Fibrous Dysplasia: Understanding Your Pathology Report

Section Editor: Bibianna Purgina, MD FRCPC
October 1, 2026

Fibrous dysplasia is a noncancerous condition in which part of a bone develops abnormally. Instead of normal, solid bone, the affected area consists of soft fibrous tissue with small, weak, irregular pieces of bone. This can weaken the bone so that it may bend, break, or change shape. Fibrous dysplasia can affect a single bone or several bones. It is most often found in the skull, face, jaw, ribs, and long bones of the legs.

Fibrous dysplasia is not cancer, and it very rarely turns into cancer. Many people have no symptoms and need no treatment. This article explains what causes fibrous dysplasia, how a pathologist confirms the diagnosis, and what the findings in your pathology report mean.

What causes fibrous dysplasia?

Fibrous dysplasia is caused by a change, or mutation, in a gene called GNAS. This gene helps control how bone-forming cells grow and mature. The mutation keeps the gene switched on, so the cells that normally build bone instead form fibrous tissue and poorly made bone.

The GNAS mutation in fibrous dysplasia happens by chance in a single cell early in development, before birth. Only the cells that come from that first cell carry the mutation. This type of change is called a somatic mutation. Fibrous dysplasia is therefore not inherited from a parent and cannot be passed on to children. It is not caused by anything a person or their parents did.

What are the types of fibrous dysplasia?

Fibrous dysplasia is grouped by how many bones are affected and whether other parts of the body are involved. The types are:

Monostotic fibrous dysplasia. Only one bone is affected. This is the most common type and is often found incidentally on an X-ray taken for another reason.
Polyostotic fibrous dysplasia. Two or more bones are affected. This type usually appears in childhood and is more likely to cause pain, fractures, or changes in bone shape.
McCune-Albright syndrome. Fibrous dysplasia occurs together with conditions outside the bones. These include flat, light brown patches on the skin called café-au-lait spots and glands that make too much hormone. In children, this can cause early puberty. Other problems can include an overactive thyroid, excess growth hormone, and phosphate loss through the kidneys.
Mazabraud syndrome. Fibrous dysplasia occurs together with one or more noncancerous muscle tumors called intramuscular myxomas.

The type of fibrous dysplasia depends on how early the GNAS mutation occurred in development and which tissues the affected cells went on to form. The same mutation causes all four types.

What are the symptoms of fibrous dysplasia?

Many people with fibrous dysplasia have no symptoms. This is especially true of monostotic fibrous dysplasia, which is often found by chance. When symptoms occur, they depend on which bones are affected and may include:

Bone pain. Pain in the affected bone is common, particularly in adults.
Fractures. Weak bone can break with little or no injury. Fractures are most common in the long bones of the legs.
Changes in shape. An affected bone may bend or enlarge. In the leg, this can cause a limp or an uneven leg length. In the face or skull, it can cause swelling or uneven features.
Dental changes. Fibrous dysplasia in the jaw can move or crowd the teeth.
Nerve problems. Rarely, fibrous dysplasia in the skull presses on nearby nerves, which can affect vision or hearing.

Fibrous dysplasia is usually most active during childhood and the teenage years, when the skeleton is growing. In many people, the lesions become less active in adulthood.

How is the diagnosis of fibrous dysplasia made?

Fibrous dysplasia is often first suspected on imaging. On an X-ray or CT scan, the affected bone typically has a hazy, uniform appearance that radiologists describe as “ground-glass.” When the imaging findings are typical, a tissue sample may not be needed. A bone scan may be used to look for other affected bones.

When the imaging is not typical, or when another condition needs to be ruled out, a biopsy is performed. The sample may be taken with a needle or during surgery. Sometimes, doctors remove tissue during an operation to treat a fracture or scrape out the lesion. A pathologist then examines the tissue under the microscope to confirm the diagnosis of fibrous dysplasia.

Several other bone conditions can look similar to fibrous dysplasia under the microscope. These include other fibro-osseous lesions, healing fractures, and a slow-growing bone cancer called low-grade central osteosarcoma. To distinguish them, the pathologist may order molecular testing. Finding a GNAS mutation in the tissue supports a diagnosis of fibrous dysplasia. Testing for extra copies of the MDM2 gene helps rule out low-grade central osteosarcoma, which usually has extra MDM2 copies and no GNAS mutation.

