C-Cell-Derived Calcitonin-Free Neuroendocrine Carcinoma of the Thyroid: The Diagnostic Importance of CGRP Immunoreactivity

In the thyroid, primary neuroendocrine tumors encompass medullary thyroid carcinoma (MTC) and, rarely, other tumors such as paragangliomas. MTCs are derived from C-cells and express calcitonin and neuroendocrine markers. Besides classic MTC, some reports have documented thyroid neuroendocrine tumors, which show no calcitonin expression and raise difficult diagnostic problems. A 76-year-old man presented with a mass in the left thyroid with neither serological calcitonin elevation nor familial history. A thorough clinico-laboratorial study did not disclose any other mass elsewhere. A left hemithyroidectomy was performed, and the histological examination revealed a neuroendocrine carcinoma resembling a paraganglioma-like MTC displaying unequivocal signs of vascular invasion. Immunohistochemically, the tumor cells showed reactivity for chromogranin A, synaptophysin, thyroid transcription factor-1 (TTF-1), paired box gene 8 (PAX8), cytokeratins (AE1/AE3 and CK8/18), and calcitonin gene-related peptide (CGRP) and negativity for calcitonin, carcinoembryonic antigen, TTF-2, thyroperoxidase, and thyroglobulin. In situ hybridization showed that the tumor cells lacked expression for calcitonin and thyroglobulin mRNA. Genetic analysis did not disclose any RET mutation. A diagnosis of C-cell-derived primary neuroendocrine carcinoma of the thyroid without calcitonin expression was made, and the patient remains free of metastasis or recurrence 18 months after surgery.