Lipoma

What it means

A lipoma is a slow-growing lump made entirely of mature fat cells, packaged inside a thin fibrous capsule that separates it from the surrounding tissue. It's the most common soft tissue growth in adults, and most people who have one never need a scan to find it — many are noticed as a soft, movable lump under the skin. When imaging is done, usually to confirm what a lump is or because it was found incidentally while scanning for something else, a lipoma has a very characteristic, reassuring appearance.

Why it appears on a CT or MRI report

On both CT and MRI, fat has a distinctive signature that's different from muscle, fluid, or solid tumor tissue, which makes lipomas easy to recognize with confidence. Reports describe a well-defined, encapsulated mass that matches the density or signal of normal fat throughout, with a thin wall and no thick internal septations, solid nodules, or areas that light up strongly with contrast. Radiologists will note its size and location — common spots include the back, shoulders, arms, and thighs, though they can occur almost anywhere fat exists, including deeper locations between or within muscles. This uniform, fat-matching appearance is what allows a lipoma to be called benign on imaging alone, without a biopsy, in most cases.

What it usually means

A lipoma with this classic appearance is benign and typically needs no treatment at all — it's tracked, if at all, simply by noting it's there. They grow very slowly, if they grow at all, and don't invade surrounding tissue or spread elsewhere. The rare look-alike to be aware of is a liposarcoma, a cancer of fat tissue, which imaging distinguishes by features the classic lipoma lacks: thick or irregular septations, solid non-fat components, strong contrast enhancement, or rapid growth on repeat imaging. Larger lesions (generally above about 5 cm), those found deep within or between muscles rather than just under the skin, and any lesion with an atypical feature are more often referred for a biopsy or removal to confirm the diagnosis with certainty. The overwhelming majority of lipomas, however, are small, superficial, and unmistakably benign on the scan itself.

When to follow up

If your report describes a small, typical, superficial lipoma with no concerning features, no urgent action is usually needed — you can mention it at your next routine visit if it bothers you cosmetically or is uncomfortable. Removal is elective and usually done only if the lump is painful, restricts movement, is growing, or is unwanted for appearance's sake. See your doctor sooner if a lump is growing quickly, feels firm or fixed in place rather than soft and mobile, is painful without an obvious cause, or if the report flags any atypical imaging feature.

A plain-language way to picture it

Picture a small, soft beanbag tucked just under the skin, filled with the same padding material your body already uses to cushion and insulate itself, wrapped in a thin, smooth cover that keeps it separate from the tissue around it. It doesn't put down roots into anything nearby, doesn't change shape from one week to the next, and mostly just sits there quietly — which is exactly why, on a scan, it tends to be one of the most reassuring things a radiologist can point to and name with confidence.