Abstract
BACKGROUND: Single soft tissue metastasis of medullary thyroid carcinoma is extremely rare. In addition, several occult medullary thyroid carcinomas with distant metastasis were reported, but undetectable primary lesion at diagnosis was also extremely rare. CASE PRESENTATION: A 74-year-old man was admitted to our hospital because of a painful nodule in his left buttock for over 1 year. Needle biopsy was performed, and the histological findings revealed adenocarcinoma positive for thyroid transcription factor-1. No evidence of a primary tumor, including the lung and thyroid gland, could be found elsewhere despite detailed examinations, including thyroid echography, chest computed tomography, and fluorodeoxyglucose-positron emission tomography. The soft tissue tumor was resected with a wide margin. Immunohistochemical analysis showed the tumor cells to be positive for cytokeratin-AE1/3, cytokeratin 7, synaptophysin, chromogranin A, calcitonin, and carcinoembryonic antigen, but negative for cytokeratin 20, Napsin A, Pax8, and p40, resulting in a diagnosis of metastasis of medullary thyroid carcinoma. CONCLUSION: Initial presentation with a single metastasis to soft tissue and undetectable primary tumor in the thyroid gland is an extremely rare clinical manifestation in patients with medullary thyroid carcinoma.