Medullary thyroid carcinoma metastatic to breast masquerading as infiltrating lobular carcinoma

Metastatic tumors to the breast from an extramammary site are rare entities and may present diagnostic difficulties for the surgical pathologist because of frequent histological similarities to primary neoplasms in this location. A case is reported of medullary thyroid carcinoma metastatic to the breast in a 28-year-old woman with a family history of MEN IIA (Sipple's) syndrome. Histological features resembled infiltrating lobular carcinoma and included the so-called "targetoid" and "Indian file" patterns. Immunostaining revealed the true nature of the lesion and was diffusely positive for calcitonin, chromogranin, and carcinoembryonic antigen. Electron microscopy disclosed typical neurosecretory granules confirming the diagnosis. A brief review of the literature and differential diagnosis is also presented.