ABSTRACT
Haemangiopericytomas are cellular tumours composed of monotonous oval to spindle-shaped cells with inconspicuous borders, dense chromatin and no intranuclear pseudoinclusions (Figure 37.1b). The characteristic vascular pattern features numerous branched thin-walled blood vessels lined by a single layer of flat endothelium, likened to a deer antler or ‘staghorn’ appearance (Figure 37.1c). A reticulin-rich network surrounding individual tumour cells is usually present, though with significant intertumoural and intratumoural variability.44 Focal hyalinization and myxoid change may be present, but is more characteristic of SFT. Mitotic activity is variable. The diagnosis of anaplastic HPC (WHO grade III) requires a mitotic rate in excess of 5 per 10 high-power fields and/or necrosis plus at least two of the following: haemorrhage, at least moderate nuclear atypia, and hypercellularity (Figure 37.1d).39
