Answer: Invasive lobular carcinoma, solid variant

Histology: This is a sheet-like proliferation of neoplastic cells associated with necrosis. Gland formation is not evident and lymphocytes are not prominent. The neoplastic cells have generally round nuclei, with relatively little pleomorphism given the high mitotic rate that is seen. Focally, intracellular mucin vacuoles are evident. Neoplastic cells label for cytokeratin (not shown), but show loss of expression of E-cadherin supporting the diagnosis of lobular carcinoma.

Discussion: Ductal carcinomas would have intact E-cadherin staining and typically form tubules. Medullary carcinoma would have a similar sheet-like pattern; however, it would show prominent lymphoid reaction and more pleomorphic neoplastic cells. Granulocytic sarcoma frequently has more angulated nucleoli, and finely granular cytoplasm. Granulocytic sarcoma would be negative for cytokeratin and would label for myeloperoxidase.

Almost all lobular carcinomas have a characteristic single-file architectural growth pattern, but the solid variant of lobular carcinoma is the exception to this rule. These lesions frequently have high mitotic rates, allowing one to categorize these tumors as Elston grade III as opposed to the usual lobular carcinoma which is Elston grade II.