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# Lipoma and liposarcoma

- Lesion

Lipomas are soft, common and harmless. Liposarcomas arise deep in the thigh or abdomen; their types differ in genes and behaviour.

Updated Nov 12, 2026

## Key points

- Lipoma: superficial, soft, mobile; mature adipocytes with a thin capsule; HMGA2 rearrangements in many.
- Lipoblast: cell with lipid vacuoles that indent and scallop the nucleus; seen in liposarcoma.
- Well-differentiated liposarcoma / atypical lipomatous tumour: MDM2 and CDK4 amplification on 12q13–15 (ring and giant marker chromosomes); MDM2 FISH confirms it.
- Dedifferentiated liposarcoma: well-differentiated tumour with a high-grade non-lipogenic sarcoma; retroperitoneum.
- Myxoid liposarcoma: t(12;16) FUS-DDIT3; myxoid matrix with a delicate 'chicken-wire' capillary network; unusual spread to soft tissue and spine.
- Pleomorphic liposarcoma: high-grade pleomorphic cells with pleomorphic lipoblasts; aggressive.

## What it is

Lipoma is the commonest soft tissue tumour of adults: a soft, mobile, encapsulated mass of mature fat in the subcutis of the trunk, neck and limbs, with variants such as angiolipoma (painful, small vessels with thrombi), spindle cell lipoma and hibernoma (brown fat). Liposarcoma is one of the commonest sarcomas of adults, usually in the deep thigh and retroperitoneum. Its well-differentiated form (atypical lipomatous tumour in limbs) carries amplification of MDM2 and CDK4 and recurs locally; dedifferentiation produces a high-grade sarcoma. Myxoid liposarcoma has the FUS-DDIT3 translocation, and pleomorphic liposarcoma has complex genetics and the worst outlook.

## Structure

- Lipoma: lobules of mature adipocytes of uniform size, thin fibrous capsule.
- Atypical lipomatous tumour: mature fat with variation in adipocyte size, fibrous septa containing atypical hyperchromatic stromal cells, few lipoblasts.
- Myxoid liposarcoma: abundant mucoid matrix, small uniform round to oval cells, branching thin capillaries, small lipoblasts; round cell areas mean higher grade.
- Dedifferentiated: abrupt transition to a non-fatty spindle or pleomorphic sarcoma.

## Step by step

- Amplification of 12q13–15 increases MDM2, which degrades p53, and CDK4, which inactivates RB.
- Adipocytic precursors proliferate with partial differentiation into atypical fat.
- Further changes can cause dedifferentiation to a high-grade sarcoma.
- In myxoid liposarcoma, the FUS-DDIT3 fusion protein blocks adipocyte differentiation and keeps cells immature.
- Deep tumours grow large before they are noticed and recur after incomplete removal.

## Adipose tumours compared

| Tumour | Site | Genetics | Behaviour |
| --- | --- | --- | --- |
| Lipoma | Subcutis of trunk and limbs | HMGA2 rearrangement in many | Benign; cured by excision |
| Atypical lipomatous tumour / well-differentiated liposarcoma | Deep thigh; retroperitoneum | MDM2 and CDK4 amplification | Recurs; no metastasis unless dedifferentiated |
| Dedifferentiated liposarcoma | Retroperitoneum | MDM2 amplification plus more changes | Recurrence and metastasis |
| Myxoid liposarcoma | Deep thigh of young adults | FUS-DDIT3 t(12;16) | Intermediate to high grade by round cell content |
| Pleomorphic liposarcoma | Deep limbs, older adults | Complex karyotype | Aggressive |

## Markers

- MDM2 and CDK4 (nuclear IHC or FISH amplification)
- S100 in adipocytes and lipoblasts
- DDIT3 rearrangement

## Clinical notes

- Superficial soft mobile lumps are usually lipomas; deep or large fatty masses need imaging and MDM2 testing.
- Retroperitoneal liposarcoma presents as a huge abdominal mass with vague symptoms.

## Memory aid

MDM2 for the well-differentiated, FUS-DDIT3 for the myxoid; the lipoblast's nucleus is dented by its fat.

## Test yourself

Think of the answer, then tap the question to see it.

## What is a lipoblast?

A cell with cytoplasmic lipid vacuoles that indent the nucleus; it marks liposarcoma.

## Which genes are amplified in well-differentiated liposarcoma?

MDM2 and CDK4 on chromosome 12q13–15.

## Which translocation defines myxoid liposarcoma?

t(12;16), forming FUS-DDIT3.

## Where does dedifferentiated liposarcoma usually arise?

In the retroperitoneum.

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## Connections

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- [Benign vs malignant tumours: the comparison table](https://durgical.com/en/general-pathology/benign-vs-malignant-tumours-the-comparison-table/)
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- [Synovial sarcoma and undifferentiated pleomorphic sarcoma](https://durgical.com/en/general-pathology/synovial-sarcoma-and-undifferentiated-pleomorphic-sarcoma/)

## Related conditions 1

- [Soft tissue sarcomas (liposarcoma, leiomyosarcoma, rhabdomyosarcoma)](https://durgical.com/en/conditions/soft-tissue-sarcomas/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Soft Tissue Sarcoma Treatment (PDQ®)–Health Professional Version](https://www.cancer.gov/types/soft-tissue-sarcoma/hp/adult-soft-tissue-treatment-pdq) (National Cancer Institute, 2025).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/lipoma-and-liposarcoma/