Imaging Studies
In addition to a complete physical examination, all patients should undergo local imaging (including intravenous contrast-enhanced magnetic resonance imaging [MRI] and plain radiography), as well as chest radiography or computed tomography (CT). [14] A solitary fibrous tumor (SFT) demonstrates a heterogeneous appearance on MRI, with high signal on T2-weighted series and intermediate to moderately high signal on T1-weighted images. [15] (See the images below.) Cellular SFTs also show rapid initial enhancement on dynamic contrast-enhanced MRI. [16]
MRI of solitary fibrous tumor demonstrates a deep soft-tissue mass in the anteromedial aspect of the thigh with heterogenous high signal intensity on coronal STIR imaging.
Similar signal intensity as skeletal muscle on precontrast axial T1-weighted fat-suppressed turbo-spin echocardiography.
SFTs are generally well-circumscribed. Small satellite nodules separate from the main lesion are not uncommon, and 12% of patients may present with metastasis or locally recurrent tumors. [5] No distinctive radiologic criteria for malignancy are recognized, other than distant metastasis.
Biopsy
Biopsy should be performed by a musculoskeletal oncologist. If the mass is palpable and accessible, a core needle biopsy may be an alternative to open incisional biopsy. The fibrous nature, potential low cellularity, and usually deep location of the mass may not allow fine-needle aspiration biopsy (FNAB). The biopsy should be placed in line with the incision for definitive tumor resection to allow en-bloc resection of the biopsy tract. Adequate tissue should be acquired to allow for histologic analysis, molecular genetics, [17] and immunohistochemistry (IHC). [18]
Histologic Findings
SFTs must be carefully distinguished from other entities, such as synovial sarcoma and angiosarcoma, which have a worse prognosis and a different treatment course.
The cell of origin for SFTs is controversial. [3] Fibroblasts are elongated, spindle-shaped cells with long processes; on electron microscopy, they are devoid of a basement membrane and demonstrate a prominent rough endoplasmic reticulum. Pericytes also have elongated cell processes and are spindle-shaped. They are contractile and surround capillaries and postcapillary venules. However, few tumors formerly labeled as hemangiopericytomas (HPCs) had actual pericytic differentiation, and most had nonspecific ultrastructural features. [1]
On histology, one can appreciate a staghorn-type branching of vessels, a bland cell morphology, and positive staining for CD34. (See the images below.)
Histology of solitary fibrous tumor demonstrates a bland cell morphology and staghorn-type branching of vessels on low magnification of hematoxylin and eosin stained slides. Image courtesy of Paul E. Wakely, Jr, MD.
Histology of solitary fibrous tumor demonstrates a bland cell morphology and staghorn-type branching of vessels on high magnification of hematoxylin and eosin stained slides. Image courtesy of Paul E. Wakely, Jr, MD.
Before the use of IHC, HPC and SFT were commonly confused with synovial sarcoma, particularly monophasic variants. Epithelioid differentiation can help distinguish biphasic synovial sarcomas. Approximately 90% of synovial sarcomas demonstrate the chromosomal translocation t(X;18)(p11.2;q11.2) and the fusion gene SYT-SSX1 or SYT-SSX2, detectable with cytogenetics and reverse transcription polymerase chain reaction (RT-PCR). [17] Nuclear STAT6 immunoreactivity can be helpful in cases where the diagnosis is uncertain. [18, 19]
Staging
SFTs are generally regarded as benign but can demonstrate aggressive behavior even if they appear morphologically benign. Histologic criteria for malignancy include the following [3, 9, 7] :
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Cellular pleomorphism
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Tumor necrosis
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Hemorrhage
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Infiltrative margins with surrounding tissues
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Hypocellularity
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Moderate-to-severe nuclear atypia
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Sharply demarcated anaplastic or poorly differentiated foci
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High mitotic count (>4 mitoses/10 hpf)
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MRI of solitary fibrous tumor demonstrates a deep soft-tissue mass in the anteromedial aspect of the thigh with heterogenous high signal intensity on coronal STIR imaging.
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Similar signal intensity as skeletal muscle on precontrast axial T1-weighted fat-suppressed turbo-spin echocardiography.
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Intense uptake on postcontrast comparison images.
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Histology of solitary fibrous tumor demonstrates a bland cell morphology and staghorn-type branching of vessels on low magnification of hematoxylin and eosin stained slides. Image courtesy of Paul E. Wakely, Jr, MD.
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Histology of solitary fibrous tumor demonstrates a bland cell morphology and staghorn-type branching of vessels on high magnification of hematoxylin and eosin stained slides. Image courtesy of Paul E. Wakely, Jr, MD.
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Positive staining for CD34. Image courtesy of Paul E. Wakely, Jr, MD.
