Breast MRI for Evaluating Residual Tumor Size Following Neoadjuvant Chemotherapy: Clinicopathologic Factors and MRI Imaging Features Affecting its Accuracy

Objectives: The aim of the study was to investigate the accuracy of breast magnetic resonance imaging (MRI) for evaluating residual tumor size following neoadjuvant chemotherapy (NAC) and to identify clinicopathologic and MRI features affecting its accuracy. Materials and Methods: We retrospectively assessed 109 women who underwent preoperative dynamic contrast-enhanced (DCE) MRI following NAC and subsequent surgery between April 2016 and August 2020. Preoperative MRI features, including Breast Imaging Reporting and Data System lexicon characteristics, size of residual enhancing lesion, tumor shrinkage pattern, and clinicopathologic features, were investigated, and MRI and pathology findings were compared. Results: Residual tumor size on MRI showed high agreement with residual invasive tumor size on pathologic examination (ICC, 0.808, p<0.001). The residual tumor size measured by MRI and final pathologic size were concordant in 63/109 cases (57.8%), while MRI overestimated the size in 35/109 cases (32.1%). For estrogen receptor (ER)-positive/human epidermal growth factor receptor 2 (HER2)-negative tumors, MRI tended to underestimate the residual tumor size compared with HER2-positive cancers (p=0.002) and triple-negative cancers (p=0.12). On MRI, tumors with concentric shrinkage patterns after NAC showed less size discrepancy with final pathologic tumor size than those with non-concentric patterns (p=0.026). Conclusion: In ER-positive/HER2-negative cancers, MRI tends to underestimate the residual tumor size, compared to in other subtypes. Tumors with concentric shrinkage patterns after NAC showed less MRI/pathology size discrepancy.