0.848 (Model D). Model B (interim MRI) demonstrated the highest sensitivity (0.99) and negative predictive value (0.97), enabling early identification of pCR, while Models C and D showed only modest improvements. In decision curve analysis, calibration and clinical utility were superior in models incorporating MRI compared with Model A. Conclusion: Interim MRI demonstrated pCR-predictive performance comparable to that of post-NAC MRI, supporting its potential utility as a clinically meaningful tool for early treatment decision-making. However, its relatively high false-positive rate suggests that caution is warranted when applying it to guide surgical de-escalation.