Answer: Sarcomatoid breast carcinoma

Histology: This is an anaplastic, high grade neoplasm which features pleomorphic cells with a somewhat nested architecture, and undifferentiated small round cells. The pleomorphic cells with nested architecture label for cytokeratin, supporting the diagnosis of anaplastic carcinoma. The small cell component labels focally for synaptophysin, raising the possibility of small cell carcinoma. However, these cells are negative for cytokeratin, and instead label for myogenin and desmin. This supports the diagnosis of sarcomatoid carcinoma with a prominent primitive rhabdomyosarcomatous component.

AE1/AE3

Myogenin

Synaptophysin

Discussion: Basal-like carcinomas are triple negative high grade carcinomas. These tumors typically have a solid nested pattern with peripheral palisading of nuclei, and label for either basal cytokeratins such as cytokeratin 5/6 or EGFR. Most high grade sarcomatoid mammary carcinomas would fall under the category of basal-like carcinomas by gene expression profiling. A combined high grade carcinoma/lymphoma is excluded by the labeling for desmin in the small cell component. The absence of cytokeratin labeling and the labeling for desmin excludes a small cell neuroendocrine carcinoma component.