Liposarcoma

Liposarcomas are the most common subtype of soft tissue sarcomas, accounting for at least 20% of all sarcomas in adults. Soft tissue sarcomas are rare neoplasms with over 150 different histological subtypes or forms. Liposarcomas arise from the precursor lipoblasts of the adipocytes (i.e. fat cells) in adipose (i.e. fat) tissues. Adipose tissues are distributed throughout the body, including such sites as the deep and more superficial layers of subcutaneous tissues as well as in less surgically accessible sites like the retroperitoneum (i.e. space behind the abdominal cavity) and visceral fat inside the abdominal cavity.

All liposarcomas consist of at least some cells that bear a resemblance to fat cells when examined for their histopathologic appearances under a microscope. However, the liposarcomas do have several forms based on differences in their clinical presentations (e.g. ages, gender preferences, sites of tumors, signs, and symptoms), severities (i.e. potential to invade local tissues, recur after surgical removal, and metastasize to distal tissues), genetic abnormalities, prognoses, and preferred treatment regimens. The World Health Organization in 2020 reclassified liposarcomas into five more or less distinct forms: 1) atypical lipomatous tumor/well-differentiated liposarcoma (WD-LPS); 2) dedifferentiated liposarcoma (DD-LPS); 3) myxoid liposarcoma; 4) pleomorphic liposarcoma; and 5) myxoid pleomorphic liposarcoma. (Pleomorphic indicates the presence of cells that have abnormal and often large variations in their size and shape and/or the size and shape of their nuclei.)

While liposarcoma forms are classified as being aggressive and malignant or, in the case of the atypical lipomatous tumor/well-differentiated liposarcoma, as relatively non-aggressive and benign, all five liposarcoma forms can infiltrate locally to injure nearby tissues and organs, occur in surgically inaccessible sites adjacent to vital organs (e.g. the retroperitoneum), recur after surgical removal, and progress to life-threatening diseases. Studies to date find that all five liposarcoma forms, while usually treatable at least initially by surgical resection, are often only marginally responsive to currently used chemotherapy and radiotherapy regimens. The liposarcomas require a wide range of further studies to determine their responsiveness to various radiotherapy, chemotherapy, and more novel treatment regimens as used individually and in various combinations that would include, where possible, surgical removal.

"fatty tumor" (plural lipomata), 1830, medical Latin, from Greek lipos "fat" (n.), from PIE root *leip- "to stick, adhere", also used to form words for "fat", + -oma.

1650s, "fleshy excrescence", (plural liposarcomata), Medical Latin, from Latinized form of Greek sarkoma "fleshy substance" (Galen), from sarkoun "to produce flesh, grow fleshy", from sarx (genitive sarkos) "flesh", + -oma.

Liposarcomas are generally large tumors (>10 cm) but can be of almost any size. They occur mainly in adults with only 0.7% of cases occurring in those <16 years old. In adults, liposarcomas occur predominantly in and after middle-age. The very rare cases occurring in children and adolescents are diagnosed predominantly as being the myxoid liposarcoma form.

The five liposarcoma forms must be distinguished not only from each other but also from certain other soft tissue tumors. These other tumors along with some of their distinguishing histopathologic features are: 1) dysplastic lipomas (i.e. benign humors that have sites of tissue necrosis and neoplastic, variably sized fat cells containing variable sized/shaped nuclei; these neoplastic cells, unlike most neoplastic cell in the liposarcomas, do not overexpress the MDM2 gene); 2) atypical spindle cell lipomas (i.e. benign tumors with mildly atypical spindle-shaped cells in a fibrous-to-myxoid stroma intermixed with vacuolated lipoblasts and variable-sized adipocytes with atypical nuclei; 3) pleomorphic lipomas (i.e. benign tumors characterized by giant cells with overlapping nuclei); and 4) solitary fibrous tumors (i.e. tumors, up to 22% of which exhibit malignant behavior, consisting of spindle- or ovoid-shape cells within a collagenous background stroma intermixed with blood vessels with a characteristic staghorn shape).

Together, atypical lipomatous tumors (ALTs) and well-differentiated liposarcomas (WDLs) account for 40–45% of all liposarcomas. They rarely if ever metastasize and therefore are regarded as benign or premalignant tumors. However, they are locally invasive and may transform to a more aggressive and potentially metastasizing liposarcoma, i.e. a dedifferentiated liposarcoma. Furthermore, a surgically removed atypical lipomatous tumor/well-differentiated liposarcoma may recur as a dedifferentiated liposarcoma.