Malignant lymphomas and undifferentiated small cell carcinoma of the thyroid: a clinicopathological review in the light of the Kiel classification for malignant lymphomas

The paper reports a re‐evaluation—based on the Kiel classification for non‐Hodgkin lymphomas—of a group of cases initially diagnosed as undifferentiated small cell carcinomas or primary lymphomas of the thyroid. Twelve such cases were found among the 155 cases of primary malignant tumours of the thyroid recorded at the Institut Jules Bordet between 1955 and 1975. The review of the clinical charts and the histology showed that all the cases were in fact malignant lymphomas fitting easily into one of the groups described in the Kiel classification. These findings support the growing opinion that undifferentiated small cell carcinoma of the thyroid does not exist as a distinctive clinicopathological entity. Furthermore, the Kiel classification proved to be an excellent prognostic indicator, since all the cases classified as highly malignant were indeed fatal, whereas the surviving cases—three of which had shown tumoral extension beyond the thyroid capsule—fell into the group of low malignancy. Lastly, this study acknowledges the frequently observed association of malignant lymphoma of the thyroid with stigmata of Hashimoto's disease, and thus supports the concept that the continuous antigenic stimulation observed in the latter could trigger the development of a malignant lyphoma.