A lipoma is a noncancerous tumor made of mature fat cells, which pathologists call adipocytes. It is the most common soft tissue tumor in adults.
Lipomas grow slowly, stay in one place, and do not spread to other parts of the body. They are different from cancerous fat tumors such as liposarcoma, and a lipoma does not turn into one over time.
Most lipomas are diagnosed in people between 40 and 60 years old, and they are slightly more common in men. They are uncommon in children.
This article explains how a lipoma is diagnosed, what the findings in your pathology report mean, and when a fatty tumor needs further testing.
Most lipomas develop just beneath the skin, in a layer called the subcutaneous tissue. Common locations include the neck, upper back, shoulders, upper arms, and upper thighs. Lipomas are less common in the hands, feet, and lower legs.
Some lipomas develop deeper in the body. A lipoma inside a muscle is called an intramuscular lipoma, and a lipoma on the face usually sits beneath the layer of tissue covering the facial muscles. Deep lipomas are harder to feel and are usually found on imaging.
A lipoma usually appears as a soft lump under the skin that moves easily when you press it. It grows slowly and is typically painless. Many people notice lipomas by chance or because of their appearance.
Some lipomas cause discomfort. This happens when the lump presses on a nearby nerve or sits in an area that is exposed to repeated pressure.
For most people, the cause of a lipoma is unknown. The tumor is not caused by diet, weight, or injury, and a lipoma does not shrink with weight loss.
At the genetic level, many lipomas have a change involving a gene called HMGA2, which helps control the growth of fat cells. This change develops in the tumor cells during a person’s lifetime and is not inherited.
Some people develop several lipomas over time. This may happen on its own or as part of an inherited condition:
Having more than one lipoma does not by itself mean you have an inherited condition. Your doctor may consider one if lipomas developed at a young age, if several family members are affected, or if other growths are present.
Doctors often identify many lipomas during a physical exam, and imaging such as ultrasound or MRI often shows a typical appearance. A pathology report is produced when the lump is removed in a procedure called an excision.
Under the microscope, a lipoma is made of mature fat cells that look like normal fat. The cells are arranged in groups called lobules and are separated by thin bands of fibrous tissue. A thin capsule usually surrounds the tumor, which is why a lipoma can often be removed in one piece.
The fat cells in a lipoma look uniform. Their nuclei, the part of the cell that holds the genetic material, are small and evenly sized. This uniform appearance is what separates a lipoma from a fatty tumor that needs closer attention.
Your report may describe changes within the lipoma, such as scarring, areas where fat has broken down, or gel-like myxoid change. Rarely, a lipoma contains bone or cartilage, and these are called osteolipoma and chondrolipoma. None of these findings change the noncancerous nature of the tumor.
The most important tumor to distinguish from a lipoma is an atypical lipomatous tumor, also known as well-differentiated liposarcoma. It is also made of fat, but it grows into surrounding tissue and can come back after surgery.
Under the microscope, an atypical lipomatous tumor contains fat cells of widely varying size and scattered cells with enlarged, dark nuclei. When the distinction is unclear, doctors test for extra copies of the MDM2 gene. A lipoma does not have extra copies of MDM2, and an atypical lipomatous tumor does.
Several noncancerous tumors are also made partly of fat and have their own names. Your report may use one of these instead of lipoma:
Most fatty lumps are simple lipomas. Certain features make doctors look more carefully, because they can also be seen with an atypical lipomatous tumor:
These features do not mean a tumor is cancerous. They are reasons to get imaging, see a specialist, or do MDM2 testing before or after removal.
A pathology report for a lipoma is usually short. It confirms the tissue is mature fat and that no worrisome features are present.
Your report may comment on margins, which are the edges of the tissue cut by the surgeon. For a lipoma, a positive margin is not a cause for concern in the way it is for cancer. It only means some fat may remain, which slightly raises the chance the lump will come back. Many lipomas are removed in pieces, and in that situation margins cannot be assessed at all.
The outlook after lipoma removal is excellent. Complete removal is usually curative, and fewer than 5% of lipomas come back. When one does come back, it grows slowly and does not damage surrounding tissue.
A lipoma does not spread to other parts of the body and does not become cancer. A typical lipoma that has been removed needs no follow-up imaging.
Most lipomas need no treatment. You can leave a small, painless lump alone and monitor it.
If more than one lipoma is present and an inherited condition is suspected, your doctor may suggest a review of your family history or a referral for genetic assessment.