Answer: Metaplastic carcinoma

Histology: The lesion shows variable cellularity. There are broad areas of collagenization and a prominent chronic inflammatory infiltrate composed predominately of lymphocytes and plasma cells. The lesional cells are mostly spindled, with minimal cytologic atypia. These cells tend to cluster into “angiomatoid” type spaces. A storiform pattern is also focally evident. The spindled cells are strongly immunoreactive for high molecular weight cytokeratin 903 and broad spectrum cytokeratin AE1/AE3, confirming the diagnosis of metaplastic carcinoma. Interestingly, p63 nuclear labeling is also identified in the spindled cells.

Discussion: Inflammatory pseudotumor is a serious consideration here, given the bland nature of the spindled cells and the prominent inflammatory infiltrate. One can avoid this pitfall by insisting upon performing a cytokeratin on any spindled cell lesion in the breast, given how bland metaplastic carcinomas can appear. Fibromatosis typically lacks the inflammatory infiltrate of the current case, and would show a more fascicular, regular growth pattern. Fibromatosis of the breast is characterized by nuclear labeling for beta-catenin, and should be non-reactive for cytokeratins. Nodular fasciitis of the breast would share the somewhat myxoid background, spindle cell predominance and inflammatory infiltrate of the current lesion. However, nodular fasciitis is rare in the breast, usually affects younger patients, and would not label for cytokeratin.

Metaplastic carcinomas may demonstrate heterologous differentiation into osteoid or cartilage. The current case is an example of a pure spindled metaplastic carcinoma; these are frequently associated with squamous carcinoma. In general, metaplastic carcinomas are negative for estrogen and progesterone receptors, and frequently are aneuploid. It is important to grade both the spindled and epithelial components since the prognosis for metaplastic carcinoma is linked to both stage and grade. Most have considered these lesions to be derived from neoplastic epithelium, in that one frequently sees a poorly-differentiated ductal carcinoma intermixed with the tumor. However, the finding of p63 immunoreactivity in some of these lesions suggests that some may in fact demonstrate myoepithelial differentiation.