Less Common Triple Negative Breast Cancers

Abstract

This chapter encompasses four uncommon types of triple negative breast cancer, which include adenoid cystic carcinoma (ACC), apocrine carcinoma, metaplastic carcinoma, and medullary carcinoma. Also included in the discussion for differential diagnosis are three additional rare subtypes: acinic cell carcinoma, oncocytic carcinoma, and mucoepidermoid carcinoma. Most of these tumors are “basal-like cancers,” as defined by gene expression analysis. Although sharing the triple negative and basal-like phenotype, these tumors demonstrate remarkably different clinical behaviors, which range along a spectrum from indolent to aggressive, emphasizing the notion that triple negative or basal-like breast cancer comprises a heterogeneous group of diseases. While ACC has well-defined histopathologic features and unique molecular alterations, metaplastic carcinomas represent a morphologically and genetically diverse group of tumors. Furthermore, uniform agreement on diagnostic criteria is lacking for medullary carcinoma and apocrine carcinoma. Accordingly, diagnosis of these uncommon lesions can be challenging, especially in core needle biopsy (CNB) specimens.