Summary: Medullary thyroid carcinoma (MTC) has a varying clinical course; distant metastases are frequently present even at diagnosis. We present two MTC cases with unusual metastatic sites. Two female patients are presented with slow progressive MTC. The first case developed distant metastases 23 years after diagnosis and underwent locoregional therapies. At the same time a breast mass developed representing MTC metastasis. Treatment with vandetanib led to long-term disease stabilization. The second patient is presented with metastases in the pancreas 13 years after diagnosis. Shortly, a painful mass developed in the mandible and metastasis of MTC was diagnosed. Disease progression was recorded 20 months after the initiation of local and systemic therapy. Such cases have only rarely been reported in the literature and highlight the need for prompt recognition of unexpected MTC metastases.
Learning points: Unusual sites of metastasis may appear in patients with medullary thyroid carcinoma (MTC) years after the initial diagnosis. Although rare, unexpected MTC metastases highlight the need for prompt recognition and appropriate treatment. Local recurrences accompanied by inappropriately low calcitonin levels should prompt further investigation for possible distant metastatic disease. Systemic treatment with tyrosine kinase inhibitors may be effective even in patients with unusual metastases from MTC.