Canine spindle cell tumor mimicking human classical hemangiopericytoma

The neoplastic mass developed in the left flank of a Border Collie dog. The tumor was resected surgically and evaluated histologically and immunohistochemically. Histologically a moderate number of spindle cells were proliferated with staghorn, placentoid, and myxoid growth patterns and a lack of perivascular whirling. Immunohistochemically, the tumor cells were positive to vimentin, laminin, S-100 protein, CD34 and CD117 antibodies. They were negative to cytokeratin AE1/3, desmin, α-SMA and calponin antibodies. Endothelial cells of the staghorn channels were positive for vWF antibody. The present case was diagnosed as spindle cell tumor, but it was similar to human classical hemangiopericytoma (HEP) and canine HEP classified by Avallon and others.