Liposarcoma is a malignancy of fat cells (see Liposarcoma in the Pediatric Medicine section and Liposarcoma, Soft Tissue in the Radiology section). In adults, it is the most common soft tissue sarcoma. Liposarcoma normally appears as a slowly enlarging, painless, nonulcerated submucosal mass in a middle-aged person, but some lesions grow rapidly and become ulcerated early. Virchow first described liposarcoma in the 1860s.
The development of a liposarcoma from a preexisting benign lipoma is rare. Most cases arise de novo. Liposarcomas most frequently arise from the deep-seated stroma rather than the submucosal or subcutaneous fat. Dermal lesions are rare and may resemble pleomorphic fibroma. 
The most recent World Health Organization classification of soft tissue tumors recognizes 5 categories of liposarcomas: (1) well differentiated, which includes the adipocytic, sclerosing, and inflammatory subtypes; (2) dedifferentiated; (3) myxoid; (4) round cell; and (5) pleomorphic.
A spindle-cell variant of well-differentiated liposarcoma is also described. The concept that round-cell liposarcoma represents the high-grade counterpart of myxoid liposarcoma is generally accepted. Spindle-cell liposarcoma is a rare variant of an atypical lipomatous tumor (ie, well-differentiated liposarcoma), and it must be distinguished from a dedifferentiated liposarcoma with metastatic potential and a benign spindle-cell lipoma. The advent of cytogenetic and molecular investigations has contributed to better categorization of this subset of mesenchymal neoplasms. Not only have they provided new insights into the biology of these tumors, but they have also validated the current classification schemes based on conventional morphologic observations. [2, 3, 4, 5]
Liposarcoma occurs in 3 main biologic forms: (1) well-differentiated liposarcoma; (2) myxoid and/or round cell; and (3) pleomorphic. In rare circumstances, lesions can have a combination of morphologic types; these are classified as combined or mixed-type liposarcomas.
The anatomical distribution of liposarcoma appears to be partly related to the histologic type. Well-differentiated liposarcoma tends to occur in deep soft tissues of both the limbs and the retroperitoneum. Myxoid and/or round-cell liposarcomas and pleomorphic liposarcomas have a striking predilection for the limbs, and dedifferentiated liposarcoma occurs predominantly in the retroperitoneum. Although any liposarcoma subtype occasionally arises in the subcutis, involvement of the dermis appears to be exceedingly rare.
Liposarcoma is a lipogenic tumor of large deep-seated connective tissue spaces. Fusion proteins created by chromosomal abnormalities are key components of mesenchymal cancer development. An abnormality of band 12q13 has been associated with the development of liposarcomas. The most common chromosomal translocation is the FUS-CHOP fusion gene, which encodes a transcription factor necessary for adipocyte differentiation. These and other distinct genetic aberrations may aid in the diagnosis of particular liposarcoma subtypes, and they can potentially be targets that can be exploited therapeutically. 
Soft tissue sarcomas occur in approximately 5000 patients in the United States per year. Overall, liposarcomas account for less than 20% of all soft tissue sarcomas, and the average patient age at presentation is 50 years. However, in children, liposarcomas account for less than 5% of all soft tissue sarcomas; fewer than 60 cases in children have been reported.
With an annual incidence of 2.5 cases per million population, liposarcoma is the most common soft tissue sarcoma, accounting for approximately 17% of all soft tissue sarcomas and 3% of all liposarcomas in the head and neck region (usually the neck and the cheek). Oral involvement is rare; as of the year 2000, fewer than 50 oral cases had been reported. The trunk and the lower extremities are the most likely sites of tumor development.
No association with race or geography is known.
Liposarcomas are slightly more common in males than in females.
The mean patient age at onset is 50 years. Although liposarcomas account for about 17% of all soft tissue sarcomas, they are involved in only 4% of childhood soft tissue sarcomas. Cases of liposarcoma are reported in young adults and teenagers, but cases in children are rare. 
The prognosis is affected by the type of liposarcoma present. In general, survival for extremity tumors is favorable.  The histologic grade of localized liposarcomas of the extremities may be the most important prognostic factor. 
The well-differentiated type and most myxoid types have favorable prognoses, with 100% and 88% 5-year survival rates, respectively.  However, these tumors are poorly circumscribed and locally recur after incomplete excision. Although they rarely metastasize, repeated local recurrences may cause the tumor to evolve into a higher grade of sarcoma or to dedifferentiate, in which case metastasis is possible.
The form that a dermatologist is most likely to examine at biopsy is 1 of the 3 subtypes of a well-differentiated liposarcoma, that is, the lipomalike liposarcoma or atypical lipoma; the other 2, inflammatory and sclerosing, are less likely. The clinical outcome of this subtype is best predicted by the anatomical location. In the subcutis, lipomalike liposarcomas are often cured with local excision. Lipomalike liposarcomas rarely recur, and they do not metastasize. Use of the term atypical lipomatous tumor, a less serious designation for atypical lipoma, has been suggested.
Round-cell and poorly differentiated types have a poor prognosis. Each has a 5-year survival rate of about 50% because they recur locally and tend to metastasize quickly and widely, especially in poorly differentiated liposarcomas. The lungs and the liver are the most common sites of metastasis. .
Despite high-grade morphology, cutaneous liposarcomas have a favorable clinical prognosis.
For soft tissue sarcoma involving the popliteal fossa, despite a high rate of microscopically positive margins, the local recurrence rate reported is only 7%. 
Cutaneous and subcutaneous pleomorphic liposarcomas have a more favorable outcome compared with their deep-seated counterparts, a fact attributable to their small size and superficial location. 
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