Occult medullary thyroid carcinoma. Unusual histologic variant presenting with metastatic disease

A 66-year-old man presented with an enlarged cervical lymph node, interpreted on fine-needle aspiration as squamous cell carcinoma. Histologic assessment of the excised mass demonstrated a mucin-positive glandular neoplasm that, by immunohistochemistry, was positive for carcinoembryonic antigen, calcitonin, and neuron-specific enolase. A subsequent total thyroidectomy revealed a grossly normal organ with a 2-mm microscopic focus of medullary carcinoma and foci of C-cell hyperplasia in the immediate vicinity of the tumor. No amyloid could be demonstrated in either lesion. The diagnostic difficulty encountered in this case is related to the presence of a rare morphologic variant (devoid of amyloid and forming mucin-positive glands) occurring as an occult primary but presenting as a metastasis. Although an occult primary in medullary carcinoma is recognized in familial cases, such a phenomenon is rare in sporadic cases.