Answer: Malignant solitary fibrous tumor

Histology: The tumor has essentially two components. One is typical of usual solitary fibrous tumor. This are shows variable cellularity with extensive collagenous sclerosis. Dilated staghorn blood vessels are present. The tumor cells are non-pleomorphic and spindled. This area merges with a highly cellular area, which shows pleomorphism, mitotic activity and necrosis. These features qualify the lesion as a malignant solitary fibrous tumor. The tumor was immunoreactive for CD34 but not for CD117 (c-kit).

Discussion: A schwannoma shows alternating cellularity of spindled tumor cells but is characteristically strongly and diffusely immunoreactive for S100 protein and not for CD34. Gastrointestinal stromal tumors label consistently with CD34 but typically lack the hyaline sclerosis and alternating cellularity characteristic of the current tumor. Gastrointestinal stromal tumors label more consistently with c-kit (CD 117). This lesion meets definitional criteria for malignant solitary fibrous tumor in that is shows high cellularity and a mitotic rate of over 4 per 10 high power fields. The diffuse CD34 reactivity is helpful in confirming the diagnosis and excluding other entities in the differential diagnosis such as synovial sarcoma.