Undifferentiated thyroid tumors of diffuse small cell type. Histological and immunohistochemical evidence for their lymphomatous nature.

A histological review of 72 undifferentiated thyroid tumors was performed in order to discover small cell anaplastic carcinomas and Non-Hodgkin lymphomas. Cases suspected to be lymphoma were examined for the presence of Ig and keratin and lectins with a PAP-procedure. Among the 72 cases, 68 (94,5%) were anaplastic carcinomas of various types. Four cases (5,5%) were diffuse small cell tumors, which had previously been regarded as anaplastic carcinomas. All four could be identified as Non-Hodgkin lymphomas by histology, immunohistochemistry, repeat biopsy or autopsy. The findings suggest that the majority of small cell anaplastic thyroid tumors are lymphomas and that true anaplastic small cell carcinoma of the thyroid must be extremely rare. Its diagnosis requires electronmicroscopy and/or immunohistochemistry to demonstrate the epithelial nature of tumor cells.