Answer: Low grade metaplastic carcinoma

Histology: Within the substance of the breast there is a squamous and spindle cell proliferation. The squamous proliferation does not show severe cytologic atypia; however, the nests of squamous epithelium are irregularly dispersed, and have an infiltrative pattern. Similarly, the fibroblastic proliferation which merges with the squamous proliferation has an irregular border. Immunostains for cytokeratin (cytokeratin 34*E12, AE1/AE3) highlight the squamous nests and their irregular, infiltrative border.

Discussion: Papillomas often show squamous metaplasia, particularly after infarction. However, there is no evidence of a large duct papilloma in this lesion, and the squamous metaplasia has an irregular growth pattern as opposed to what would be seen within a sclerotic papilloma. Fibromatosis resembles the spindle cell component of this case; however, it does not explain the squamous proliferation. Fibromatoses of the breast demonstrate nuclear labeling for *-catenin due to a disruption of the APC/*-catenin pathway within these lesions. Fasciitis is incredibly rare within the breast, and should be associated with a more myxoid background, extravasated red blood cells and a “tissue culture”-like appearance to the fibroblasts.

This is a lesion that is in the low-grade end of the spectrum of metaplastic carcinoma. Metaplastic carcinomas of the breast include heterologous (matrix-producing lesions, sometimes known as carcinosarcomas), any lesion with squamous differentiation, and spindle-cell proliferations. There is no evidence that a metaplastic carcinoma has any worse outcome than a pure ductal carcinoma once grade is accounted for. Low-grade lesions such as this would be expected to have minimal, if any, metastatic capability, but significant capacity for local recurrence.