Intramuscular Myxoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina, MD FRCPC
October 3, 2026

An intramuscular myxoma is a noncancerous tumor that develops inside a muscle. It consists of scattered cells in a soft, gel-like background called myxoid tissue, which gives the tumor its name.

An intramuscular myxoma does not spread to other parts of the body and does not turn into cancer. Removing it cures almost everyone.

These tumors are usually found in adults between 40 and 70 years old, and they are about three times more common in women. The most common locations are the large muscles of the thigh, shoulder, buttock, and upper arm. Intramuscular myxoma is one type of myxoma.

This article explains how an intramuscular myxoma is diagnosed and what the findings in your pathology report mean.

What are the symptoms of an intramuscular myxoma?

The usual finding is a slow-growing lump deep within a muscle. Most are painless, and because they sit beneath the muscle surface, people often notice them only once they have grown large.

Some people feel a deep ache, fullness, or discomfort when using the muscle. A tumor near a nerve can cause tingling or numbness, although this is uncommon.

What causes an intramuscular myxoma?

Most intramuscular myxomas are caused by a change in a gene called GNAS. This change develops in the tumor cells during a person’s lifetime. It is not inherited and cannot be passed on to children.

The same gene change causes a bone condition called fibrous dysplasia, in which fibrous tissue replaces an area of normal bone. When a person has one or more intramuscular myxomas together with fibrous dysplasia, the combination is called Mazabraud syndrome.

Mazabraud syndrome is often described as a genetic condition, which can be misleading. The gene change happens early in development rather than being passed down. The condition does not run in families, and children of an affected parent are not at increased risk. People with Mazabraud syndrome tend to develop several myxomas, often in the same limb as the affected bone.

How is the diagnosis made?

Because an intramuscular myxoma sits deep in a muscle, imaging is usually the first step. On MRI, it appears as a well-defined mass with a very high water content, often with a rim of fat and a bright signal in the surrounding muscle.

A pathologist confirms the diagnosis by examining the tissue, either from a biopsy or after the whole tumor has been removed. A small biopsy can be difficult to interpret, because several cancers also contain myxoid tissue. Your report may therefore recommend removal before a final diagnosis.

What does an intramuscular myxoma look like under the microscope?

Under the microscope, an intramuscular myxoma is made of long, thin spindle cells spread thinly through abundant myxoid tissue, which looks light blue on a routine slide. The cells are few and uniform, and dividing cells are rare or absent. Blood vessels are sparse.

At the edge of the tumor, the myxoid tissue extends between individual muscle fibers, and the trapped fibers often look thinned. This is why the tumor can be difficult to separate cleanly from the muscle at surgery.

Your report may describe the tumor as hypocellular (few cells) or hypercellular (many cells). It may also mention degenerative changes that develop over time, including bleeding, scarring called fibrosis, fluid-filled spaces called cysts, and clumps of calcium called calcification. None of these make the tumor cancerous.

What is a cellular myxoma?

Some intramuscular myxomas contain more cells and more blood vessels than usual. Your report may call this a cellular myxoma or a cellular variant.

A cellular myxoma is still noncancerous. The extra cells make it resemble a myxoid cancer more closely, so additional care is taken with the diagnosis. These tumors also come back after surgery somewhat more often than the usual type.

How is this different from a cancer?

Several cancers contain myxoid tissue, including myxofibrosarcoma, myxoid liposarcoma, and low-grade fibromyxoid sarcoma. Separating them from an intramuscular myxoma is the main task for the pathologist.

An intramuscular myxoma has few cells, uniform cells, almost no dividing cells, and sparse blood vessels. Myxoid cancers contain more cells, normal-looking cells, and a network of small, branching blood vessels running through the tissue. A report naming an intramuscular myxoma means these possibilities have been considered and excluded.

Margins

A margin is the edge of tissue cut by the surgeon. Because the tumor extends between muscle fibers at its edge, your report may say that it reaches a margin.

For an intramuscular myxoma, a positive margin does not carry the meaning it does for cancer. It raises the chance that the tumor comes back in that spot, and surgeons weigh this against preserving the strength and function of the muscle.

What is the prognosis?

The outlook is excellent. An intramuscular myxoma is noncancerous, does not spread, and does not become cancer.

Removal is curative for almost everyone, and the tumor rarely comes back. Recurrence is more likely after incomplete removal and with the cellular variant. A tumor that comes back is still noncancerous.

What happens after the diagnosis?

Surgery. Removing the tumor along with a small rim of surrounding muscle is the usual treatment. A wider margin isn’t necessary and would cost muscle function.
Observation. A small tumor confirmed on biopsy and causing no symptoms may be watched instead of removed.
Follow-up. Routine long-term follow-up is usually not needed after complete removal of a single tumor.
Assessment for Mazabraud syndrome. Consider this when more than one myxoma is present or when imaging shows a bone abnormality. People with this syndrome are followed for both the myxomas and the bone condition.

Questions to ask your doctor

Was the whole tumor removed, and did it reach a margin?
Does my report describe a cellular myxoma?
How was this separated from a cancer containing similar tissue?
Is there a chance the tumor will come back?
Will the surgery affect the strength of that muscle?
Do I have more than one of these tumors, and should my bones be checked?
Do I need any follow-up imaging?