MEDULLARY-THYROID CARCINOMA METASTATIC TO BREAST MASQUERADING AS INFILTRATING LOBULAR CARCINOMA

Metastatic tumors to the breast from an extramammary site are rare ent ities and may present diagnostic difficulties for the surgical patholo gist because of frequent histological similarities to primary neoplasm s 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 infilt rating lobular carcinoma and included the so-called ''targetoid'' and ''Indian file'' patterns. Immunostaining revealed the true nature of t he lesion and was diffusely positive for calcitonin, chromogranin, and carcinoembryonic antigen. Electron microscopy disclosed typical neuro secretory granules confirming the diagnosis. A brief review of the lit erature and differential diagnosis is also presented.