Ectopic breast tissue and breast-like sweat gland metaplasias: an overlapping spectrum of lesions.

There are many similarities in the morphology of benign and malignant lesions of the sweat glands and the breasts. The recently described cutaneous mammary-like sweat glands, also known as mixed sweat glands or apoeccrine glands, are also a likely source of selected proliferations that closely mimic those of the breast. We present three cases of breast-like lesions arising in the skin that demonstrate the ways in which the morphologic and pathologic continuum of the mammary glands, cutaneous mammary-like glands, and sweat glands can produce difficulties in precise diagnosis. The examples demonstrate that an anatomic location outside the milk line does not preclude the diagnosis of ectopic mammary tissue, and that lesions closely resembling those of the breast may also arise outside the milk line from conventional sweat glands or mixed sweat glands. The concept of homologous lesions of the breast, breast-like glands and sweat glands, in which morphology is partially mirrored by biochemical similarities, provides a perspective for classification of problematic cases of breast-like cutaneous lesions.