Microscopic description and IHC:
The tissue sections demonstrate a compact neoplasm dense in collagen with frequent areas of necrosis. The tumor cells show a moderate degree of pleomorphism, and show prominent nucleoli. A mitotic count is <1 mitoses in 10 high power fields. The tumor cells are diffusely positive for STAT6, and focally positive for S100 and EMA. Prominent staining for reticulin is noted. The tumor was negative for synaptophysin, Melan -1, HMB45, Sox 10, cytokeratin AE1/AE3 and Chromogranin.
Based on morphology and architecture, the tumor is best classified as Solitary fibrous tumor/ hemangiopericytoma, WHO Grade II.
Discussion:
Solitary fibrous tumor/ hemangiopericytoma are highly vascular usually dural based malignant neoplasm. The histological spectrum encompass to main morphologic variants: solitary fibrous tumor phenotype characterized by a pattern less architecture or short fascicular pattern with alternating hypo and hyper cellular areas with thick collagen bands; and hemangiopericytoma phenotype characterized by high cellularity and a delicate rich network of reticulin fibres investing individual cells. Hemangiopericytoma like vessels are shared by both phenotypes.
Outside the CNS, the term hemangiopericytoma has been subsumed into the designation “solitary fibrous tumor”. The criteria for malignancy outside CNS is hyper cellularity, necrosis and mitoses > 4 per 10 high power fields.
In the CNS, there is a three tiered system of grading. Grade I tumors are considered benign and treated by surgical resection. Grade II and III tumors are considered malignant and are treated by adjuvant radiotherapy. Classic solitary fibrous phenotype is considered to correspond histologically to grade I, whereas hemangiopericytoma phenotype are sub classified as grade II or grade III depending on mitotic count (<5 vs >= 5 mitoses per 10 high power fields.
Fibrous meningioma is a close mimic of SFT but typically express EMA and is negative for nuclear STAT6 expression.
Dural- based Ewing’s sarcoma/ peripheral primitive neuroectodermal tumor shares the hyper cellularity and CD99 positivity of hemangiopericytoma, but lacks nuclear STAT6 and has EWSR1 gene rearrangement in majority of cases.
Schwannoma occur at sino-dural angle and patients often present with hearing loss. The current case morphologically is not consistent with schwannoma.