[Hyalinizing trabecular adenoma of the thyroid gland. Histologic and immunohistochemical study. Report of 2 cases]

Hyalinizing trabecular adenoma of the thyroid gland is an uncommon neoplasm that was recently described by Carney et al. (1987). It is important to recognize this tumor, considered benign, as it may mimic papillary carcinoma, medullary carcinoma or paraganglioma. We present two cases with histological and immunohistochemical criteria and discuss the diagnostic problems. Grossly, this small tumor is yellow tan and well circumscribed. The epithelial cells, polygonal or elongated and sharply outlined, have an eosinophilic or clear cytoplasm. The nuclei are oval or elongated, with often non-visible nucleoli. Grooved nuclei are quite frequent. Mitotic figures are very uncommon. The cells are arranged in clusters or trabeculae or pseudofollicles containing colloid material. Characteristically the cells, arranged shoulder to shoulder, can show a palisade pattern. The fibrovascular stroma appears hyaline, pseudoamyloid (Congo red staining is negative). The tumor cells show intense staining for thyroglobulin and no staining for thyrocalcitonin. Immunoreactivity for cytokeratin, vimentin, protein S100, NSE is positive but staining is negative for EMA and desmin. In one case, the tumor cells show little positivity for chromogranin A. The variable patterns of this tumor can lead to diagnostic problems. We can exclude the diagnosis of papillary carcinoma as this tumor lacks any true papillary architecture. We reject the diagnosis of medullary carcinoma because of its thyroglobulin and calcitonin immunoreactivity. Paraganglioma never shows immunoreactivity for thyroglobulin. Very often the surrounding thyroid tissue shows focal thyroiditis. The association of the tumor with Hashimoto's thyroiditis can be explained by an autoimmune process.