Retroperitoneal hemangiopericytoma (HPC) is a rare vascular tumor arising from pericytes (Zimmerman cells). Since WHO 2013, it is considered the same spectrum as solitary fibrous tumor (SFT) — both show NAB2-STAT6 fusion and nuclear STAT6 positivity, which is the gold standard for diagnosis. It is a hypervascular tumor; the 'staghorn' vessel pattern on histology is a diagnostic feature, and this vascular structure is responsible for intense enhancement and prominent feeding arteries on CT/MRI. Typically in ages 40-60 with low malignancy potential, and may be slightly female-predominant. Hypoglycemia (Doege-Potter syndrome — paraneoplastic IGF-II production) may rarely accompany. Preoperative embolization significantly reduces surgical blood loss.
Age Range
30-60
Peak Age
45
Gender
Equal
Prevalence
Rare
HPC originates from pericytes (Zimmerman cells) in capillary vessel walls. Pericytes are contractile cells surrounding endothelial cells that regulate vascular wall tone; they express contractile proteins such as actin and desmin. NAB2-STAT6 gene fusion is pathognomonic, resulting from paracentric inversion at the 12q13 locus — this fusion protein causes constitutive STAT6 activation driving cell proliferation. This genetic aberration shared with SFT is the molecular evidence that both tumors are in the same spectrum. The tumor shows dense 'staghorn' vascularity — non-branching, dilated, thin-walled vessels surrounded by pericytes are characteristic. This vascularity provides the tumor's very high perfusion capacity and is responsible for intense early enhancement and prominent feeding arteries on CT/MRI. Tumors are usually well-circumscribed with a fibrous pseudocapsule. In malignant HPC, necrosis, high mitotic activity (>4 mitoses/10 HPF), infiltrative margins, and cellular atypia develop — these features correlate with aggressive biological behavior and metastasis risk. Doege-Potter syndrome pathognomonic mechanism: the tumor produces large amounts of IGF-II → IGF-II activates insulin receptors → hypoglycemia develops.
The triad of intense arterially enhancing mass + prominent feeding arteries + early venous drainage is the most characteristic imaging finding for HPC/SFT spectrum. This triad reflects the hypervascular network of staghorn vessels, low-resistance arteriovenous connections, and the tumor's high perfusion capacity, not seen in this combination in any other benign retroperitoneal tumor.
Intense homogeneous enhancement in arterial phase with enhancement intensity measurable in the 100-160 HU range. Large feeding arteries appear as tortuous, dilated vessels at the mass periphery, and early venous drainage (AV shunt pattern) is evidenced by opacification of draining veins in arterial phase. This hypervascular pattern is the most characteristic CT finding of HPC/SFT spectrum and provides an important clue in the differential diagnosis of retroperitoneal masses. Necrosis is not expected in benign form; necrosis suggests malignant HPC. Slight heterogeneity may be seen in large tumors (>10 cm) but the dominant pattern is homogeneous enhancement.
Report Sentence
An intensely homogeneously enhancing hypervascular mass is seen in the retroperitoneal space with prominent feeding arteries and early venous drainage; HPC/SFT spectrum should be considered.
Shows intermediate-to-high signal on T2, and unlike SFT, T2 signal is generally higher because HPC has less collagen and more cellular component. Intratumoral flow voids (dilated staghorn vessels) are characteristically seen as signal voids on T2-weighted images confirming hypervascularity. Necrosis and cystic change are not expected in benign form; presence of such findings should suggest malignant transformation. Tumor borders are usually smooth and a pseudocapsule may be seen as a thin hypointense band on T2.
Report Sentence
The mass shows intermediate-to-high T2 signal with multiple intratumoral flow voids; consistent with hypervascular tumor.
Intense homogeneous enhancement on contrast-enhanced MRI with rapid and intense gadolinium accumulation. Dynamic contrast-enhanced sequences show early arterial enhancement (rapid wash-in) pattern. Feeding arteries and early draining veins can be prominently evaluated on MR angiography, and this information is critically important for surgical and embolization planning. Enhancement pattern is concordant with CT findings supporting hypervascular tumor. Persistent enhancement may be seen in delayed phase.
Report Sentence
The mass shows intense enhancement with prominent feeding arteries and arteriovenous shunt findings on contrast-enhanced MRI; consistent with hypervascular tumor.
Well-defined, homogeneous, soft tissue density (40-60 HU) mass on non-contrast CT. Calcification is rare, reported in 5-10% of HPC — this rate is significantly lower than the 20-30% calcification rate in Castleman disease. Necrosis is not expected in benign form; presence of necrotic areas suggests malignant HPC. Mass contours are smooth and pseudocapsule may be clearly visible. No invasion of surrounding tissues in benign form.
Report Sentence
A well-defined, homogeneous soft tissue density mass is seen on non-contrast CT with discernible pseudocapsule.
