Mesenchymal chondrosarcoma of the thyroid -- a rare tumour at an unusual site.

We describe the light microscopic, immunohistochemical and ultrastructural features of the first case in the literature of a primary mesenchymal chondrosarcoma (MC) of the thyroid and discuss its differential diagnosis at unusual extraskeletal sites. A nodular lesion of the thyroid with no evidence of extrathyroid disease showed the bimorphic pattern and haemangiopericytoma-like areas typical of MC. In the undifferentiated areas, the cells were CD99 positive/CD117 negative, while the stroma showed focal positivity for alpha-inhibin. In spite of its rarity, it is important to diagnose primary mesenchymal chondrosarcoma in a parenchymatous organ such as the thyroid because its biological behaviour may be different from that of tumours of similar morphology and complete resection is the treatment of choice. The patient is free of disease nearly 66 months after its first presentation. Cytogenetic and immunohistological markers may play important roles in diagnosis of this lesion in future, especially with limited tissue samplings; however, for the present a thorough sampling of the tumour remains the best diagnostic strategy.