Malignant haemangioendothelioma of the thyroid, immunohistochemical evidence of heterogeneity.

26 cases of malignant haemangioendothelioma (MHE) of the thyroid gland were investigated immunohistochemically with the endothelial marker UEA-1 lectin and the panepithelial marker Lu-5. The results were compared with the results of staining for factor VIII-related antigen in the same cases observed in a previous study of Pfaltz et al. The 26 cases were classified on light microscopic grounds without reference to the immunohistochemical results as classical MHE (15 cases) and borderline cases intermediate between MHE and undifferentiated carcinoma (11 cases). 7 of the 15 classical MHE revealed one or both of the vascular markers, but did not express the epithelial marker. One case showed no staining and another reacted only with Lu-5. Vascular and epithelial markers were found in 6 cases of the 15 classical MHE and in 2 of the 11 borderline cases. These findings indicate that MHE of the thyroid may represent a heterogeneous group of lesions. Tumours positive only for endothelial markers strongly support the hypothesis that MHE is of endothelial origin, whereas tumours which reacted only to the epithelial marker may be undifferentiated carcinomas. Cases with both epithelial and endothelial features on immunohistochemical investigation may represent either tumours in which the malignant cells are in transition from epithelial to mesenchymal differentiation as suggested by Eckert et al. or are tumours of malignant endothelial cells with epithelial differentiation particularly of their cytoskeleton.