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# Fibrous dysplasia and other fibrous and cystic bone lesions

- Lesion

Common tumour-like lesions of young bones: a fibrous defect in the cortex, bone replaced by fibrous tissue, and two kinds of cysts.

Updated Nov 12, 2026

## Key points

- Nonossifying fibroma: very common in children; eccentric, bubbly, sclerotic-rimmed metaphyseal lesion; storiform spindle cells, giant cells, foamy macrophages; often regresses.
- Fibrous dysplasia: activating GNAS mutation (high cAMP) after conception; 'Chinese letter' trabeculae of woven bone without osteoblastic rimming; ground-glass X-ray.
- McCune–Albright: polyostotic fibrous dysplasia, café-au-lait spots with jagged 'coast of Maine' borders and endocrine hyperfunction (precocious puberty, hyperthyroidism, GH excess).
- Mazabraud syndrome: fibrous dysplasia plus intramuscular myxomas.
- Shepherd's crook deformity of the proximal femur in polyostotic disease.
- Aneurysmal bone cyst: USP6 rearrangement, blood-filled spaces with fluid levels on MRI; secondary ABC changes occur in other tumours.

## What it is

Several benign lesions of children and young adults are often found by chance or after a fracture. Nonossifying fibroma (fibrous cortical defect when small) is an eccentric metaphyseal lesion of spindle cells that usually heals by itself. Fibrous dysplasia is a developmental lesion in which bone is replaced by fibrous tissue with curved trabeculae of woven bone, caused by a postzygotic activating GNAS mutation; it can be monostotic, polyostotic, part of McCune–Albright syndrome or of Mazabraud syndrome. Aneurysmal bone cyst is a blood-filled multicystic tumour driven by USP6 rearrangements, and the simple (unicameral) bone cyst is a fluid-filled cavity of the proximal humerus or femur.

## Structure

- Nonossifying fibroma: cellular fibroblastic tissue in a whorled pattern with scattered giant cells, haemosiderin and foamy macrophages.
- Fibrous dysplasia: bland fibrous tissue with curvilinear trabeculae of woven bone that lack an osteoblast rim; cartilage nodules sometimes.
- Aneurysmal bone cyst: blood-filled spaces separated by septa containing fibroblasts, osteoclast-type giant cells and reactive woven bone.
- Simple bone cyst: single cavity with clear fluid, a thin fibrous lining and sometimes a 'fallen fragment' after fracture.

## Step by step

- In fibrous dysplasia, an activating GNAS mutation arises in an early embryonic cell after conception (mosaicism).
- The mutant Gsα keeps adenylyl cyclase on, so cAMP stays high in osteoprogenitor cells.
- These cells proliferate but fail to mature, laying down fibrous tissue and irregular woven bone.
- The earlier the mutation arises in development, the more tissues carry it: bone only, or bone, skin and endocrine glands (McCune–Albright).
- The weak bone bends and fractures, and lesions stop growing in adulthood.

## Fibrous and cystic lesions compared

| Lesion | Age and site | Genetics | Key features |
| --- | --- | --- | --- |
| Nonossifying fibroma | Children; metaphysis of distal femur or tibia, cortical | Often KRAS or FGFR1 mutations | Eccentric lucency with sclerotic rim; regresses |
| [Fibrous dysplasia](https://durgical.com/en/conditions/fibrous-dysplasia-of-bone-and-mccune-albright-syndrome/) | Children and young adults; ribs, femur, skull and jaw | GNAS activating (mosaic) | Ground-glass, Chinese-letter trabeculae; McCune–Albright |
| Aneurysmal bone cyst | Under 20; metaphysis, vertebrae | USP6 rearrangement | Expansile blood-filled cysts, fluid levels |
| Simple bone cyst | Children; proximal humerus and femur | Not a clonal tumour | Single fluid-filled cavity, fallen fragment |

## Clinical notes

- Many lesions are found by chance on X-rays taken for injury; typical features allow observation without biopsy.
- Fibrous dysplasia of the skull and jaw causes facial asymmetry and can compress cranial nerves.
- Girl with precocious puberty, café-au-lait spots and limping: think of McCune–Albright syndrome.

## Memory aid

In fibrous dysplasia, GNAS stays switched on, the bone writes in Chinese letters, and the X-ray looks like frosted glass.

## Test yourself

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

## Which mutation causes fibrous dysplasia?

A postzygotic activating mutation of GNAS, the gene for the Gsα protein.

## What are the features of McCune–Albright syndrome?

Polyostotic fibrous dysplasia, café-au-lait spots with irregular borders and endocrine hyperfunction such as precocious puberty.

## Which gene is rearranged in primary aneurysmal bone cyst?

USP6.

## What is the natural history of nonossifying fibroma?

It usually heals and fills with bone on its own as the child grows.

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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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- [Giant cell tumour of bone](https://durgical.com/en/general-pathology/giant-cell-tumour-of-bone/)
- [Nodular fasciitis, fibromatoses and desmoid tumour](https://durgical.com/en/general-pathology/nodular-fasciitis-fibromatoses-and-desmoid-tumour/)
- [Multiple endocrine neoplasia (MEN) syndromes](https://durgical.com/en/general-pathology/multiple-endocrine-neoplasia-syndromes/)

## Related conditions 1

- [Fibrous dysplasia of bone and McCune–Albright syndrome](https://durgical.com/en/conditions/fibrous-dysplasia-of-bone-and-mccune-albright-syndrome/)

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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: [McCune-Albright syndrome: MedlinePlus Genetics](https://medlineplus.gov/genetics/condition/mccune-albright-syndrome/) (MedlinePlus, US National Library of Medicine, 2025); [Primary Bone Cancer - NCI](https://www.cancer.gov/types/bone/bone-fact-sheet) (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/fibrous-dysplasia-and-other-fibrous-and-cystic-bone-lesions/