What is the pathology of medullary thyroid carcinoma (MTC)?

Updated: May 14, 2020
  • Author: Pramod K Sharma, MD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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On gross examination, MTCs are fairly well circumscribed, though they are unencapsulated. They are typically tannish pink and often contain yellow granular regions, which represent focal calcification. Most tumors arise in the middle and upper third of the thyroid lobes, commensurate with the location of the parafollicular C cells in the thyroid gland. Sporadic tumors are unilateral, and inherited forms usually involve both thyroid lobes.

MTCs can have a varied microscopic appearance. The tumors typically have a lobular, trabecular, insular, or sheetlike growth pattern. Some tumors have a fibrotic character. Malignant cells may appear round, polygonal, or spindle shaped. The cytoplasm is eosinophilic and finely granular. In the stroma, characteristic deposits of amyloid are commonly observed. This amyloid has typical green birefringence on Congo red staining, and this is a feature unique to MTC among thyroid malignancies. Immunohistochemical stains for calcitonin and carcinoembryonic antigen are microscopically useful for differentiating MTC from other tumors.

A unique feature to the familial cases of MTC is the finding of C-cell hyperplasia, which can help in distinguishing familial cases from sporadic ones. C-cell hyperplasia is considered a precursor to MTC and is usually adjacent to foci of MTC. The finding of C-cell hyperplasia with MTC should raise the suspicion for familial disease.

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