Answer: Infiltrating ductal carcinoma, with carcinoid-like morphology

Histology: This is an invasive carcinoma with a prominent desmoplastic reaction. The neoplastic cells are arranged in cords and trabeculae. The surrounding desmoplastic stroma is highly vascular. The neoplastic cells have nuclei with uniform fine chromatin. By immunohistochemistry, the neoplastic cells are diffusely immunoreactive for synaptophysin, and demonstrate strong estrogen receptor positivity. These features support a primary mammary carcinoma with endocrine differentiation.

Discussion: A micropapillary carcinoma would feature clusters of carcinoma cells separated from stroma by clear spaces. Micropapillary carcinomas of the breast resemble serous carcinomas of the ovary, and are associated with a high incidence of nodal metastases. Metastatic carcinoid tumor is essentially excluded by the strong estrogen receptor immunoreactivity of this neoplasm. Infiltrating lobular carcinoma would feature more discohesive cells with more single file infiltrative patterns, and would lack expression of E-cadherin protein.

By itself, the presence of endocrine differentiation is of little significance in mammary carcinoma, with the exception of small cell carcinomas. When compared with other mammary carcinomas of similar grade and stage, no prognostic significance of endocrine differentiation has been demonstrated.