Hemangiopericytoma
Synonyms and keywords: HPC; Solitary fibrous tumor

# Overview
Hemangiopericytoma (HPC) is a type of soft tissue vascular sarcoma that originates in the pericytes in the walls of capillaries. Typically,  hemangiopericytomas have large vessels especially located at its periphery, and commonly involve the lower limbs (35% of cases), especially the thigh, pelvis, and retroperitoneum (25%). Hemangiopericytoma was first described by Arthur Purdy Stout and Margaret Ransone Murray, two American pathologists, in 1942. mThe NF2 gene mutation has been associated with the development of hemangiopericytoma. 
If left untreated, the majority of patients with hemangiopericytoma may progress to develop bone invasion.  On MRI is the imaging modality of choice for hemangiopericytoma. On MRI, characteristic findings of hemangiopericytoma, include: solitary fibrous tumor, demonstrates a deep soft-tissue mass, and heterogenous high signal intensity on coronal STIR imaging.  Surgery is the mainstay of therapy for hemangiopericytoma. Arterial preoperative embolisation is highly recommended to avoid bleeding among patients with hemangiopericytoma.

# Historical Perspective
- Hemangiopericytoma was first described by Arthur Purdy Stout and Margaret Ransone Murray, two American pathologists, in 1942.

# Classification
- Hemangiopericytoma may be classified into 4 groups:
- Hemangiopericytoma of the spleen
- Dural hemangiopericytomas
- Myopericytoma
- Infantile myofibromatosis
- Sinonasal hemangiopericytomas
- Other associated conditions, include: Doege-Potter syndrome

# Pathophysiology
- Hemangiopericytomas arise from pericytes, which are normally involved in the formation of connective tissue around small vessels walls and are also associated in flow regulation.
- The NF2 gene mutation has been associated with the development of hemangiopericytoma.
- On gross pathology, characteristic findings of hemangiopericytoma, include:
- Large mass
- Poorly demarcated margins
- On microscopic histopathological analysis, characteristic findings of hemangiopericytoma, include:
- Hypervascular lesion
- Abundant thin-walled branching small vessels of variable size.
- Spindle or ovoid shaped cells in nests or sheets
- Cells may "onion-skin" around thin blood vessels
- Hemangiopericytoma-like area (staghorn vessels)
- Keloid-like collagen bundles (key feature)

# Causes
- There are no established causes for hemangiopericytoma.

# Differentiating Hemangiopericytoma From Other Diseases
- Hemangiopericytoma must be differentiated from other diseases that cause a slow growing painless mass, such as:
- Liposarcoma
- Sinovial sarcoma
- Angiosarcoma

# Epidemiology and Demographics
- Hemangiopericytoma represents only about 1-2% of all soft-tissue tumors.

## Age
- The median age at diagnosis is 45 years.
- Hemangiopericytoma is more commonly observed among patients aged between 45 to 50 years old.
- Hemangiopericytoma is less commonly observed among children.

## Gender
- Hemangiopericytoma affects men and women equally.

## Race
- There is no racial predilection for hemangiopericytoma.

# Risk Factors
- There are no associated risk factors in the development of hemangiopericytoma.

# Natural History, Complications and Prognosis
- The majority of patients with hemangiopericytoma are asymptomatic.
- Early clinical features include painless mass, or slow enlargement.
- If left untreated, the majority of patients with hemangiopericytoma may progress to develop bone invasion.
- Common complications of hemangiopericytoma, may include:
- May profusely bleed during resection.
- The mean survival rate of patients with hemangiopericytoma is approximately 13 years.
- 1-year survival rate is 95%
- 5-year survival rate is 82%
- 10-year survival rate is 60%
- 20-year survival rate is 23%

# Diagnosis

## Symptoms
- Hemangiopericytoma is usually asymptomatic.
- There are no hallmark symptoms of hemangiopericytoma.

## Physical Examination
- Patients with hemangiopericytoma usually are well-appearing.
- Physical examination may be remarkable for:
- Palpable mass
- No tenderness
- Located in the extremities (femur, proximal tibial, axilla or pelvis)

## Laboratory Findings
- There are no specific laboratory findings associated with hemangiopericytoma.

## Imaging Findings
- MRI is the imaging modality of choice for hemangiopericytoma.
- On MRI, characteristic findings of hemangiopericytoma, include:
- Solitary fibrous tumor
- Demonstrates a deep soft-tissue mass
- Heterogenous high signal intensity on coronal STIR imaging
- Brightly enhancing soft tissue mass
- Often hyperintense on T2WI, with prominent flow void (most marked in haemangiopericytomas)
- Other imaging findings for hemangiopericytoma, may include angiography.
- On angiography, findings of hemangiopericytoma, include:
- Dense
- Well-circumscribed areas of enhancement with early draining veins and shunting.
- Hemangiopericytoma are described as having a pedicle formed by the arteries supplying the tumor from which vessels branch to encircle the tumor

# Treatment

## Medical Therapy
- Common medical therapy for hemangiopericytoma may include is adjuvant radiation therapy.

## Surgery
- Surgery is the mainstay of therapy for hemangiopericytoma.
- Arterial preoperative embolisation is highly recommended among patients with hemangiopericytoma.

## Prevention
- There are no primary preventive measures available for hemangiopericytoma.