Your pathology report for aneurysmal bone cyst (ABC)

by Jason Wasserman MD PhD FRCPC
January 10, 2026

An aneurysmal bone cyst is a benign (non-cancerous) tumour of bone composed of multiple blood-filled spaces separated by thin walls of tissue. Despite the name, it is not related to blood vessel aneurysms. The expanding cystic spaces can weaken bone and cause pain or swelling.

This article explains the pathology report for aneurysmal bone cyst, including how it is diagnosed, what features pathologists look for, and how these findings relate to prognosis and treatment.

Where does an aneurysmal bone cyst develop?

An aneurysmal bone cyst can develop in any bone, but it most commonly arises in:

  • The metaphysis (growing end) of long bones such as the femur, tibia, and humerus.

  • The posterior elements of the spine.

Rarely, similar tumours can occur in soft tissues outside of bone.

What are the symptoms of an aneurysmal bone cyst?

The most common symptoms are pain and swelling in the affected area. These symptoms usually develop gradually as the cyst expands and weakens the bone.

Fractures are uncommon but can occur if the bone becomes very thin. When the spine is involved, the cyst may compress nerves or the spinal cord, leading to neurological symptoms such as weakness, numbness, or changes in sensation.

Who gets an aneurysmal bone cyst?

Aneurysmal bone cysts can occur at any age, but they are most common in children and adolescents, with about 80% of cases occurring in the first two decades of life. Males and females are affected equally. Overall, these tumours are rare.

What causes an aneurysmal bone cyst?

The exact cause is not fully understood. However, many aneurysmal bone cysts are now recognised as true bone tumours rather than reactive changes.

Most cases arise from a genetic rearrangement involving the USP6 gene. A genetic rearrangement occurs when pieces of DNA break and reattach abnormally, causing a gene to become overactive. In aneurysmal bone cysts, this leads to increased growth of a population of spindle-shaped cells within the tumor.

This genetic change is found in about 70% of aneurysmal bone cysts and helps distinguish them from other bone tumours that may look similar. Importantly, the presence or absence of this rearrangement does not appear to affect the aggressiveness of the tumour or the risk of recurrence.

How is the diagnosis made?

The diagnosis is made by combining imaging findings with microscopic examination of tissue removed from the tumor. In selected cases, molecular testing may support the diagnosis.

Imaging features

Imaging plays a vital role in suggesting the diagnosis of an aneurysmal bone cyst:

  • X-rays and CT scans typically show a well-defined, expansile, lytic lesion with a thin shell of reactive bone.

  • MRI often shows characteristic fluid–fluid levels, created by blood products of different ages settling within the cystic spaces.

  • Contrast-enhanced imaging may highlight internal septa and help identify solid areas.

  • Bone scans usually show increased uptake around the edges of the lesion with reduced activity in the centre.

While these features are characteristic, they are not diagnostic on their own and must be correlated with pathology.

Microscopic (pathologic) features

Under the microscope, an aneurysmal bone cyst is usually well circumscribed and composed of large blood-filled spaces that lack a proper lining. Fibrous septa separate these spaces.

The septa contain:

  • Bland spindle-shaped fibroblasts.

  • Multinucleated osteoclast-type giant cells.

  • Reactive woven bone, often rimmed by osteoblasts.

The newly formed bone often follows the contours of the septa. In some cases, the bone appears blue under the microscope (“blue bone”), although this feature is not required for diagnosis.

Cell division may be frequent, but atypical mitotic figures are not seen, which helps distinguish aneurysmal bone cyst from malignant bone tumours. Necrosis (tissue death) is uncommon unless there has been a fracture.

Some aneurysmal bone cysts have more solid areas. These solid variants show the same components and behaviour as the cystic form.

Immunohistochemistry

Immunohistochemistry is a laboratory test that uses special stains to detect specific proteins within cells. These stains help pathologists better understand which types of cells are present and can distinguish between conditions that look similar under the microscope.

Immunohistochemistry is not routinely required to diagnose an aneurysmal bone cyst. However, in difficult cases, the absence of staining for a protein called H3.3 p.Gly34Trp can help distinguish aneurysmal bone cyst from giant cell tumour of bone, which may show similar cystic changes but typically expresses this protein.

Molecular testing

Molecular testing looks for changes in the DNA of tumour cells. These tests are usually performed on a tissue sample using techniques such as fluorescence in situ hybridization (FISH).

In selected cases, molecular testing may be used to look for a USP6 gene rearrangement, which is found in about 70% of aneurysmal bone cysts. Identifying this genetic change supports the diagnosis, especially when microscopic features overlap with those of other bone tumours. The percentage of tumour cells carrying this rearrangement can sometimes be low.

Molecular testing is supportive but not required for diagnosis, and the presence or absence of a USP6 rearrangement does not affect prognosis.

What conditions can look similar?

Aneurysmal bone cyst–like areas can occur in other bone tumours that have undergone bleeding and cystic change, most commonly:

  • Giant cell tumour of bone.

  • Osteoblastoma.

  • Chondroblastoma.

  • Fibrous dysplasia.

A critical diagnosis to exclude is telangiectatic osteosarcoma, a malignant bone tumour that can look similar on imaging and gross examination. Unlike aneurysmal bone cyst, telangiectatic osteosarcoma contains malignant tumour cells within the septa.

What is the prognosis for a person with an aneurysmal bone cyst?

Aneurysmal bone cysts are benign, but they can recur locally, especially if not completely removed. Recurrence rates after curettage range from 20% to 70%, depending on location and treatment method.

Spontaneous regression after incomplete removal is very rare. The presence or absence of a USP6 gene rearrangement does not affect prognosis.

Questions to ask your doctor

  • Which bone is affected, and how large is the aneurysmal bone cyst?
  • Do the imaging findings support this diagnosis, or are other conditions being considered?
  • Was immunohistochemistry or molecular testing performed, and what did the results show?
  • What treatment options are recommended, and what is the risk of recurrence?
    How often will follow-up imaging or clinic visits be needed?
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