To evaluate the relevance of breast cancer subtypes for magnetic resonance imaging (MRI) markers for monitoring of therapy response during neoadjuvant chemotherapy (NAC).

MRI examinations were performed in 188 women before and during NAC. MRI interpretation included lesion morphology at baseline, changes in morphology, size, and contrast uptake kinetics (initial and late enhancement). By using immunohistochemistry, tumors were divided into three subtypes: triple negative, human epidermal growth factor receptor 2 (HER2) positive, and estrogen receptor (ER) positive/HER2 negative. Tumor response was assessed dichotomously (ie, presence or absence of residual tumor in the surgical specimen). Complementary, a continuous scale assessment was used (the breast response index [BRI], representing the relative change in tumor stage). Multivariate regression analysis and receiver operating characteristic analysis were employed to establish significant associations.

Residual tumor at pathology was present in 31 (66%) of 47 triple-negative tumors, 23 (61%) of 38 HER2-positive tumors, and 96 (93%) of 103 ER-positive/HER2-negative tumors. Multivariate analysis of residual disease showed significant associations between breast cancer subtype and MRI (area under the curve [AUC], 0.84; P < .001). BRI also showed significant correlation among breast cancer subtype, MRI, and age (Pearson's r = 0.465; P < .001). In subset analysis, this was only significant for triple-negative tumors (P < .001) and HER2-positive tumors (P < .05). Residual tumor after NAC in the triple-negative and HER2-positive group is significantly associated with the change in largest diameter of late enhancement during NAC (AUC, 0.76; P < .001). No associations were found for ER-positive/HER2-negative tumors.

MRI during NAC to monitor response is effective in triple-negative or HER2-positive disease but is inaccurate in ER-positive/HER2-negative breast cancer.