Pure apocrine carcinoma of the breast (PACB) is a rare carcinoma derived from the epithelial cells in the lactiferous ducts of the mammary gland. The mammary gland is an apocrine gland. 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. 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. 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. 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. The World Health Organization (2019) did not define any of the other invasive apocrine carcinomas as diagnostically distinct entities, but did recognize invasive ductal breast carcinoma in which >90% of its cancer cells were apocrine gland cells as "carcinoma with apocrine differentiation." 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. Invasive carcinomas consisting of 10%–90% apocrine tumor tissue have been termed apocrine-like invasive carcinoma; these carcinomas sometime have estrogen receptor-positive and/or progesterone receptor-positive tumor cells. 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, occurs primarily in postmenopausal females, and usually presents as an asymptomatic mass that in many cases has spread to nearby axillary (i.e. armpit) lymph nodes. While earlier studies used only some of these criteria to diagnose PACB, the studies reviewed here used all four of them.
Presentation Due to its rarity and more recent definition, PACB studies have generally consisted of small numbers of patients. In these studies, most individuals diagnosed with PACB presented with an asymptomatic breast tumor discovered by self-palpation or screening methods such as mammography. The mammographic features of PACB included a spectrum of dense masses with associated microcalcifications; these findings are non-specific and similar to those seen in, e.g. invasive carcinoma of no special type. Uncommonly, PACB tumors caused breast symptoms such as a pulling sensation, skin puckering, pain, nipple discharge, or overt ulceration. In one study, 41 individuals presenting with PACB were females aged 32–75 years (mean: 59 years) with 12 (53.7%) being over 59 years old, 35 (85.4%) being postmenopausal, and 6 (14.6%) being of childbearing age; their tumors widest diameter was 0.47–10.00 centimeters (cm) (average: 2.4 cm); and axillary lymph node metastases on the same side as the tumor were present in 20 (48.8%) cases. A study of 15 individuals found 5 (33%) with lymph node metastases, again on the same side as the tumor. A study of 18 females all of whom had HER2/new-positive PACB tumors reported that they were 44–83 years old (median age: 55.5 years); 14 (77.8%) were postmenopausal; the widest diameter of their tumors ranged from 0.8 to 5 cm (average: 4 cm); and 9 (50%) had axillary lymph node metastases. These metastases were detected in from 1 to 10 lymph nodes on the same side as the tumor although one individual had metastases in the lymph nodes of both axillae and another had simultaneous metastases in infraclavicular, i.e. below the clavicle, lymph nodes. Rarely, individuals with PACB have presented with metastases in more distant tissues such as in the lymph nodes of the mediastinum (i.e. central compartment of the thoracic cavity) or in the deltopectoral lymph nodes, i.e. lymph nodes situated below the clavicle and between the pectoralis major and deltoid muscles. Also rarely, individuals previously treated for PACB have later presented with recurrence of their disease in sites where the tumors were surgically removed, in the nearby skin or chest wall, and/or in more distant tissues such as a bone or the brain.
Pathology
Histopathology
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