An intramuscular lipoma is a noncancerous tumor made of mature fat cells that develops inside a muscle. It is a type of lipoma, the most common soft tissue tumor in adults, and it is the form most often found deep in the body.
An intramuscular lipoma does not spread to other parts of the body and does not turn into cancer. It differs from an ordinary lipoma under the skin in two ways. It often blends into the surrounding muscle rather than sitting in a neat capsule. It is also deep and can grow large, so it must be carefully separated from a fatty tumor that can behave like a cancer.
These tumors are usually found in adults, most often between 40 and 60 years old. The most common locations are the large muscles of the thigh, shoulder, upper arm, and trunk.
This article explains how an intramuscular lipoma is diagnosed and what the size, MDM2 result, and margins in your pathology report mean.
Your report may describe how the tumor relates to the surrounding muscle. This affects how easy the tumor is to remove:
Both patterns are noncancerous. The word infiltrating sounds alarming, but here it describes only how the fat grows between muscle fibers. It does not mean the tumor is invading like cancer does.
An intramuscular lipoma usually causes a firm, slow-growing lump deep in a muscle. Because it sits beneath the muscle surface, it is harder to feel than a lipoma under the skin and is often noticed only once it is large.
Most intramuscular lipomas are painless. Some cause aching, tightness in the muscle, or difficulty moving when the tumor is large or close to a joint. Occasionally the lump becomes more noticeable when the muscle is tensed.
Because an intramuscular lipoma is deep, imaging is usually the first step. An MRI shows a fat-filled mass and helps determine how far it extends within the muscle. Imaging alone cannot always distinguish an intramuscular lipoma from an atypical lipomatous tumor, so doctors usually examine tissue.
The diagnosis may be made after a core needle biopsy or after the whole tumor has been removed. Under the microscope, an intramuscular lipoma is made of mature fat cells that look like normal fat. The fat surrounds and separates bundles of skeletal muscle fibers, and the trapped muscle fibers often look thinned.
The fat cells look uniform. No cells have enlarged, dark nuclei, and no immature fat cells called lipoblasts are present. These absences are what separate an intramuscular lipoma from a fatty tumor that can behave like a cancer.
MDM2 is a gene that controls cell growth. Normal cells and lipoma cells have two copies of this gene. Atypical lipomatous tumor, also called well-differentiated liposarcoma, has many extra copies. Having extra copies is called amplification.
Because a deep or large fatty tumor can be either of these, your pathologist may order a test to count the copies of MDM2. The usual test is fluorescence in situ hybridization (FISH). A normal number of copies supports the diagnosis of an intramuscular lipoma, and extra copies point to an atypical lipomatous tumor instead.
Not every fatty tumor needs this test. It is most often used when the tumor is large, deep within a muscle, has returned after previous surgery, or looks unusual under the microscope.
Tumor size is the greatest dimension of the intramuscular lipoma, measured in centimeters (cm). The size is reported after the whole tumor has been removed. The tumor is measured in three dimensions, and the largest of these appears in the report.
Size does not change the noncancerous nature of an intramuscular lipoma. It matters for two other reasons. Larger tumors are more difficult to remove completely, and a large deep fatty tumor is one of the reasons a pathologist orders MDM2 testing.
A margin is the edge of tissue cut by the surgeon. A negative margin means no tumor cells are seen at the cut edge, and a positive margin means tumor cells reach it.
Positive margins are common with the infiltrating pattern, because the fat blends into the muscle and the surgeon works to preserve muscle function. A positive margin here does not mean what it does for cancer. It raises the chance that the tumor will come back in the same place, and it is one reason your surgeon may want to follow you for a period afterward.
The outlook is excellent. An intramuscular lipoma is noncancerous, does not spread, and does not become cancer over time.
The main issue is that it can come back in the same place, particularly when the infiltrating pattern made complete removal impossible. Reported recurrence rates vary widely between studies, from a few percent to a substantial share of cases in older reports. When an intramuscular lipoma does come back, pathologists usually repeat MDM2 testing, because a tumor that returns is occasionally found to have been an atypical lipomatous tumor from the start.
Treatment depends on the size of the tumor, where it is, and whether it causes symptoms.