Adipocytic tumours include lipoma, angiolipoma and liposarcoma. Lipomas are the most common soft tissue tumour, at least one hundred times more common than all malignant sarcomas combined.
Lipomas are common tumours that typically occur in 40-60 year olds. Exact incidence rates are unknown.
Angiolipomas are less common and typically occur in young male adults.
Well differentiated liposarcomas are often considered separately to poorly differentiated (grade 2-3) liposarcomas due to their inability to metastasise. They represent over 50% of malignant adipocytic tumours and occur in 40-60 year olds with equal sex predilection.
Dedifferentiated liposarcoma arises from well differentiated liposarcoma in about 10% of cases. They occur in a similar patient group to the well differentiated liposarcomas; most commonly in the retroperitoneal site.
Myxoid liposarcoma usually develops in the thigh (2/3rds of cases). About 1/3rd of malignant apidocytic tumours are myxoid liposarcoma; they make up 10% of all malignant soft tissue lesions. It occurs, on average, ten years earlier than the well differentiated and dedifferentiated liposarcomas.
Pleomorphic liposarcoma is the least common malignant adipocytic tumour, forming 5% of cases. It occur in older adults.
Aetiology and Pathogenesis
The aetiology of lipomas is unknown but they occur more frequently in obese people.
Angiolipoma has even less well understood aetiology; it has been noted in immunosuppressed patients.
The aetiology of liposarcomas is also unknown. Dedifferentiated liposarcoma commonly arises from well differentiated sarcoma.
Lipoma is not destructive but expands locally. More deep seated lipomas can displace or infiltrate adjacent muscle tissue.
Angiolipoma may be single or multiple and usually forms a small nodule. Although it is benign, multiple nodules can be problematic.
Well differentiated liposarcomas are locally aggressive but do not metastasise. Cure is obtainable in the limbs with wide margins; this is unfortunately not usually possible for retroperitoneal lesions and recurrences in that site are common. Dedifferentiated liposarcomas are far more likely to recur locally and also possess the ability to metastasise in about 20% of patients.
Myxoid liposarcoma has an unusual metastatic pattern, often spreading to soft tissue in the abdomen, contralateral limb, or spinal column. It has typically poorer outcomes when compared to the more common well differentiated liposarcoma.
Pleomorphic liposarcoma is a rapidly growing tumour and presents more quickly than other types. It is associated with high metastasis rate (50%) and rapid demise.
For general imaging and pathological testing see the general sarcoma topic.
Patient present complaining of a mass in the subcutaneous tissue, usually not in the distal extremities or face.. It is has often been present for some time. 5% of patients have multiple tumours. On examination, the tumour is soft and mobile. Concern is only warranted when the lesion is fixed, firm, larger than 5 cm or in deep tissue.
Angiolipomas typically occur on the chest wall or forearm, usually 0.5 - 2 cm in size and tender/painful.
Well Differentiated and Dedifferentiated Liposarcoma
Patients present with a gradually enlarging and painless mass. Retroperitoneal lesions can be particularly large due to their asymptomatic nature. Dedifferentiated liposarcoma may present with metastases but otherwise follows a similar clinical course.
These tumours frequently arise in the thigh. About 1/3rd of patients develop distant metastases.
These are rapidly growing tumours that almost always occur in the extremities. Distant metastases are common (30-50%).
Macroscopically, the tumour is encapsulated and golden yellow in colour on the cut surface. Microscopically the tumour is comprised of adipocytes that are similar to normal adipose tissue, with the exception of slight variation in the size and shape of cells. The cell is mostly taken up by lipid with an eccentric nucleus and minimal cytoplasm. Immunohistochemistry demonstrates positivity for S100, vimentin and leptin. The most common genetic finding is a translocation of 12q13 and abnormal chromosome numbers are not seen; this typically involves the HGMIC gene.
Macroscopically, these lesions are reddish yellow. Angiolipomas are formed by two cell types; adipocytes similar to lipoma and a branching capillary network containing thrombus. They are a differential diagnosis of Kaposi sarcoma and angiosarcoma. They have a normal karyotype.
Well Differentiated Liposarcoma
Tumours vary in macroscopic appearance from yellow to white and are typically lobulated; tumours containing increased amounts of fibrous tissue tend to be white and firm. On microscopy, the adipocytes vary considerably in size and shape. Other microscopic features include lipoblasts (multivaculated cells) and hyperchromatic stromal cells with fibrosis. Immunohistochemistry is usually positive for S100. Karyotype often demstates additional ring or marker chromosomes which characteristically contain amplification of the MDM2 gene. MDM2 binds TP53 and prevents it from exerting its effects on cell cycle progression and DNA repair.
These tumours often share features with the well differentiated liposarcoma, forming a nodular mass with areas of tan to white nodules that represent the dedifferentiated areas. Microscopically, dedifferentiated areas appear as any type of non-liposarcoma histology, often pleomorphic fibrous histiocytoma which consists of atypical and pleomorphic spindle cells. Immunohistochemistry can distinguish dedifferentiated regions of the tumour. Genetics are similar to well differentiated liposarcoma with ring or giant marker chromosomes; the dedifferentiated region may have loss of function of TP53.
Tumours are nodular and tan to grey in colour. These tumours consist of variably sized malignant adipocytes on a myxoid (mucous-like) background. Large pools of mucin may form. Areas of so called 'round cell liposarcoma' may develop within myxoid liposarcoma, containing small round cells with minimal stroma.
On genetic evaluation, myxoid liposarcomas almost always have a characteristic translocation of chromosomes 12q13 and 16p11. This translocation creates a hybrid protein, FUS-DDIT3, that is thought to act as an unregulated transcription factor.
The pleomorphic liposarcoma is yellow to white in colour and nodular. There is usually no capsule. Microscopically, the tumour consists of pleomorphic spindle cells (similar to pleomorphic fibrous histiocytoma) mixed with pleomorphic adipocytes. Unlike the other tumours above, these adipocytes are bizzare, with larger nuclei, significant variation in size, and multiloculated lipid droplets. Immunohistochemistry is positive for vimentin and occasionally S100. Genetic studies show multiple complex rearrangements and specific genes or findings have not been elucidated.