Metastatic medullary carcinoma of thyroid to breast; A case initially diagnosed as primary invasive lobular carcinoma - A case report -

Metastasis to the breast from medullary carcinoma of the thyroid is extremely rare. We report a case of metastatic medullary carcinoma of the thyroid which presented as multiple breast masses with ipsilateral axillary lymphadenopathy in a 48-year-old woman. Six years ago, she underwent total thyroidectomy and neck dissection because of palpable neck masses, with a diagnosis of medullary thyroid carcinoma. Histological features of breast masses showed single-file or linear-cord arrangements, with plasmacytoid appearance, and the initial diagnosis was invasive lobular carcinoma. She underwent modified radical mastectomy. The tumor cells were diffusely positive for E-cadherin, calcitonin and thyroid transcription factor-1 (TTF-1) and were metastatic medullary carcinoma of thyroid. In the patients with a history of medullary carcinoma of the thyroid, a careful examination is necessary for a breast mass composed of solid and cord-like clusters of small round to ovoid cells with plasmacytoid appearance. Immunohistochemical staining for E-cadherin, calcitonin and TTF-1 could be helpful for differential diagnosis.