Pure apocrine carcinoma of the breast

Pure apocrine carcinoma of the breast (PACB) is a rare carcinoma derived from the epithelial cells in the lactiferous ducts of the mammary gland.^[1] The mammary gland is an apocrine gland.^[2] Its lactiferous ducts have two layers of epithelial cells, a luminal layer which faces the duct's lumen (i.e. passageway) and a basal layer which lies beneath the luminal layer. There are at least four subtypes of epithelial cells in these ducts: luminal progenitor cells and luminal mature cells which reside in the luminal layer and mammary stem cells and basal cells which reside in the basal layer.^[3] Examination of the genes expressed in PACB cancer cells indicate that most of these tumors consist of cells derived from luminal cells but a minority of these tumors consist of cells derived from basal cells.^[4][5]

Invasive apocrine carcinomas of the breast with an appreciable amount of apocrine gland-like tissue was termed apocrine breast tumors for decades when in 1988 d'Amore et al named a subset of these carcinomas in which >90% of its tissue consisted of apocrine gland-like tissue as pure invasive apocrine carcinoma.^[6] In 2010, Vranic and colleagues added further to the definition of PACP to establish the criteria currently used for its diagnosis. They defined it as a carcinoma that: a) has a component(s) which invades adjacent tissue; b) consists of >90% apocrine gland-like tissue; c) has at least 10% of its cancer cells that express androgen receptors; and d) lacks cancer cells that express estrogen receptors and progesterone receptors. Many PACB tumors have cancer cells which overexpress the HER2/neu receptor-like protein.^[7] The World Health Organization (2019) did not define any of the other invasive apocrine carcinomas as diagnostically distinct entities,^[8] but did recognize invasive ductal breast carcinoma in which >90% of its cancer cells were apocrine gland cells as "carcinoma with apocrine differentiation."^[9] Non-invasive apocrine carcinoma of the breast, i.e. apocrine ductal carcinoma in situ, is classified as a variant of ductal carcinoma in situ breast tumors.^[9] Invasive carcinomas consisting of 10%–90% apocrine tumor tissue have been termed apocrine-like invasive carcinoma; these carcinomas sometime have estrogen receptor-positive ^[1] and/or progesterone receptor-positive tumor cells.^[9] Both of these carcinomas are described in the last section (Other types of apocrine carcinomas) of this article.

As defined by the criteria established by Vranic and colleagues, PACB is a rare carcinoma that accounts for about 1% of all breast cancers,^[10] occurs primarily in postmenopausal females,^[11] and usually presents as an asymptomatic mass that in many cases has spread to nearby axillary (i.e. armpit) lymph nodes.^[12] While earlier studies used only some of these criteria to diagnose PACB, the studies reviewed here used all four of them.