Unusual findings in papillary thyroid microcarcinoma suggesting partial regression: a study of two cases.

Histologic changes of regression have been recognized in many malignant tumors, although they have not been documented in thyroid carcinoma. We analyze here the histologic features of 2 cases of papillary thyroid microcarcinoma, one of the diffuse sclerosing variant and another multicentric of the follicular variant, that suggest partial regression. The histologic and clinical features of the 2 thyroid microcarcinomas were compared with the features of regression reported in other tumors. Hematoxylin and eosin stains were examined in each case. In addition, immunoperoxidase stains for thyroglobulin and thyroid transcription factor-1 were performed on the metastatic and primary lesions of each case. Immunostains for CD4 and CD8 performed on the thyroid lesions were also examined. Both patients presented with metastatic papillary thyroid carcinoma in regional lymph nodes with no clinical or radiologic evidence of primary tumor in the thyroid glands. When examined, the thyroid glands revealed only microscopic foci of papillary thyroid carcinoma measuring less than 1.5 mm, diffuse sclerosis and a lymphocytic infiltrate mainly composed of cytotoxic T lymphocytes around the neoplastic microfollicles. Numerous psammoma bodies were also found in the thyroid of case 1, and venulitis similar to that seen in acute hepatic rejection was also seen in the microcarcinoma of case 2. The 2 patients reported had papillary thyroid microcarcinomas measuring less than 1.5 mm and features suggestive of regression characterized by extensive fibrosis, an accompanying T lymphocyte infiltrate, venulitis, and only a few clusters of neoplastic cells or a few neoplastic follicles. Thus, we believe that these histologic changes suggest regression because papillary thyroid microcarcinomas measuring less than 1.5 mm do not metastasize and have been considered innocuous or medical curiosities.