Primary clear cell thyroid carcinoma with squamous features.

A clear cell neoplasm of the thyroid with indistinguishable light microscopic features and similar tinctorial reactions for lipid and glycogen to those of clear cell renal carcinoma is described. However, electron microscopic examination disclosed cytoplasmic tonofibrils and keratohyaline granules as well as suggestive transitions between the tumor cells and acinar epithelium. The former appear to represent helpful "markers" for the distinction of this type of thyroid cancer from parathyroid or metastatic renal clear cell tumors. There is some evidence that not all clear cell thyroid neoplasms possess such squamous features. Nevertheless, it is apparent that electron microscopy may be necessary for the pathological discrimination of primary and secondary clear cell neoplasms of the thyroid, a situation analogous to that for clear cell tumors of other sites such as the lung.