Liposarcoma

Anthony G. Ryan and Peter L. Munk

A 25-year-old woman presented with a large palpable swelling behind her knee. This case was referred to us to rule out a deep venous thrombosis (DVT).

Figure 45A

Figure 45B

Liposarcoma: myxoid subtype.

• Dedifferentiated liposarcoma
  
• Malignant fibrous histiocytoma

Liposarcomas are a heterogeneous group of lesions comprising malignant tumors of mesenchymal origin containing variable adipose components. They are the second most common soft-tissue sarcoma in adults after malignant fibrous histiocytoma, comprising 12 to 18% of all malignant soft-tissue sarcomas. They occur most frequently in the fifth and sixth decades and are almost unheard of in the pediatric population.

The crucial decisions on imaging are the differentiation of a lipomatous or myxomatous lesion with malignant potential from benign lipomatous or myxomatous lesions, and to stratify those lesions with malignant potential into low-grade (e.g., nonmetastasizing), well-differentiated liposarcoma and high-grade lesions (e.g., the pleomorphic variant), although the final arbiter of this decision is invariably histological.

All liposarcomas demonstrate a genetic clonal abnormality resulting in amplification of the 12q13–15 region. Lipomas do not share this genetic lesion, making this one of the criteria used by pathologists to differentiate these entities.

Dedifferentiation is thought to occur in roughly 10% of well-differentiated liposarcomas, occurring three times as often in retroperitoneal lesions (15% risk of differentiation) than in extremity lesions (5% risk). This difference is thought to be a result of the longer “time to discovery” for retroperitoneal lesions.

As with other sarcomas, hematogenous metastasis to the lungs followed by the visceral organs is the expected pattern.

Myxoid liposarcomas have a predilection for metastases to scerosal and pleural surfaces, subcutaneous tissues, and bone.

A concomitant mass in the retroperitoneum or thigh may be present in up to 10% of myxoid liposarcomas, presenting as either a multicentric lesion or a primary lesion with a secondary deposit at initial diagnosis.

• The second most common type of liposarcoma
  
• Equal sex incidence, with peak prevalence in the fourth to fifth decades of life
  
• Painless soft-tissue mass reaching a large size (> 15 cm) before presentation
  
• 70 to 80% of cases are intermuscular, and 75 to 80% of cases affect the lower extremity.
  
• This is the most common type of liposarcoma to affect children, accounting for almost 80% of liposarcomatous lesions in patients ages 10 to 16 years.

• Least common subtype, accounting for 5 to 15% of all liposarcomas
  
• Equal sex distribution, most arising in patients older than 50 years
  
• The lower extremity is most commonly affected (56%), the upper extremity is affected in 20% of cases, and the retroperitoneum in 9% of cases.
  
• A firm, painless soft-tissue mass that demonstrates rapid enlargement
  
• Most frequently intramuscular

• Dedifferentiated liposarcoma comprises two distinct histological populations: well-differentiated liposarcoma and nonadipose cellular sarcoma, such as malignant fibrous histiocytoma.
  
• Equal sex distribution, with peak prevalence in the seventh decade
  
• Presents as a painless, slow-growing mass
  
• The clinical history of a rapid increase in size in an apparently well-differentiated liposarcoma suggests possible dedifferentiation.

• Five to 12% of all liposarcomas
  
• Usually affects older patients and is most commonly found in the retroperitoneum and the abdominal cavity

Liposarcomas are subclassified by the World Health Organization into four major subtypes and a fifth subtype comprising a mixture of elements of the other subtypes:

1. Well-differentiated
   
2. Myxoid
   
3. Pleomorphic
   
4. Dedifferentiated
   
5. Mixed

Well-differentiated liposarcoma represents the lowest, and dedifferentiated liposarcoma the highest, grades of tumor.

• Clearly marginated, large, pale, lobular mass

• Low-grade neoplasm with five described histologic variants, of which lipoma-like is the most common, containing abundant mature adipose tissue that may appear identical to classic lipoma
  
• The adipose tissue is studded by scattered lipoblasts or cells with enlarged hyperchromatic nuclei representing lipocytic atypia, a sine qua non of the lesion.
  
• The fibrous interlobular septae are seen to contain increased cellularity, including skeletal muscle fibers (particularly at the margin with adjacent muscle), in comparison to lipoma, where the septae remain fibrous.

• Well-circumscribed, multinodular mass
  
• Lesions consisting mostly of myxoid material are gelatinous with opaque white nodules representing focal cellular aggregates.
  
• Lesions consisting mostly of round cells have a white, fleshy appearance.

• 40 to 50% (most common): well-delineated lobules of myxoid matrix; plexiform capillary pattern; primitive proliferating fibroblasts with variable numbers of usually mono- and sometimes bivacuolated lipoblasts
  
• Overall, these tumors typically consist of less than 10% fat.
  
• Myxoid liposarcomas are generally considered intermediate-grade lesions if predominantly myxoid, but high-grade lesions if they contain a substantial round cell component.
  
• 90% of myxoid liposarcomas express a reciprocal translocation of t(12;16)(q13;p11), which is thus a very specific and sensitive marker for this lesion.

• High-grade sarcoma comprising a large multinodular, pale mass
  
• Myxoid and necrotic areas may be present.

There are two distinguishable histologic types sharing cellular pleomorphism:

1. Malignant fibrous histiocytoma-like spindle cell tumor containing bizarre lipoblasts; frank adipose regions are sparse
   
2. Epithelioid pleomorphic liposarcoma
   
   
   
   • Solid sheets of epithelioid-appearing cells
     
   • Regions of fatty differentiation with bizarre lipoblasts

• Multinodular mass with yellow well-differentiated regions
  
• Dedifferentiated foci often appear as distinct admixed tan-gray regions, with an abrupt transition between the two components.