Medullary thyroid carcinoma metastatic to breast diagnosed by fine-needle aspiration biopsy.

We report a patient with a 22-yr history of medullary thyroid carcinoma in whom fine-needle aspiration biopsy of a breast mass, clinically suspected of being a primary mammary carcinoma, established the diagnosis of metastasis. This article demonstrates the importance of the use of immunocytochemical methods on fine-needle aspirates to diagnose metastases to the breast.