High-grade carcinomas of the breast showing patterns of mixed ductal and myoepithelial differentiation (including myoepithelial cell-rich carcinoma of the breast)

Aims: To assess the clinical, morphological and immunophenotypic characteristics of breast carcinomas showing patterns of mixed epithelial and myoepithelial differentiation. Methods and results: Included in the study were four carcinomas containing a mixed population of epithelial and myoepithelial cells identified using morphological features at the light microscopic level which were found amongst a review of 500 archival cases and two recently accessioned cases. The carcinomas varied in size from 20 to 38 mm and all were grade 3 ductal carcinomas. Most showed nodular and sheet-like cellular aggregates, although one case showed small solid cell aggregates with duct formation. The cells were large, round, polygonal or spindle-shaped and had areas of clear or eosinophilic cytoplasm in variable proportions. Foci of metaplasic carcinoma were present in three cases. All cases showed strong, patchy positivity for cytokeratin (CK)14, calponin, smooth actin and muscle specific actin. Epithelial membrane antigen and CK8 were positive in a similar proportion of cells. One patient died 23 months following diagnosis with metastatic carcinoma, another patient died of unrelated disease and four patients are alive with follow-up ranging from 18 months to 25 years. Conclusions: High-grade carcinomas of the breast showing patterns of mixed ductal and myoepithelial differentiation may show additional morphological features such as foci of metaplasia and appear to have a good prognosis similar to myoepithelial cell-rich carcinomas. However, young age and lymph node metastasis may portend a worse prognosis.