Small cell carcinoma of the thyroid. A reclassification of cases originally diagnosed as small cell carcinomas of the thyroid.

99 cases of undifferentiated carcinomas of the thyroid and nine cases of primary malignant Non-Hodgkin lymphomas of the thyroid were examined from 1967 to 1983 in our institute. Among the undifferentiated carcinomas nine cases were classified as small cell subtype. Over the years, the histopathological handling in regard to small cell subtype of undifferentiated carcinoma and primary malignant Non-Hodgkin lymphoma has changed. The frequency of primary malignant Non-Hodgkin lymphoma has increased conspicuously in the last few years, whereas the number of small cell carcinomas decreased. A reclassification, based on immunohistochemical investigation for tumor markers of the nine cases originally diagnosed as small cell carcinomas from 1967 to 1983 revealed that five cases were poorly differentiated carcinomas or undifferentiated carcinomas of the spindle cell type. In three cases the primary diagnosis had to be revised into malignant Non-Hodgkin lymphoma of the diffuse "histiocytic" type. The postmortem examination of the patient with the remaining small cell carcinoma "of the thyroid" revealed a clinically undetected small cell carcinoma of the lung with metastases to the cervical lymph nodes and the thyroid gland. These findings are in agreement with the results of several recently published papers indicating that true small cell carcinoma of the thyroid must be a very rare tumor.