Medullary Thyroid Carcinoma Presenting as Metastatic Disease to the Breast.

Medullary thyroid carcinoma (MTC) is a neuroendocrine tumor that is derived from C cells of the thyroid gland. It is a rare aggressive tumor, known to metastasize to lymph nodes, liver, bones, and lungs. A 41-year-old female, who presented with a breast mass, was initially diagnosed with invasive ductal carcinoma. She was also found to have a thyroid mass which was later diagnosed as MTC. On a rereview of the breast pathology, the morphologic features were strikingly similar to the MTC. Further investigation revealed that this was in fact a very rare case of MTC that had metastasized to the breast. We have identified 20 cases of MTC metastasizing to the breast in the literature that supports its occurrence as a real possibility. Albeit rare, medullary thyroid carcinoma should be considered in the differential diagnosis of a breast mass.