Anaplastic thyroid carcinoma with osteosarcomatous differentiation.

A case of a thyroid tumour with the cytological and histological pattern of anaplastic carcinoma with extensive osteosarcomatous differentiation in a 54-year-old Kaukasian woman is presented. Immunohistochemical examination revealed keratin-vimentin co-expression in anaplastic tumour areas. According to the WHO classification of thyroid tumours the present tumour has to be classified as an anaplastic carcinoma. A retrospective survey revealed only twenty-four comparable cases in the literature. The present tumour most likely represents an example of a neoplastic epithelial-mesenchymal metaplasia. The possible mechanisms of the occurrence of thyroid tumours with mixed epithelial-mesenchymal differentiation are briefly discussed.