A GNAS test that does not find a mutation does not rule out fibrous dysplasia. The mutation may be present in only some cells, and some samples may not contain enough of them to detect it. The final diagnosis combines pathology, imaging, and the patient’s age and symptoms.

What does fibrous dysplasia look like under the microscope?

Under the microscope, fibrous dysplasia has two main components. The first is fibrous tissue made of spindle cells, which are long, thin cells similar to those that form scar tissue. In fibrous dysplasia, these cells look bland, meaning they lack the abnormal features seen in cancer cells.

The second part is small pieces of immature bone scattered through the fibrous tissue. These pieces of bone are thin, curved, and irregular. Pathologists sometimes describe their shapes as resembling letters of the alphabet or Chinese characters. In normal growing bone, a row of bone-making cells called osteoblasts lines the surface of each piece. In fibrous dysplasia, this lining is usually missing or incomplete.

Fibrous dysplasia can also contain other findings, such as fluid-filled spaces called cysts, small areas of cartilage, or collections of immune cells. In the bones of the face and skull, the pattern of the bone may look slightly different. These variations are expected and do not change the diagnosis.

What is the long-term outlook for fibrous dysplasia?

Fibrous dysplasia is a noncancerous condition, and most people live a normal lifespan. The long-term outlook depends mainly on how many bones are affected and where. Important points include:

Activity over time. Fibrous dysplasia usually grows most during childhood. In many people, the lesions become stable in adulthood, although pain can continue.
Regrowth after surgery. Fibrous dysplasia often grows back in the same bone after it is scraped out. For this reason, surgery usually focuses on supporting or straightening the bone rather than removing the lesion.
Fractures and bone shape. People with more than one affected bone, especially in the legs, have a higher risk of fractures and bone shape changes.
Change to cancer. Very rarely, a cancer develops in an area of fibrous dysplasia. This happens in less than 1% of people. The most common type is osteosarcoma. The risk is higher in people with many affected bones and in those who have had radiation treatment to the area.

A new or worsening pain, rapid growth, or a change on imaging in a known area of fibrous dysplasia should be checked by a doctor. A new biopsy may be considered to make sure the lesion has not changed.

What happens after a diagnosis of fibrous dysplasia?

Once pathology confirms fibrous dysplasia, the next steps depend on which bones are affected and whether there are symptoms. Involvement of other parts of the body also matters. Possibilities include:

Observation. Many people, especially those with a single affected bone and no symptoms, are followed with periodic check-ups and imaging.
Surgery. Surgery may be considered to treat or prevent a fracture, to correct a bone that has changed shape, or to relieve pressure on a nerve. In the leg, this often involves placing metal rods or plates to support the bone.
Medicines for bone pain. Bisphosphonates, which slow bone breakdown, may be considered for people with bone pain. The FDA has not approved them specifically for fibrous dysplasia, but they are commonly used for this purpose. Denosumab, another medicine that slows bone breakdown, has been used in some people whose pain does not improve and is still being studied.
Hormone and blood tests. When more than one bone is affected, doctors may use blood and urine tests to look for features of McCune-Albright syndrome. If these are found, an endocrinologist, a doctor who treats hormone conditions, is often involved in care.

A team that may include surgeons, dentists, and eye and ear specialists often manages fibrous dysplasia in the face, skull, or jaw. Radiation treatment is generally avoided because it has been linked to a higher risk of the rare change to cancer.

Questions to ask your doctor

Is my fibrous dysplasia in one bone or in several bones?
Was a biopsy needed, or was the diagnosis made from imaging?
Was GNAS testing done on my sample, and what did it show?
Were tests done to rule out a bone cancer such as low-grade central osteosarcoma?
Do I need a bone scan to look for other affected bones?
Should I be tested for McCune-Albright syndrome?
Is the affected bone at risk of breaking?
Does the lesion need surgery, or can it be watched?
What can be done to manage my bone pain?
How often will I need follow-up imaging?
What symptoms should prompt me to come back sooner?
Can fibrous dysplasia be passed on to my children?