Bone and Soft-Tissue Disorders
= malignant tumor of mesenchymal origin rarely arising from lipoma
Frequency: 10–16–35% of all soft-tissue sarcomas; 2nd most common soft-tissue sarcoma in adults (after malignant fibrous histiocytoma)
Prevalence: 4.1÷1,000,000 in USA (2001)
Age: 5th–6th decade; M÷F = 1÷1
Path: wide spectrum of pathologic appearances varying from circumscribed lesions consisting predominantly of adipose tissue to circumscribed / infiltrating masses without any macroscopically visible adipose elements
Histo:<50% of liposarcomas contain lipid material; nonadipose components include fibrosis, inflammation, areas of myxoid change, fat necrosis ± calcification
Categories:
- well-differentiated = lipogenic subtype (50%)
- dedifferentiated round cell subtype (10%)
- myxoid subtype (20–50%)
- pleomorphic subtype (5–15%)
- mixed subtype (5–12%)
Molecular features: ring chromosome derived from chromosome 12; MDM2 + CDK4 amplification
Location: lower extremity (45%), abdominal cavity and retroperitoneum (14%), trunk (14%), upper extremity (7.6%), head & neck (4–6.5%), miscellaneous (13.5 %)
Spread: hematogenous to lung, visceral organs; myxoid liposarcoma shows tendency for serosal + pleural surfaces, subcutaneous tissue, bone; regional nodal metastases (in <10%)
- typically painless, slow-growing (months – years) soft-tissue mass; pain + tenderness (10–15%)
- unifying radiologic feature = adipose tissue (vast majority) of nodular / finely reticulated / amorphous appearance
- associated nonlipomatous components and lesion location often allow specific diagnosis of subtype
Criteria favoring liposarcoma over lipoma:
- male patient age >60 years
- size of lesion >10 cm
- <75% fat component
- associated nonadipose mass
- presence of thick septa >2 mm of nodular / globular appearance with contrast enhancement
- deep lesions in mediastinum / retroperitoneum
Rx: wide surgical excision ± radiation therapy ± adjunct chemotherapy
Well-differentiated Liposarcoma (50%)
= ATYPICAL LIPOMATOUS TUMOR
= locally aggressive tumor of intermediate malignant potential with <25% of tissue volume consisting of fat
Peak age: 6th–7th decade; M÷F = 1÷1
Path: large multilobulated well-circumscribed mass
Histo: malignant lipoblasts with large amounts of lipid + scanty myxoid matrix; 5 variants (lipoma-like, sclerosing, inflammatory, spindle cell, liposarcoma with meningothelial whorls)
Location:
- deep soft tissues of extremities (65–75%): thigh
- retroperitoneum (20–33%): presenting quite large with >20 cm in diameter
- upper extremity (14%)
- head & neck (5%)
Site: intramuscular >intermuscular / subcutaneous
- round / lobulated mass of fat density displacing surrounding structures
X-Ray:
- soft-tissue mass of >10 cm in diameter:
- fat detectable in extremities, not in retroperitoneum
- calcification / metaplastic ossification (10–32%)
US:
- heterogeneous multi-lobulated well-defined mass
- hyperechoic / variably echogenic fat
CT / MR:
- adipose mass with low proportion of fat content (<25% of tumor volume)
- presence of nonadipose focal globular masslike areas and enhancing septa >2 mm ± nodularity:
- high attenuation on CT
- hypo- / isointense relative to skeletal muscle on T1WI
- hyperintense relative to skeletal muscle on T2WI
CEMR:
- moderate (¼) / marked (¾) septal enhancement
Prognosis:NO metastatic potential; risk of local recurrence (43% in extremity, 70% in groin, 91% in retroperitoneum)
Dedifferentiated Liposarcoma (10%)
Path: high-grade fibrosarcoma / malignant fibrous histiocytoma (90%) mixed with well-differentiated areas
Histo: supernumerary ring / giant chromosomes derived from 12q13–15 regions
Age: 7th decade; M÷F = 1÷1
Location: deep (retroperitoneum÷extremity = 3÷1); head & neck + trunk + spermatic cord (<20%); subcutis (<1%)
Development: 7–8 years from well-differentiated liposarcoma
- large components identical to well-differentiated liposarcoma
- additionally focal nodular nonlipomatous regions >1 cm in size of low to intermediate SI on T1WI + high SI on T2WI:
- fibrous collagenized tissue
- metaplastic mineralization
- fat necrosis
Prognosis: more aggressive than well-differentiated liposarcoma with local recurrence in 41% + distant metastasis (lung, liver, bone) in 21%
= tumor of intermediate differentiation with varying degrees of mucin + fibrous tissue + relatively little lipid (<10%)
Path: fat content often <10–25% obscuring typical features of lipomatous tumors
Histo: plexiform vascular network + mixture of stellate spindle-shaped mesenchymal cells + lipoblasts in a basophilic myxoid ground substance
Genetics: reciprocal translocation of t(12;16)(q13;p11)
Peak age: 4th–5th decade (most common subtype to affect children); M÷F= 1÷1
Location: intermuscular fascial plane (70–80%) in deep-seated area
Site:
- lower extremity (75–80%): medial thigh, popliteal region, groin, buttock, calf
- retroperitoneum (8%)
- upper extremity (5%)
- well-defined multilobulated intermuscular mass:
- density between water + muscle on CT mimicking a cyst
- T1 prolongation with a cystic appearance (in 5–10%)
- very long relaxation times of large component of myxoid matrix (22%)
- PATHOGNOMONIC lacy / linear amorphous foci of fat intensity (in 90–95%) of approx. 10% of entire lesion
- peripheral nodular (61%) / central nodular (44%) / diffuse (17%) enhancement
US:
- anechoic mass ± thin septa (disproving fluid content)
Dx: biopsy of adipose area
Prognosis: frequent recurrence; metastasis in unusual locations (opposite extremity, retroperitoneum, chest wall, pelvis); 5-year mortality of 47–77%
DDx:
- Myxoma (intramuscular, only thin rind of perilesional fat on T1WI, surrounding muscle atrophy + edema)
- Synovial cyst / ganglion (typical location, thick wall and surrounding edema, clinical history of liquefied hematoma / abscess)
Histo: marked cellular pleomorphism, paucity of lipid + mucin = highly undifferentiated
- well-defined mass ± infiltrative margins:
- intensity of muscle (no distinguishing imaging features from other soft-tissue sarcomas) in 84%
- small lacy / linear / amorphous foci containing <1% fat in 62–75% (fat suppression technique!)
- prominent heterogeneity with tumor necrosis in 81%
Dx: biopsy of adipose foci
Prognosis: aggressive high-grade sarcoma with marked propensity for tumor recurrence + metastases (lung); 5-year survival of 21–63%