Histological and molecular classification of breast cancer: what do we know?

Breast cancer is the neoplasm most diagnosed malignancy and the leading cause of mortality among women on a global scale. A profound increase in the understanding and clinical management of breast cancer has occurred over the past two decades, which has led to significant progress in prevention, early detection, and personalized breast cancer therapy. However, the biggest obstacle still faced in clinical practice is the complete understanding of intertumoral and intratumoral heterogeneity, in addition to the mechanisms of multiple drug resistance in the systemic treatment of the disease. In view of this, many studies focus on analyzing morphological and, mainly, molecular patterns of breast cancer, with the purpose of grouping these tumors into classes or entities to assist in clinical management, in the elaboration of epidemiological and functional studies, and in the performance of clinical trials. The most common special histological types of breast cancer include: medullary carcinoma, metaplastic carcinoma, apocrine carcinoma, mucinous carcinoma, cribriform carcinoma, tubular carcinoma, neuroendocrine carcinoma, classic lobular carcinoma, and pleomorphic lobular carcinoma, in addition to the non-specific type of invasive ductal carcinoma, which constitutes the majority of newly diagnosed cases. As to their molecular aspect, intrinsic subtypes were identified based on global studies of gene expression profiles. Today, four molecular subgroups are widely reproduced and well established in the clinical routine, namely: Luminal A, Luminal B, HER2 +, and Triple Negative. Thus, the present article aims to briefly address the histological and molecular classification of breast cancer.