Oxyphil and squamous variants of medullary thyroid carcinoma.

Two unusual morphologic variants of medullary thyroid carcinoma not previously described are reported. The first tumor was composed predominantly of large eosinophilic cells indistinguishable at the light microscopic level from Hürthle cells. Focal areas of conventional medullary carcinoma were also present. Electron microscopic study showed mitochondrion-rich cells containing round neurosecretory granules. Immunoreactive calcitonin and carcinoembryonic antigen (CEA) were demonstrated within most tumor cells. The second tumor had areas of typical medullary carcinoma but exhibited extensive squamous differentiation. The tumor was positive for CEA, neuron-specific enolase, and chromogranin but negative for calcitonin. Oxyphil and squamous cells must be added to the long list of cell types that have been described in medullary thyroid carcinoma.