Myoepithelial carcinoma of the breast: a clinicopathological and immunohistochemical study of 15 diagnostically challenging cases

Abstract

Myoepithelial carcinoma (MEC) is a rare type of breast cancer composed purely of myoepithelial cells. Most often it presents with a spindle cell morphology that can mimic several benign and malignant lesions and may be misdiagnosed by the pathologist. We report 15 cases of MEC, which were sent to our consultation practice: Five of them were initially diagnosed as benign. The patients, all female, ranged from 45 to 86 years in age (mean 69.5) and-with one exception-presented with a breast mass. The tumor size measured between 1 and 4.8 cm (mean 2.6 cm). Microscopically, the tumors had infiltrative growth pattern most frequently with thin anastomosing cords of tumor cells associated with an intimately admixed reactive spindle cell stroma. The neoplastic myoepithelial cells were emanating from the myoepithelial cell layer of entrapped ductules in every case. The nuclei showed mild to moderate pleomorphism, and the mitotic activity ranged from 0 to 9/10 high power field. Immunohistochemical stains for p63, CD10, CK903, and CK5/6 reacted strongly and diffusely with the tumor cells, and mainly the reactive stroma had weak positivity for calponin, S-100, and smooth muscle actin. Estrogen receptor, progesteron receptor, and Her2 immunostains were negative, but strong epidermal growth factor receptor expression was observed. Follow-up was available for seven patients: All of them were alive at last contact; one patient had local recurrence, and one developed pulmonary metastases. MEC is a potentially aggressive malignant neoplasm sharing many features with metaplastic carcinomas. Morphologically, it is often difficult to distinguish it from benign spindle cell proliferations.

A Targetable Androgen Receptor–Positive Breast Cancer Subtype Hidden Among the Triple-Negative Cancers

“…Adenoid cystic carcinomas are very indolent and low-grade cancers with no known case that has had axillary node metastases; these tumors probably would not require any additional therapies following surgical excision. Myoepithelial 10 and squamous cell 11 carcinomas of the breast are generally EGFR positive; EGFR could be explored as a therapeutic target for these tumors. Carcinomas with osseous or chondroid differentiation, carcinosarcomas, and carcinomas arising from microglandular adenosis are developmentally complex lesions that require substantially more assessment at the molecular level for a better understanding of their evolution and identification of potential targets for therapy.…”

“…4,10,11 EGFR has been explored as a potential target for therapy based on its expression in some variants of TNBC and the demonstration of EGFR dependence for growth and proliferation on BLBC cell lines. 48 TBCR 001, a randomized phase 2 trial, evaluated the role of EGFR inhibition in metastatic TNBC.…”

“…Among carcinomas of special types, the squamous and myoepithelial carcinomas are epidermal growth factor receptor (EGFR) positive and could potentially benefit from EGFR-targeted therapy. [10][11][12] About 25% to 75% of TNBCs show androgen receptor (AR) expression; many, but not all, of these tumors are apocrine carcinomas. [13][14][15][16][17] It is this latter group (AR þ TNBCs) that is of special interest in this review for a potential benefit from AR-targeted therapies.…”

Metaplastic Carcinoma of the Breast

“…Because MCB and myoepithelial carcinoma are so histologically similar and behave similarly, it is very difficult to differentiate the two. 27 Phyllodes tumors are mostly benign breast lesions that resemble fibroadenomas, but additionally there are hypercellular mesenchymal elements arranged in a pattern resembling a leaf. These tumors may also contain spindle cells and share MCB's positivity of vimentin and actin (in nonsquamous types only).…”

CD10 Immunohistochemical Expression in Apocrine Lesions of the Breast

“…MEC) markers (cytokeratin 5 and/or EGFR) and this feature has been suggested for the delineation of the basal-like gene-expression profile-based subgroup of breast cancers on IHC [36]. Not surprisingly, some of these carcinomas may also express CD10, an MEC marker in a substantial number of cases (16/20 of spindle-cell metaplastic carcinomas and carcinosarcomas) [37], similarly to the rare cases that demonstrate straightforward myoepithelial differentiation [38]. Apocrine carcinomas are also generally ER- and PR-negative [39], and might have been included in previous studies of ER-negative carcinomas, but without distinct identification of this subset.…”