Solitary fibrous tumor (SFT) is a rare fibroblastic mesenchymal neoplasm that is usually benign but has malignant potential. While pleural SFT is most well-known, retroperitoneal SFT represents an important subgroup of extrapleural SFTs. Typically seen between ages 40-70 without gender predilection. Histologically characterized by patternless pattern, collagen bands, and staghorn vessels (hemangiopericytoma-like); STAT6 and CD34 positive. NAB2-STAT6 fusion gene is pathognomonic and confirms the diagnosis. On imaging, it appears as a well-defined, intensely enhancing mass; low T2 signal (collagen) is distinguishing and shows the lowest T2 signal among retroperitoneal tumors. Paraneoplastic hypoglycemia (Doege-Potter syndrome — IGF-II overproduction) may occur in 5% of patients.
Age Range
30-70
Peak Age
50
Gender
Equal
Prevalence
Rare
SFT is a fibroblastic cell proliferation driven by NAB2-STAT6 gene fusion. This fusion results from inversion of two adjacent genes at the 12q13 locus leading to constitutive activation of STAT6 transcription factor — triggering cell proliferation. Histologically, collagen-rich stroma and dilated staghorn vessels are characteristic — staghorn vessels are described as a hemangiopericytoma-like pattern. The high density of collagen fibers forms the basis of SFT's low T2 signal — restricted water molecules between collagen fibers cause short T2 relaxation time. Staghorn vessels are responsible for the tumor's intense enhancement — these wide-lumened, thin-walled vessels provide dense perfusion allowing rapid contrast distribution into the tumor. In malignant transformation, mitotic activity increases (≥4 mitoses/10 HPF), necrosis develops, collagen ratio decreases and T2 signal rises — these imaging changes provide clues for early diagnosis of malignant SFT. Doege-Potter syndrome results from the tumor producing large amounts of IGF-II (insulin-like growth factor 2) — this peptide activates insulin receptors causing refractory hypoglycemia.
The most distinguishing finding of SFT is the coexistence of markedly low signal on T2 with intense homogeneous enhancement on post-contrast series. While the vast majority of retroperitoneal sarcomas show high T2 signal, SFT shows low T2 signal due to collagen content — this combination is highly specific for SFT and no other retroperitoneal tumor shows this profile.
On contrast-enhanced CT arterial phase, SFT shows intense, homogeneous enhancement. This intense enhancement reflects the tumor's rich staghorn vascularity. In large tumors, enhancement may be slightly heterogeneous but necrosis is rare in benign SFT. Feeding arteries and draining veins may be prominent. Calcification is rare and may appear centrally. Enhancement intensity may reach 100-150 HU — one of the most intensely enhancing retroperitoneal tumors.
Report Sentence
A well-defined, intensely homogeneously enhancing mass is seen in the retroperitoneal space; hypervascular tumor (SFT) should be primarily considered.
On T2-weighted MRI, SFT shows markedly low signal — isointense to or lower than muscle. This low T2 signal reflects the tumor's dense collagen content and is the lowest T2 signal among retroperitoneal sarcomas. In large tumors, myxoid degeneration or cystic areas may appear as T2 high signal foci but the dominant feature is low T2 signal. In malignant transformation, collagen ratio decreases and T2 signal rises — this change is an important warning for clinicians.
Report Sentence
The mass demonstrates markedly low signal on T2-weighted images consistent with collagen-rich tumor (SFT).
On contrast-enhanced MRI, SFT shows intense, nearly homogeneous enhancement. In large tumors, intratumoral flow voids (signal voids) may be seen — representing dilated staghorn vessels. Feeding arteries may be prominent. This combination of intense enhancement + flow voids is highly characteristic of SFT and suggests hypervascular tumor. Enhancement persists in delayed phase but intensity slightly decreases — slow washout pattern.
Report Sentence
The mass shows intense enhancement with intratumoral flow voids on contrast-enhanced MRI, consistent with hypervascular SFT.
