A case of metastatic haemangiopericytoma to the thyroid gland: Case report and literature review.

Haemangiopericytoma (HPC) is a mesenchymal neoplasm accounting for a minority of all vascular tumours. HPC mostly arises in the lower extremities and the retroperitoneum, while the head and neck area is the third most common site. The majority of HPCs are histologically benign. However, a small percentage possess atypical features, such as a high mitotic rate, high cellularity and foci of necrosis. We report a case of classical abdominal HPC that presented 7 years after the first surgical resection with thyroid metastases of malignant HPC. Microscopic examination revealed multiple hypercellular nodules with an infiltrative growth pattern. These nodules consisted of tightly packed fusiform or spindle-shaped cells with nuclear polymorphism and an increased mitotic rate. The tumour cells exhibited a marked expression of CD34. Cells were arranged around a prominent vascular network, occasionally with a 'staghorn' configuration. The results of this study support and confirm the theory that HPC is a rare neoplasm with unpredictable behaviour, as largely debated in the international literature. Therefore, this study emphasized the importance of applying strict diagnostic criteria in making the most appropriate diagnosis.