Medullary Thyroid Carcinoma Without Calcitonin: A Case Linking Ultimobranchial Bodies to Tumor Evolution.

Medullary thyroid carcinoma (MTC) is a thyroid tumor with neuroendocrine properties purportedly derived from C-cells. The biochemical activity of medullary thyroid carcinoma includes the production of calcitonin and carcinoembryonic antigen, which are sensitive tumor markers, facilitating diagnosis, follow-up, and prognostication. Calcitonin-negative medullary thyroid carcinoma is a rare, poorly understood primary neuroendocrine carcinoma of the thyroid characterized by classic medullary thyroid carcinoma morphology without raised serum calcitonin and with or without the expression of calcitonin detected by immunohistochemistry. Previous studies reported that C-cells were derived from the neural crest; however, more recently, C-cells have been indisputably shown to be derived from the pharyngeal endoderm and ultimobranchial bodies. Ultimobranchial body (UBB) remnants can persist in the thyroid and express p63, but their function is poorly understood. Some have postulated that ultimobranchial bodies may be the "stem" cell of the thyroid and may be precursors for thyroid tumors, particularly mixed tumors with follicular and medullary components. We present a unique case of calcitonin-negative MTC in a 58-year-old male arising in an inflamed and fibrotic thyroid with numerous scattered ultimobranchial body remnants and concomitant C-cell hyperplasia/medullary microcarcinoma (CCH/MMC). The ultimobranchial body remnants, C-cell hyperplasia, and medullary thyroid carcinoma were MTC classifier positive according to ThyroSeq. The areas representing CCH/MMC expressed calcitonin by IHC while the main MTC tumor was negative. An additional unique feature was an area demonstrating a "mixed" C-cell/thyroid follicular epithelial phenotype. In this review we review the possible etiologies of calcitonin-negative MTC, the possibility of a neoplastic sequential progression from ultimobranchial bodies to CCH/MMC to medullary thyroid carcinoma with the individual elements (UBB, CCH/MMC, MTC) demonstrated in this thyroid, and previous postulations that ultimobranchial bodies may be the source of some follicular thyroid cancers, medullary thyroid cancers, and mixed tumors of medullary and follicular epithelial types.