Morphologic Features

Grossly, the hemangiopericytoma is a well-circumscribed lesion. The lesion is red and has an appearance similar to cavernous hemangioma. The tumor may or may not have a capsule surrounding the tumor. Histopathologically, the tumor is composed of pericytes with ill-defined borders, large cytoplasm, and round to oval nuclei. Pericytes usually are ovoid, but spindle cells can also be seen. The his-topathologic pattern is mixed and even varies between different zones in a tumor.57,59 These cells surround the sinusoidal or staghorn-shaped blood channels lined by a single row of flattened endo-thelial cells. The pericytes are surrounded by a re-ticulin framework. The reticulin deposits surround individual cells in hemangiopericytoma. This is in contrast to other vascular tumors, in which the re-ticulin framework surrounds a cluster of cells, not each cell individually.57

The pericyte does not react with CD31, factor VIII related antigen, and/or Ulex europaeus agglutinin I, which stain the endothelial cells. However, it does react with vimentin and CD34 (Figure 14.12).66

FIGURE 14.13. Massive proptosis and inferior dislocation of the right globe secondary to large hemangiopericytoma extending into the cranium, perinasal sinuses, and nasal cavity. The histopathol-ogy revealed sarcomatous changes with high mitotic rate (6 per HPF).

Croxatto and Font counted the number of mitotic figures per 40 high power fields (HPF) and found that in the benign group of tumors there were 4 mitotic figures per HPF and in the borderline group there were 14 mi-totic figures per HPF. The malignant tumors averaged 35 mitotic figures per HPF.59 Hemangiopericytomas that are capable of recurrence and metastasis usually have more than 4 mitotic figures per 10 HPF. However, hemangiopericytomas with fewer than 4 mitotic figures per HPF can also demonstrate malignant potential.59 Other histopathologic features that may indicate malignant potential are cellular anaplasia, foci of necrosis and hemorrhage, increased cellularity, and scarcity of reticulin fibers.59 However, the biologic behavior of he-mangiopericytoma cannot be reliably predicted from the histopathologic appearance of the tumor.

Ultrastructurally, the hemangiopericytoma demonstrates different cell types. The pericyte-like cell is the type most frequently encountered and is considered to be diagnostic of hemangiopericytoma in transmission electron microscopic studies (Figure 14.12).57

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