Medullary Thyroid Carcinoma Metastasis to the Breast and Axillary Lymph Nodes

Breast metastases of medullary thyroid carcinoma (MTC) are extremely rare, and only a few cases have been reported in the literature so far. Here, we report a case of metastatic MTC to the breast and axillary lymph nodes (LN). The case illustrates that (1) metastatic MTC of the breast could be clinically and pathologically misdiagnosed as primary breast cancer, such as invasive lobular carcinoma with axillary LN involvement; (2) unlike other metastatic breast cancer patients, who have very poor prognoses, our patient survived for more than 5 years after the breast and axillary surgery; and (3) metastasis of MTC to the breast is accompanied by axillary LN metastasis, which requires thorough axillary LN dissection, as in most primary breast cancers.