Dermatologic Mimickers of Breast Lesions Arising in the Breast Skin, Subcutis, or Axilla

The subanatomical compartments of the breast, nipple, and axilla are located within close proximity to one another, resulting in an overlap of different histopathological regions. As a consequence, lesions of the skin, subcutaneous tissue, and fascia can present clinically as breast masses. Moreover, a breast neoplasm can be initially managed as a dermatological process. Clinical information, including radiologic imaging, is of critical importance in the evaluation of these lesions. In limited biopsy samples, the distinction between mammary and non-mammary entities can be problematic, as these can share similar histomorphology. As the management of cutaneous and mammary neoplasms is vastly different, proper management depends on an accurate interpretation of this limited material. In this chapter, we propose an algorithmic approach to the differential diagnosis of cutaneous mimics of breast neoplasms for the initial evaluation of needle biopsies. Relevant immunohistochemical stains and their patterns of expression are discussed. A review of cutaneous tumors reported to present in the breast and axilla is provided. These lesions are of sweat gland, surface epithelial, neuroendocrine, melanocytic, or mesenchymal derivation. The pertinent mammary differential diagnosis for each entity is discussed with an emphasis on possible diagnostic pitfalls and important clues to the diagnosis.