Bone and Soft-Tissue Disorders
Lipoma composed of mature adipose tissue is the most common mesenchymal tumor (16% of soft tissue tumors).
Histo: mature fat cells (adipocytes) of uniform size and shape with a small portion of surrounding / intervening connective tissue stroma; fat unavailable for systemic metabolism
- Variants: fibrolipoma, osteolipoma, chondrolipoma, intramuscular lipoma, angiolipoma, chondroid lipoma, spindle cell lipoma, pleomorphic lipoma, sialolipoma
Molecular features: translocation of 12q13–15; absence of MDM2 + CDK4 amplification (= liposarcoma)
- stable size after initial period of discernible growth
Age: 5th–6th decade; M >F
Location:
- superficial = subcutaneous lipoma (more common) in posterior trunk, head & neck (25%), proximal extremities
- deep lipoma in retroperitoneum, mediastinum, chest wall, deep soft tissue of hands + feet; multiple in 5–7% (up to several hundred tumors)
◊ The deeper + more centrally located a fatty mass resides the more likely it is malignant!
- well-defined encapsulated mass of fat opacity / density / intensity identical to subcutaneous fat
- no enhancement
- cortical thickening (with adjacent parosteal lipoma)
CT:
- well-defined + homogeneous tumor with low attenuation coefficient (–65 to –120 HU)
- no enhancement following IV contrast material
MR:
- well-defined + homogeneous, often with septations
- SI characteristics similar to subcutaneous fat: hyperintense on T1WI + moderately intense on T2WI
- thin septations of low SI (not uncommon) correspond to fibrous connective tissue
- differentiation from other lesions by fat suppression technique
Atypical features:
- septa >2 mm thick + septal nodularity
- soft tissue component with attenuation / SI of muscle / fibrous tissue / hemorrhage / calcification ± contrast enhancement
Rx: resection with <5% recurrence rate
DDx: normal fatty deposit (no internal architecture, no mass effect on adjacent structures and their metabolic behavior)
= lesion composed of fat separated by small branching vessels
Age: 2nd + 3rd decade; 5% familial incidence
- tender
Location: upper extremity, trunk
- signal characteristics of fat + mixed with varying numbers of large / small vessels
- mostly encapsulated lesion, may infiltrate
= long-standing lipoma with chondroid + osseous metaplasia
= INTRAMUSCULAR LIPOMA
= relatively common benign lipomatous tumor extending between muscle fibers that become variably atrophic
Peak age: 5th–6th decade; M >F
Location: thigh (50%), shoulder, upper arm
= DIFFUSE SYNOVIAL LIPOMA
= rare idiopathic intraarticular lesion characterized by replacement of subsynovial tissue by mature fat cells with villous synovial proliferation
Cause: nonspecific proliferative synovial reaction to chronic irritation from inflammation / trauma
Frequently associated with:
degenerative joint disease, chronic rheumatoid arthritis, prior trauma
Mean age: 59 years (range, 5th–7th decade); M÷F = 1÷1
Path: frondlike appearance resembling a tree in leaf [arborescens, Latin = treeforming / treelike]
Histo: fatty proliferation with hypertrophic synovial villi distended by fat + dense focally nodular lymphocytic and plasmacellular infiltrate
Location: knee >>other joints; monoarticular (94%)
Site: suprapatellar pouch of knee
- long-standing painless slowly progressive swelling
- recurrent joint effusions; joint pain + swelling (47%)
X-ray:
- joint fullness with radiolucent areas
- (frequently) osteoarthritic changes
US:
- hyperechoic frondlike mass bending and waving in real time with joint manipulation
MR:
- frondlike synovial mass isointense to fat on all sequences (PATHOGNOMONIC)
- joint effusion
- lack of magnetic susceptibility artifact
Rx: synovectomy
= Fibrolipomatous hamartoma of nerve
= rare tumorlike condition characterized by sausage-shaped / fusiform enlargement of a nerve by fibrofatty tissue
Age: early adulthood <30 years / at birth
Histo: infiltration of epineurium + perineurium by fibrofatty tissue with separation of nerve bundles
- soft slowly enlarging mass
- pain, tenderness, decreased sensation, paresthesia
Location: volar aspect of hand, wrist, forearm
Site: median n. (most frequently), ulnar n., radial n., brachial plexus
May be associated with:
macrodactyly (in ⅔) = macrodystrophia lipomatosa
- may not be visible radiographically
MR:
- longitudinally oriented, cylindrical, linear / serpiginous structures of signal void about 3 mm in diameter (= nerve fascicles with epi- and perineural fibrosis) separated by areas of fat SI (= mature fat infiltrating the interfascicular connective tissue)
US:
- “cablelike appearance” = alternating hyper- and hypoechoic bands on US
DDx: cyst, ganglion, lipoma, traumatic neuroma, plexiform neurofibroma, vascular malformation