Cytology of malignant hemangiopericytoma.

Fine-needle aspiration findings in five cases of malignant hemangiopericytoma are described. The cytologic material in three cases showed clearly malignant cells with large, oval to spindle-shaped, hyperchromatic nuclei, scanty cytoplasm and many mitoses. One case had neoplastic cellular findings, but malignancy was only suspected. One case showed cytologically benign neoplastic cells. In all the cases the cytologic material contained capillaries with benign endothelial cells and some knob-like formations of neoplastic cells lined by endothelium. The diagnosis was verified from ordinary histologic sections and from semithin epoxy resin sections stained by a modified silver impregnation method to demonstrate the basal membrane and reticulin fibers. Three cases were studied electron microscopically. In our experience the described cytologic features are common but not necessarily characteristic of malignant hemangiopericytoma, since other mesenchymal tumors may also have similar features.