On T1-weighted images, SFT shows isointense to slightly hypointense signal relative to muscle. Signal is homogeneous — hemorrhagic or fatty foci are not expected. This homogeneous T1 signal reflects the tumor's uniform tissue composition (collagen + fibroblasts) and shows similarity with benign neurogenic tumors but is differentiated by T2 and enhancement pattern.
Report Sentence
The mass shows homogeneous isointense signal to muscle on T1-weighted images.
On non-contrast CT, SFT appears as a well-defined, homogeneous, soft-tissue density (40-60 HU) mass. Calcification is rare and minimal with low diagnostic value. Necrosis is not expected in benign SFT — presence of necrosis suggests malignant transformation. The tumor typically displaces but does not invade surrounding structures — encapsulated or pseudoencapsulated growth reflects benign character.
Report Sentence
On non-contrast CT, a well-defined, homogeneous, soft-tissue density mass is seen in the retroperitoneal space.
On DWI, benign SFT usually shows no diffusion restriction or mild restriction — ADC values are in intermediate range (1.0-1.5 × 10⁻³ mm²/s). In malignant SFT, ADC may decrease (cellular density increases, collagen decreases). In collagen-dominant areas, T2 shine-through is minimal so DWI signal intensity remains low.
Report Sentence
Mild diffusion restriction with intermediate ADC values is seen in the mass.
Criteria
Low mitotic activity (<4 mitoses/10 HPF), no necrosis, well-defined, minimal nuclear atypia
Distinct Features
Homogeneous enhancement, markedly low T2, no necrosis, curative with surgery (recurrence <10%), Doege-Potter syndrome resolves after resection
Criteria
High mitotic activity (≥4 mitoses/10 HPF), necrosis, infiltrative margin, marked nuclear atypia, increased Ki-67
Distinct Features
Heterogeneous enhancement, T2 signal increases (collagen decreases, cellular density increases), necrotic areas, irregular margin, local recurrence 30-40% and distant metastasis 10-20%
Criteria
Development of anaplastic sarcoma on SFT background, high-grade morphological transformation, TP53 mutation frequent
Distinct Features
Prominent necrosis, high-grade sarcoma appearance, aggressive behavior, poor prognosis, abrupt transition from SFT area to dedifferentiated component may be seen
Distinguishing Feature
Leiomyosarcoma shows intermediate-high T2 signal, heterogeneous enhancement and necrosis; SFT shows low T2, homogeneous intense enhancement — opposite MR profiles
Distinguishing Feature
Castleman disease is usually smaller and in lymph node location, intense enhancement similar; SFT presents as extra-nodal retroperitoneal mass and is usually larger
Distinguishing Feature
Ganglioneuroma shows high T2 signal (myxoid stroma) and delayed enhancement; SFT shows low T2 (collagen) and early intense enhancement — exactly opposite profile
Distinguishing Feature
Hemangiopericytoma is now considered the same spectrum as SFT (WHO 2013); STAT6 nuclear positivity unifies both — imaging findings are identical
Urgency
routineManagement
surgicalBiopsy
NeededFollow-up
6-monthComplete surgical resection targeting R0 (negative surgical margin) is essential in retroperitoneal SFT treatment. Preoperative embolization may be applied to reduce surgical blood loss in hypervascular tumors — especially recommended in large tumors (>10 cm). Prognosis is excellent in benign SFT (recurrence <10%). In malignant SFT, local recurrence rate is 30-40% and distant metastasis risk is 10-20% — most commonly metastasizing to lung and liver. Doege-Potter syndrome (paraneoplastic hypoglycemia — IGF-II overproduction) rapidly resolves after resection — hypoglycemic episodes are treated with tumor resection. Close follow-up is required as late recurrences have been reported (even 10+ years later) — MRI follow-up every 6-12 months is recommended and adjuvant radiotherapy may be considered based on surgical margin status. Chemotherapy options in malignant SFT are limited but antiangiogenic agents (pazopanib, bevacizumab + temozolomide) may show response.
SFT generally follows a benign course but 10-15% show malignant behavior. Malignancy criteria: size >10 cm, high mitotic index, necrosis, and rapid growth. Complete surgical resection is curative. Doege-Potter syndrome (hypoglycemia) is a rare paraneoplastic finding.