Atypical lipomatous tumour / well differentiated liposarcoma

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.

Synonyms

Atypical lipoma, lipoma-like liposarcoma, sclerosing liposarcoma, spindle cell liposarcoma, inflammatory liposarcoma.

Epidemiology

There is the largest group of locally aggressive lipomatous tumours. It usually occurs in middle age.

Macroscopic appearances

Usually forming a well circumscribed mass, may sometimes be infiltrative. White to yellow, may be fatty, firm or myxoid.

Histopathology

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.

Heterologous elements may include bone, smooth muscle and skeletal muscle.

Immunohistochemistry

• S-100 is positive in adipocytes.

• HMB45 is negative.

Cytogenetics

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 represtn the same tumour type¹.

Differential diagnosis

• Inflammatory variant:

  • inflammatory myofibroblastic tumour

  • Castleman's disease

  • Lymphoma

• Heterologous elements need to be distinguished from those seen in dedifferentiated liposarcoma.

• Angiomyolipoma: positive for HMB45.

Management

If possible, wide local excision.

Prognosis

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 retroperitoneum.

References

World Health Organization Classification of Tumours, Tumours of Soft Tissues and Bone, IARC Press 2002.

¹ Rosai J, Akerman M, Dal Cin P, et al. Combined morphologic and karyotypic study of 59 atypical lipomatous tumors. Evaluation of their relationship and differential diagnosis with other adipose tissue tumors (a report of the CHAMP Study Group). Am J Surg Pathol 1996; 20:1182-9

This page last revised 19.11.2005.

©SMUHT/PW Bishop