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# Uterine leiomyosarcoma vs leiomyoma

- Lesion

A rare, malignant smooth-muscle tumour of the uterus that arises on its own, not from a fibroid; three microscopic features separate it from one.

Updated Nov 12, 2026

## Key points

- Arises de novo; malignant transformation of a leiomyoma is exceptional.
- Criteria (spindle type): at least two of tumour necrosis, significant atypia and high mitotic count.
- Gross: usually a single large, soft, fleshy tumour with haemorrhage and necrosis and poorly defined borders.
- Complex karyotypes; TP53, RB1 and ATRX alterations are common.
- Hematogenous spread, especially to the lungs; frequent recurrence.

On this page

- The basics Key points
- What it is
- Structure
- Step by step
- Leiomyoma, uncertain potential and leiomyosarcoma
- Markers
- Memory aid
- Elsewhere in DurgiPedia

- Clinical Clinical notes

- Study Test yourself

- Connections Connections

## What it is

Leiomyosarcoma is the most common pure uterine sarcoma but is rare, arising de novo rather than from a leiomyoma, usually in postmenopausal women. It is diagnosed by combining three features in spindle-cell tumours: tumour (coagulative) necrosis, moderate to severe nuclear atypia and a high mitotic count. Tumours with only one worrying feature that do not meet criteria are called smooth-muscle tumours of uncertain malignant potential. Leiomyosarcomas have complex genomes, spread by blood to the lungs, and have a poor prognosis.

## Structure

- Spindle cells in fascicles with enlarged, hyperchromatic, pleomorphic nuclei.
- Tumour cell necrosis: an abrupt transition from viable to dead tumour cells without intervening granulation tissue, unlike the hyaline infarct of a degenerating fibroid.
- Numerous and abnormal mitoses; infiltrative margins and vascular invasion may be seen.

## Step by step

- A myometrial cell acquires inactivation of tumour suppressors such as TP53 and RB1.
- Chromosomal instability produces a highly abnormal genome.
- The tumour grows rapidly with necrosis and invades the myometrium and vessels.
- Tumour cells enter veins and seed the lungs and other organs.

## Leiomyoma, uncertain potential and leiomyosarcoma

| Feature | Leiomyoma | Smooth-muscle tumour of uncertain malignant potential | Leiomyosarcoma |
| --- | --- | --- | --- |
| [Number and size](https://durgical.com/en/conditions/uterine-fibroids/) | Often multiple, variable | Usually single | Usually single and large |
| Gross | Firm, white, whorled, well circumscribed | Variable | Soft, fleshy, haemorrhagic, necrotic |
| Tumour necrosis | Absent (hyaline infarcts may occur) | Only one worrying feature or borderline values | Present in most |
| Atypia | None or bizarre cells only | As above | Moderate to severe, diffuse |
| Mitoses | Few | As above | Many, often abnormal |
| Behaviour | Benign | Usually benign, small recurrence risk | Malignant, lung metastases |

## Markers

- Desmin, smooth-muscle actin, h-caldesmon
- p53 and p16 often aberrant
- Ki-67 high

## Clinical notes

- Presents with bleeding, a rapidly enlarging uterus or a pelvic mass in an older woman; growth of a fibroid after menopause is a warning sign.
- Because a sarcoma can be mistaken for a fibroid, uncontained morcellation of presumed fibroids risks spreading an unsuspected sarcoma.

## Memory aid

Necrosis, nuclei and numbers: two of the three Ns make a leiomyosarcoma.

## Test yourself

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

## Does leiomyosarcoma usually arise from a pre-existing leiomyoma?

No; it arises de novo, and transformation of a fibroid is exceptional.

## Which three features are used to diagnose uterine leiomyosarcoma?

Tumour cell necrosis, moderate to severe nuclear atypia and a high mitotic count; at least two are needed in the usual spindle type.

## Where does uterine leiomyosarcoma usually metastasize?

Through the blood, most often to the lungs.

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## Elsewhere in DurgiPedia

- [Uterus and the endometrial cycle](https://durgical.com/en/histology/uterus-and-the-endometrial-cycle/)

## Connections

## Related 6

- [Uterine leiomyoma](https://durgical.com/en/general-pathology/uterine-leiomyoma/)
- [Endometrial stromal tumours](https://durgical.com/en/general-pathology/endometrial-stromal-tumours/)
- [Benign vs malignant tumours: the comparison table](https://durgical.com/en/general-pathology/benign-vs-malignant-tumours-the-comparison-table/)
- [Differentiation and anaplasia: how a tumour cell looks under the microscope](https://durgical.com/en/general-pathology/differentiation-and-anaplasia-how-a-tumour-cell-looks-under-the-microscope/)
- [Patterns of necrosis: coagulative, liquefactive, caseous, fat, fibrinoid, gangrenous](https://durgical.com/en/general-pathology/patterns-of-necrosis-coagulative-liquefactive-caseous-fat-fibrinoid-gangrenous/)
- [Pathways of spread and the usual metastatic sites](https://durgical.com/en/general-pathology/pathways-of-spread-and-the-usual-metastatic-sites/)

## Related conditions 3

- [Uterine fibroids (leiomyomas)](https://durgical.com/en/conditions/uterine-fibroids/)
- [Soft tissue sarcomas (liposarcoma, leiomyosarcoma, rhabdomyosarcoma)](https://durgical.com/en/conditions/soft-tissue-sarcomas/)
- [Abnormal uterine bleeding](https://durgical.com/en/conditions/abnormal-uterine-bleeding/)

[← All of general pathology](https://durgical.com/en/general-pathology/)

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: [Uterine Sarcoma Treatment (PDQ®)–Health Professional Version](https://www.cancer.gov/types/uterine/hp/uterine-sarcoma-treatment-pdq) (National Cancer Institute, 2026); [Uterine Leiomyomata](https://www.ncbi.nlm.nih.gov/books/NBK546680/) (StatPearls (NCBI Bookshelf), 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/general-pathology/uterine-leiomyosarcoma-vs-leiomyoma/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/uterine-leiomyosarcoma-vs-leiomyoma/