Breast, Metastatic well-differentiated neuroendocrine tumour from GI origin

Metastatic malignancies can be challenging to diagnose and may pose as mimickers of primary breast cancer. In this case, an infiltrative neoplasm is seen on core needle biopsy. There are solid areas that appear to have gland-forming type patterns. There are also areas with single filing architecture. The main differential diagnosis includes a primary mixed ductal and lobular carcinoma; however, this neoplasm shows more abundant cytoplasm than may be expected in lobular carcinoma with bland nuclear features.

A metastatic malignancy was confirmed with positive staining for synaptophysin, CD56, and CDX2. This neoplasm was also negative for GATA3. Further details of clinical history revealed a history of well-differentiated neuroendocrine tumour from GI origin.