Metaplastic carcinomas of the breast: diagnostic challenges and new translational insights.

Comprising less than 1% of invasive carcinomas of the breast, metaplastic carcinomas are a heterogeneous group of malignant tumors in which part or all of the carcinomatous epithelium is transformed into a nonglandular (metaplastic) growth process. Metaplastic carcinomas with a low-grade spindle cell component resembling fibromatosis, as in our present case, are worthy of particular note because they are infrequent, difficult to recognize, and have a high risk of local recurrence. The differential diagnosis of metaplastic carcinomas depends on the degree of atypia observed in the tumor and includes exuberant scars, fibromatosis, nodular fasciitis, myofibroblastomas, pseudoangiomatous stromal hyperplasia, acute and chronic abscess with fat necrosis, malignant phyllodes tumor, and primary or metastatic sarcoma. Limited studies done on the molecular pathology of metaplastic carcinomas show that activation of the Wnt signaling pathway is common in these tumors and that approximately 70% of metaplastic carcinomas show EGFR gene amplification and overexpression. This may have treatment implications because they may lead to targeted treatment for patients with metaplastic carcinomas.