Medullary Thyroid Carcinoma Showing Melanocytic Differentiation: A Report of a Rare Case

Medullary thyroid carcinoma (MTC) accounts for 5-10% of all thyroid malignancies. These tumors show variable morphological features, however, to the best of our knowledge, melanocytic differentiation is rare with only 11 reported cases. We report a 44-year-old female who presented to our clinic with neck swelling of a few months. The thyroid fine needle aspiration biopsy results were suspicious, leading to a total thyroidectomy. On microscopic examination, a malignant tumor with melanocytic features was seen. There was no amyloid deposition in the stroma. Immunohistochemical analysis revealed that the tumor cells were positive for calcitonin, melan-A, HMB45, pancytokeratin, chromogranin, carcinoembriyonic antigen; focal positive for S-100; and negative for HBME-1 and thyroglobulin. This tumor was diagnosed as medullary carcinoma with melanocytic differentiation when evaluated with the morphological and immunohistochemical findings. Turk Jem 2015; 19: 72-7