Definition
A locally aggressive tumour composed of adipocytes with at least focal cytological atypia. The term atypical lipomatous tumour is used for sites amenable to wide surgical resection. In the mediastinum and retroperitoneum, where wide resection cannot be achieved, local recurrence will occur and may be fatal, hence the designation as well differentiated liposarcoma.
There is the largest group of locally aggressive lipomatous tumours. It usually occurs in middle age.
Usually forming a well circumscribed mass, may sometimes be infiltrative. White to yellow, may be fatty, firm or myxoid.
Atypical lipomatous tumour consists of adult fat with enlarged atypical hyperchromatic nuclei. There is often multinucleation, sometimes with a "floret" arrangement. There are usually fibrous or myxoid zones of varying extent. Atypical nuclei are often more numerous in these zones. Although usually mono-vacuolated, the fat cells are rarely hibernoma-like. Uncommonly, there is a prominent chronic inflammatory cell infiltrate, sometimes with germinal centres.
Tumours are often a mix of four variants:
lipoma-like (adipocytic): unlike typical lipomas, cell size is obviously variable. There is focal nuclear atypia. The number of lipoblasts varies.
sclerosing: most commonly retroperitoneal or paratesticular. There is a fibrillary stroma with scattered lipoblasts and hyperchromatic stromal cells. Lipogenic areas may be hard to find.
inflammatory: most commonly retroperitoneal, the chronic inflammatory cell infiltrate may obscure the adipocytes. The inflammatory cells are usually predominantly lymphoplasmacytic B-cells, but may be mainly T-cells.
spindle cell: the spindle cells resemble neural tissue. The lipogenic component includes atypical cells and lipoblasts.
Cellular atypical lipomatous tumours have areas of increased cellularity but falling short of dedifferentiated or pleomorphic liposarcoma. [Some authors have regarded such tumour as showing low grade dedifferentiation.] The adipocytic cells may be small. The mitotic rate is less than 5 per 10 HPF. Myxoid areas with small fat cells, a plexiform capillary network and a microcystic pattern may resemble a myxoid liposarcoma2.
Heterologous elements may include bone, smooth muscle and skeletal muscle.
S-100 is positive in adipocytes.
HMB45 is negative.
MDM2 and CDK4 are positive
Both atypical lipomas and well differentiated liposarcoma show a ring or giant marker chromosome composed of material from the long arm of chromosome 12 , helping to prove that they represent the same tumour type1. The gene MDM2 and often teh CDK4 genes are within the amplified region 12q13-15.
Lipoma, particularly when there is fibrosis or liponecrosis.
Inflammatory variant:
Castleman's disease
Lymphoma
Heterologous elements need to be distinguished from those seen in dedifferentiated liposarcoma.
Angiomyolipoma: positive for HMB45.
If possible, wide local excision.
Complete wide excision is curative. Repeated local recurrences may be fatal, with eventual mortality of 80% for retroperitoneal tumours after 10 to 20 years. Well differentiated liposarcoma will not metastasis unless it undergoes dedifferentiation. Dedifferentiation rates vary from less than 2% in limbs to over 20% in the retroperitoneum2.
World Health Organization Classification of Tumours, Tumours of Soft Tissues and Bone, IARC Press 2002.
This page last revised 25.5.2007.
©SMUHT/PW Bishop