Fibro-osseous Lesion: Definition

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A fibro-osseous lesion is a growth made of fibrous tissue that contains small pieces of new bone. “Fibro” refers to the fibrous tissue, and “osseous” means bone. Most fibro-osseous lesions start inside a bone, especially the bones of the jaw and face. They can also be found in soft tissue. This happens when the lesion starts in soft tissue or when a bone lesion grows into the surrounding tissue.

Fibro-osseous lesion is a general description, not a final diagnosis. It covers a group of conditions that look very similar under the microscope but behave differently. Almost all fibro-osseous lesions are noncancerous. This article explains what a fibro-osseous lesion is, the main types, and why your report may use this term.

What does a fibro-osseous lesion look like under the microscope?

Under the microscope, a fibro-osseous lesion has two main parts. The first is a fibrous background made of spindle cells, which are long, thin cells similar to the cells that form scar tissue. The second is small, irregular pieces of new bone scattered through the fibrous tissue.

The new bone may be made of osteoid, the soft material that later hardens into bone, or it may already be hardened. In some lesions of the jaw, the bone forms small round deposits that resemble cementum, the hard tissue that covers the roots of the teeth. The shape and arrangement of the bone pieces can offer clues to the specific type.

What are the main types of fibro-osseous lesions?

The term covers several different conditions. The main types are:

Fibrous dysplasia. A noncancerous condition in which a bone develops abnormally. It is caused by a change in the GNAS gene that occurs during development and is not inherited. It can affect one bone or several. Fibrous dysplasia often stops growing after the teenage years.
Ossifying fibroma. A noncancerous tumor, usually found in the jaw. Unlike fibrous dysplasia, it forms a well-defined lump with a clear border and slowly grows larger. It is often called cemento-ossifying fibroma. Some types, which usually affect children and young adults, grow more quickly.
Cemento-osseous dysplasia. A noncancerous condition that affects the tooth-bearing areas of the jaw. It is most common in middle-aged women. It is often found by chance on a dental X-ray and usually does not need treatment.
Soft tissue fibro-osseous lesions. Bone can also form in soft tissue, such as muscle or tissue around a joint, usually in response to injury. This process is called heterotopic ossification. These reactive lesions are noncancerous and often shrink or stop growing over time.

These conditions are treated very differently. Fibrous dysplasia and cemento-osseous dysplasia are often only watched. An ossifying fibroma is usually removed because it continues to grow.

Why does my report say fibro-osseous lesion instead of a specific diagnosis?

The different types of fibro-osseous lesions can look almost identical under the microscope. This is especially true on a small biopsy. The features that separate them are often seen best on imaging, such as an X-ray or CT scan.

For this reason, a pathologist may describe the finding as a fibro-osseous lesion and recommend comparing the results with the imaging. Your report may say “fibro-osseous lesion, see comment” or “correlation with imaging is recommended.” The final diagnosis is then made by combining the pathology, imaging, and the lesion’s location.

How is the specific type of fibro-osseous lesion identified?

Identifying the specific type usually requires several pieces of information:

Imaging. X-rays and CT scans show whether the lesion has a clear border and whether it involves one bone or several. Imaging also shows whether the lesion started in bone or soft tissue.
Location and age. Some types are more common in certain bones, in certain age groups, or around the roots of the teeth.
Genetic testing. Finding a change in the GNAS gene supports a diagnosis of fibrous dysplasia.
Tests to rule out cancer. Rarely, a slow-growing bone cancer can look like a fibro-osseous lesion. Examples include low-grade central osteosarcoma and parosteal osteosarcoma. Testing for extra copies of the MDM2 gene helps tell these cancers apart from noncancerous fibro-osseous lesions.

What happens after a fibro-osseous lesion is found?

Next steps depend on the specific type and whether the lesion is causing symptoms. Your care team will consider the pathology report together with your imaging. Possible next steps include:

Watching the lesion. Many fibro-osseous lesions do not need treatment and are followed with imaging over time.
Removing the lesion. Lesions that are growing, causing pain or changes in appearance, or affecting the teeth may be removed.
Further testing. If the diagnosis is unclear, you may need additional imaging, genetic testing, or a larger sample.

Questions to ask your doctor

What type of fibro-osseous lesion do I have?
Did my pathology report match my imaging?
Is the lesion likely to keep growing?
Does the lesion need to be removed, or can it be watched?
Were tests done to rule out a bone cancer?
Could this lesion affect my teeth or other bones?
How often will I need follow-up imaging?