The histological spectrum of hemangiopericytoma: application of immunohistochemical analysis including proliferative markers to facilitate diagnosis and predict prognosis.

Hemangiopericytoma (HPC) is an uncommon vascular neoplasm thought to be derived from pericytes. Prediction of patient outcome is difficult based what is currently known about these tumors and histological parameters alone. We compiled 27 cases of HPC and evaluated the spectrum of histological features to investigate whether there was any correlation between histology, immunostaining, prognostic markers, and patient outcome. The following parameters were evaluated: vasculature, histological pattern (solid, myxoid, trabecular, alveolar), degree of cellular pleomorphism, necrosis, mitoses, and giant cell content. Immunohistochemistry was performed to determine the reactivity for CD 31, CD34, vimentin, actin, cytokeratin, S100, actin, and SMA. Proliferative rate was analyzed using antibodies to PCNA and MIB1. Patient's age ranged from 8 months to 75 years (mean, 35; median, 31). Twenty of 27 cases were located in the extremities. The tumors were grossly described as lobulated and well circumscribed (n=12) and nonencapsulated (n=15). By histology, the characteristic ramifying or staghorn vasculature pattern was seen in all cases. A solid histological pattern was mixed with an alveolar pattern in three cases, trabecular pattern in six cases, and myxoid pattern in two cases. Tumor cells were uniform, polygonal to spindle-shaped, often with vesicular nuclei. Tumor giant cells were present in 9 of 27 cases; necrosis, in 11 of 27. Mitoses ranged from 0 to 14 per 10 high-power fields (HPF). Cellular pleomorphism was 1+ in nine cases, 2+ in 12 cases, and 3+ in six cases. Immunohistochemistry showed reactivity for CD34 and vimentin in all cases. Actin was focally positive in one case, and SMA was focally positive in another. CD 31, cytokeratin, and S100 were uniformly nonreactive. Proliferative index measured by PCNA and MIBI ranged between less than 1% and 40% of tumor cells. Follow-up was available in 22 cases and ranged from 1 year to 15 years. Seven patients had metastases, and two recurred locally. Thirteen patients had no evidence of disease at last checkup. Parameters associated with recurrences or metastases include a trabecular pattern, the presence of necrosis, mitoses, vascular invasion, and cellular pleomorphism. Features associated with an aggressive biological behavior can be identified histologically. There was some, but not total, correlation between proliferative markers and tumor aggressiveness.