Taking one step backward to take two steps forward: the importance of breast tumor phenotype in MRI-based prediction of response

Evaluation of tumor phenotype is currently accepted to guide management of breast cancer and neoadjuvant chemotherapy (NAC). Magnetic resonance imaging (MRI) is a diagnostic imaging tool to evaluate response to NAC, mainly through morphological evaluation (RECIST criteria). It has been suggested that the capability of post-NAC MRI in the detection of complete response is greatly influenced by tumor phenotype; the interplay of the expression of estrogen and progesterone receptors and of human epidermal growth factor-2 affects deeply variable tissue characteristics. The purpose of the present review is to revise current evidence about the differential diagnostic performance of MRI according to breast tumor phenotype, both in the post-NAC setting (detection of complete response) and in the pre-NAC settings (prediction of complete response), in order to advise the radiologist in the cautious interpretation of MRIderived information. We also discuss the potential role of additional MRI techniques [diffusion-weighted imaging (DWI)] in this context. The conclusions of the present review may be helpful to achieve further personalization of management protocols to individual patients.