parosteal lipoma

Parosteal lipomas are rare benign fat-containing tumors closely related to the bone periosteum.

Epidemiology

Parosteal lipomas are rare, comprising just 0.3% of all lipomas. They typically occur in 40- to 60-year-old patients.

Clinical presentation

Patients present with a slowly enlarging, painless soft tissue mass closely related to a bone. Another clinical manifestation is nerve palsy, the most commonly affected nerve being the posterior interosseous nerve.

Pathology

The tumor is composed of mature adult fat similar to soft-tissue lipomas. There is often an osseous stalk with cortical thickening. It shows lobular growth with thin intervening septations. There is a lack of malignant potential.

Location

The most common sites are the femoral, radial and humeral diaphysis. Cases have also been reported in the tibia, scapula, clavicle, ribs, pelvis, metacarpals, metatarsals, mandible and skull.

Radiographic features

The major distinctive radiologic diagnostic feature is a juxtacortical fatty mass with variable septations, containing variable sizes of bony protuberances as well as adjacent cortical thickening.

Plain radiograph

typically appears as a large, lucent mass

CT

diagnostic for fat density
will also demonstrate lack of underlying medullary continuity: to differentiate from osteochondroma

MRI

Will present as a T1/T2 hyperintense mass with loss of signal on fat suppression. MRI provides better contrast resolution for fibrovascular septa, which are seen as low-signal-intensity strands on T1-weighted images that become higher in signal intensity on long TR images (in particular fat suppression sequences).

MRI also allows for the best evaluation of adjacent nerve involvement and muscle atrophy.