This video presents a detailed pathological examination of a Solitary Fibrous Tumor (SFT). The presenter reviews the classic morphological features, including the presence of highly characteristic, dilated, branching "staghorn" vessels, and the variable cellularity presenting as alternating zones of hypocellular sclerosis and hypercellular sheets of bland, oval-to-spindle cells arranged in a "patternless pattern." Historically, SFTs were referred to as Hemangiopericytomas (HPC), a term now largely restricted to CNS tumors. The video emphasizes that while SFT is classically CD34-positive, this marker is highly non-specific. Definitive diagnosis relies on the tumor's underlying NAB2-STAT6 gene translocation, which is most reliably confirmed using the sensitive and specific nuclear STAT6 immunostain, rather than molecular FISH testing.
Pathology of Solitary Fibrous Tumor (SFT)
0:02 Historical Terminology: SFT was formerly known as Hemangiopericytoma (HPC). The HPC term is generally obsolete in soft tissue pathology (except for rare sinonasal tumors) but remains in use within the Central Nervous System (CNS).
0:13 Characteristic Vasculature (Non-Specific): A key feature of SFT is the presence of dilated branching "staghorn" shaped vessels. Key Takeaway: This vascular pattern is not specific to SFT and can be seen in other tumors, such as synovial sarcomas and dermatofiber sarcoma protuberans (DFSP).
1:09 Variable Cellularity: The tumor shows a wide range of cellularity, alternating between hypercellular zones (more blue/cells) and hypocellular sclerotic zones (more pink/collagen).
2:16 Cellular Morphology: The cells are typically bland, plump, and oval-to-spindle shaped, embedded in abundant collagen. The vessels often feature thick, sclerotic walls.
3:05 Malignancy Risk: High-risk or malignant forms of SFT are suggested by features such as pleomorphism, high mitotic counts, or necrosis.
3:20 "Patternless Pattern": The bland cells are classically described as being arranged haphazardly in a "patternless pattern," though occasional short fascicles or palisaded areas may be observed.
3:55 Immunomarkers (CD34): SFT is classically CD34 positive. Key Takeaway: CD34 is non-specific and found in many fibroblastic and vascular tumors, requiring caution in interpretation.
4:27 Genetic Basis: SFTs are driven by the NAB2-STAT6 gene translocation.
4:44 Definitive Diagnosis (STAT6): The STAT6 nuclear immunostain is the most sensitive and specific marker for SFT and is preferred for diagnostic confirmation over molecular techniques like FISH for the fusion gene.