This video presents a detailed pathological analysis of Case 5: a Solitary Fibrous Tumor (SFT) presenting as a circumscribed thigh mass in a 60-year-old man. The discussion centers on the classic microscopic features, particularly the pathognomonic but non-specific "Staghorn vessel pattern" (formerly associated with hemangiopericytoma).
Histologically, the tumor exhibits variable cellularity, ranging from cellular zones to hypocellular, sclerotic areas rich in rope-like collagen, which often coats the dilated vessels. The cells themselves are plump, oval, and cytologically monotonous, consistent with the underlying NAB2-STAT6 fusion translocation. The definitive diagnostic marker is the strong nuclear expression of STAT6, superseding the use of less specific markers like CD34. The video concludes by emphasizing the importance of modern risk stratification (low, medium, high risk) for SFT prognosis, which factors in tumor size, mitotic activity, necrosis, and patient age, replacing the older, less reliable reliance solely on mitotic count.
Solitary Fibrous Tumor: Pathology and Diagnosis
0:09 Case Introduction: The tumor is a sharply demarcated thigh mass from a 60-year-old patient, presenting classic features of a Solitary Fibrous Tumor (SFT).
0:24 Key Vascular Pattern: The most noticeable feature is the presence of dilated, branching vessels known as the "Staghorn vessel pattern" (or antler-like). This pattern historically defined hemangiopericytoma (now an obsolete term).
1:24 Non-Specific Pattern: While classic for SFT, the Staghorn vessel pattern is not specific and can also be observed in other soft tissue tumors, including synovial sarcoma.
1:49 Cellular Features: SFT cells are generally plump, oval, or fusiform spindle cells. They exhibit uniform cytologic features, often arranged haphazardly in a "patternless pattern."
2:51 Molecular Diagnosis: SFT is defined by the NAB2-STAT6 fusion translocation.
3:04 Immunohistochemistry (IHC): The most helpful diagnostic stain is STAT6, which shows strong, crisp nuclear staining in the vast majority of SFTs. They are also often positive for CD34 (sensitive but non-specific).
4:46 Variable Cellularity: The tumor shows zones of varying cellularity, including hypocellular, sclerotic areas that are rich in dense, rope-like bundles of collagen, particularly surrounding the dilated blood vessels.
7:37 Prognostic Shift: The behavior of SFT does not fit traditional benign/malignant terminology. The older system relied purely on mitotic count (>4 mites/10 HPF), which the speaker found arbitrary.
8:36 Modern Risk Stratification: Current practice uses a risk stratification model (low, medium, and high risk) incorporating four key factors: tumor size, mitotic activity, necrosis, and patient age (age over 55 is a factor toward higher risk).
9:24 Clinical Behavior: Low-risk SFTs behave essentially like benign tumors, while high-risk SFTs behave like high-grade sarcomas with a significant chance of metastasis.
11:51 Differential Diagnosis: Variants of SFT (those with abundant myxoid change or adipose tissue) must be distinguished from myxoid liposarcoma or spindle cell lipoma (SCL). SCL usually shows loss of RB1 expression and is negative for STAT6.