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