Answer: Metaplastic carcinoma

Histology: The specimen consists of an ill-defined, predominantly spindled neoplasm of variable cellularity associated with a tubular epithelial proliferation. The tubular proliferation demonstrates focal cribriform architecture and lacks myoepithelial cells as was demonstrated using immunohistochemical stains for actin and smooth muscle myosin heavy chain. The spindled proliferation is focally highly collagenized and pauci- cellular; however, there are areas of high cellularity with nuclear atypia and mitotic activity. A storiform pattern is focally appreciated. This lesion extends out into the surrounding fat in an infiltrative fashion. Spindle cells are immunoreactive for broad-spectrum cytokeratin AE1/3, and demonstrate discrete positivity with high molecular weight cytokeratin 34BE12 (CK903).

Discussion: Fibromatosis of the breast infiltrates between and entraps the breast epithelium. Epithelial proliferation is not a component of the lesion, and fibromatosis should not label for cytokeratin. Adenomyoepitheliomas are characterized by biphasic proliferations of myoepithelial and secretory cells, often in association with papillomas. While a papilloma was identified in the current case, the proliferation in current case was not uniformly biphasic. Florid biopsy site changes are eliminated by the strong immunoreactivity for high molecular weight and broad-spectrum cytokeratin.
Metaplastic carcinomas can be of several types. These include squamous carcinomas of the breast, low-grade adenosquamous carcinomas, and spindled cell tumors, some of which are associated with matrix (cartilage, osteoid) production. Other spindled metaplastic carcinomas can have a fibromatosis-like appearance, and have a locally aggressive, non-metastasizing course. The current lesion shows more cellularity, and it has a fibroblastic/myofibroblastic phenotype. In general, stains for high molecular weight cytokeratins are the most specific for true epithelial differentiation, and seem to be the most useful in demonstrating epithelial differentiation within metaplastic carcinomas.