Small cell malignant tumors of the thyroid. A light and electron microscopic study.

Three small cell malignant tumors involving the thyroid gland were examined by light and electron microscopy. In each, numerous similarities were presented on light microscopic analysis rendering interpretation difficult. In the first patient the small cells were identified on electron microscopy as moderately well differentiated lymphocytes. This tumor occurred in an elderly woman as a locally invasive thyroid tumor, subsequently involving distant sites, including the liver, spleen, lymph nodes, and soft tissue. The tumor repeatedly regressed following radiotherapy. The patients eventually died with disseminated lymphocytic lymphoma. In the second patient the thyroid tumor, also locally invasive, was composed principally of neoplastic epithelial cells when studied on electron microscopy. This patient responded poorly to radiation and died within a year after diagnosis. The third patient presented with an enlarging thyroid mass, which ultrastructurally was found to be composed principally of well differentiated lymphocytes. Subsequent clinical evaluation established a diagnosis of chronic lymphocytic leukemia. Small cell malignant tumors of the thyroid represent a difficult diagnosis problem for surgical pathologists. Ultrastructural study, we believe, is a useful adjunct in the differentiation of these tumors.