May show mild-to-moderate diffusion restriction on DWI. ADC values in the intermediate range (1.0-1.5 x 10⁻³ mm²/s), higher than the markedly low ADC values of lymphoma (0.4-0.8 x 10⁻³ mm²/s). This intermediate ADC reflects the moderate cellular density of HPC pericyte cells. In malignant HPC, cellular density increases and ADC values decrease → more pronounced diffusion restriction is seen.
Report Sentence
Moderate diffusion restriction is seen in the mass (ADC 1.0-1.5 x 10⁻³ mm²/s) with higher ADC values than lymphoma.
Shows isointense or slightly hypointense signal to muscle on T1. Homogeneous T1 signal is expected in benign form; heterogeneous signal in malignant form may reflect necrosis or hemorrhage. Fat content or macroscopic calcification is not expected. Pseudocapsule may be seen as a thin hypointense band on T1.
Report Sentence
The mass shows isointense homogeneous signal to muscle on T1.
Enhancement continues in portal venous phase and slow washout pattern may be seen in delayed phase. This persistent enhancement reflects the high-capacity vascular network of staghorn vessels retaining contrast agent for extended periods. In benign form, homogeneous enhancement continues, while in malignant form necrotic areas may be seen as non-enhancing hypodense foci.
Report Sentence
Persistent enhancement is seen in portal venous and delayed phases.
Criteria
Low mitotic activity (<4 mitoses/10 HPF), no necrosis, well-defined, minimal cellular atypia — 60-70% of cases
Distinct Features
Homogeneous enhancement, prominent flow voids, curative with surgery (>90% cure rate), long-term follow-up needed because late recurrences (10+ years) reported; preoperative embolization improves surgical safety
Criteria
High mitotic activity (>4 mitoses/10 HPF), necrosis, infiltrative margin, prominent cellular atypia — 30-40% of cases
Distinct Features
Heterogeneous enhancement (necrotic areas non-enhancing), irregular margin, surrounding tissue invasion, 5-year survival 50-60%; lung and bone metastasis most common; adjuvant radiation may be considered
Criteria
Tumor showing features of both SFT and HPC — contains both collagen-rich areas and hypervascular areas
Distinct Features
Low T2 areas (collagen/SFT pattern) + high T2 areas and flow voids (cellular/HPC pattern) coexisting on MRI; nuclear STAT6 positive; treatment approach determined by dominant histological pattern
Distinguishing Feature
SFT shows low T2 signal due to collagen dominance (collagen's short T2 dominant); HPC shows intermediate-high T2 because cellular component is dominant with less collagen. Both are STAT6 positive and in the same spectrum; histopathological evaluation after surgery provides definitive differentiation. Flow voids are less prominent in SFT.
Distinguishing Feature
Castleman has nodal origin (lymph node hyperplasia) and calcification is common (20-30%); HPC is extra-nodal (from pericytes) and calcification is rare (5-10%). Both tumors show hypervascular enhancement but germinal center hyperplasia in Castleman and staghorn vessel pattern in HPC provide histological differentiation.
Distinguishing Feature
Leiomyosarcoma shows markedly heterogeneous enhancement, central necrosis, and irregular margins; IVC or large vessel involvement (intraluminal extension) is characteristic. HPC shows homogeneous hypervascular enhancement, no necrosis (in benign form), and well-defined structure. Leiomyosarcoma SMA (smooth muscle actin) positive, STAT6 negative; HPC nuclear STAT6 positive.
Distinguishing Feature
Paraganglioma shows paraganglionic localization at aortic bifurcation/organ of Zuckerkandl; HPC can occur at any retroperitoneal location. Paraganglioma may secrete catecholamines (hypertension, palpitations) with positive MIBG scintigraphy; HPC is functionally inactive (except Doege-Potter syndrome). Paraganglioma shows characteristic salt-and-pepper T2 pattern while HPC shows homogeneous T2.
Urgency
routineManagement
surgicalBiopsy
NeededFollow-up
6-monthSurgical resection is the primary treatment, and prognosis is excellent in benign HPC after complete resection (>90% cure rate). Because the tumor is hypervascular, preoperative embolization (selective arterial embolization) reduces surgical blood loss by 50-70% and is strongly recommended especially for masses >5 cm or cases with large feeding arteries. Nuclear STAT6 immunohistochemistry confirms the SFT-HPC spectrum and is the gold standard for definitive diagnosis. In malignant HPC, local recurrence (15-30%) and distant metastasis risk (10-20%, most commonly lung and bone) exist; adjuvant radiotherapy is controversial but may be considered for incomplete resection or high mitotic index. Long-term follow-up is mandatory because late recurrences (even 10+ years later) have been reported — contrast-enhanced CT or MRI every 6 months is recommended. Doege-Potter syndrome (paraneoplastic hypoglycemia) resolves with tumor resection and may recur with recurrence.
Hemangiopericytoma/SFT is treated with surgical resection. Preoperative embolization may reduce intraoperative bleeding. Malignant behavior occurs in 10-15%. Long-term follow-up is required (late recurrence may occur).