Fatty lump (lipoma) seen by chance on a scan
An ultrasound, CT or MRI done for something else may report a "lipoma", a "simple lipoma" or a "fatty lesion". A lipoma is a harmless lump of fat cells under the skin or, less often, in a muscle. They are common, and most found this way are simply noted. (NHS lipoma)(UK sarcoma 2025)(Natella 2023) This page covers what the report means, how common lipomas are and the signs that need a check, not treatment.
What the report words mean
- Lipoma. A soft, fatty lump that grows slowly, can be from the size of a pea to a few centimetres across, often moves a little when pressed and is not usually painful. Lipomas can appear anywhere, often on the shoulders, chest, arms, back, bottom or thighs. (NHS lipoma)
- Simple lipoma. On ultrasound a lipoma usually looks even and bright with clear edges, and on MRI it looks the same as normal body fat on every type of picture. (Natella 2023)
- Superficial or deep. Reports say whether the lump is just under the skin (superficial) or under the tough layer covering the muscles (deep). Specialists sort fatty lumps this way first. (Natella 2023)
- Atypical lipomatous tumour (ALT). Also called a well-differentiated liposarcoma. A fatty growth that is much less common than a lipoma and tends to be larger, deep and in the leg. (UK sarcoma 2025) Experts class it as intermediate (locally aggressive): it sits between a lipoma and the cancers that spread. (Natella 2023)
How common lipomas are, and at what ages
- The NHS says lipomas are common. (NHS lipoma)
- A UK sarcoma guideline says a benign lipoma is by far the most common soft tissue lump of the limbs and body seen in family practice, and that most soft tissue lumps turn out to be benign lipomas. (UK sarcoma 2025)
- Benign soft tissue growths are about 60 times more common than cancerous ones (3,000 versus 50 per million people), and at least 30% of the benign ones are lipomas. Exact numbers are unknown because many lipomas cause no symptoms and are never reported. (Natella 2023)
- Fatty soft tissue growths are seen most often between about 40 and 70 years of age; atypical lipomatous tumours mostly between about 50 and 70. (Natella 2023)
Scan features that call for a closer look
A 2023 review lists MRI features that make a fatty lump less likely to be a simple lipoma: (Natella 2023)
- lumps of non-fat tissue inside it, or fatty areas that look different from normal fat
- thick strands (septa) over 2 mm, or strands that are irregular or light up with contrast dye
- clear growth over time
- calcium deposits inside it
- a lump in the chest, the back of the belly, the pelvis or the groin cord rather than under the skin
- length over 10 cm just under the skin, or over 5 cm when deep
A 2024 review of 13 studies with 1,390 people found the features most often linked to an atypical lipomatous tumour were size of 110 mm or more, age over 60, a lump in the leg, an irregular shape, thick strands, lumps of non-fat tissue and contrast uptake. Scans were 76% to 95% accurate, so they cannot always settle the question. (Muhib 2024) Specialists may use a genetic test on tissue (MDM2) to tell the two apart. (Natella 2023)
Which checks guidance lists
- An examination. The NHS says a GP will usually be able to tell if a lump is a lipoma, and may arrange a scan if there is any doubt. (NHS lipoma)
- Ultrasound first. NICE says doctors should consider an urgent ultrasound (within 2 weeks) for an adult with an unexplained lump that is increasing in size, and a suspected cancer referral if the ultrasound suggests a sarcoma or is uncertain and concern remains. (NICE NG12)
- MRI when unsure. The UK sarcoma guideline says ultrasound depends on the skill of the person doing it, so an MRI should be done when the result is uncertain. (UK sarcoma 2025)
- Lumps inside the belly. The same guideline says a mass at the back of the belly or inside it that looks like a sarcoma on CT or MRI should go to a specialist sarcoma team. (UK sarcoma 2025)
- Worry is real. See after an abnormal screening result.
The NHS says to see a GP if you get a lump anywhere on your body, or if a lump is painful, red or hot to touch, is hard and does not move, or gets bigger. (NHS lipoma) It says a soft tissue sarcoma lump is usually deep, solid, painless and hard to move, and keeps growing. Having a lump does not mean you have cancer, but it is important to get it checked. (NHS sarcoma)
Other things the same scan may show
The same scan may mention other chance findings, each with its own page: an enlarged lymph node, a spot on a bone and a groin or belly button hernia.
Who it applies to
Adults told that a scan done for another reason shows a lipoma or fatty lump. Children and young people, and people already being seen for a soft tissue growth, have their own care and are outside this page. (NICE NG12)
Routine age-based screening
There is no screening test for lipomas or soft tissue sarcoma, and a finding like this does not replace the routine screenings for your age. See screenings by age and gender.
Where they differ:
- Which lumps need a closer look. NICE bases its advice on a lump that is unexplained and getting bigger, with ultrasound as the first test. (NICE NG12) European cancer guidelines quoted in a 2023 review suggest MRI for any deep lump, a lump over 5 cm near the surface, or one growing fast. (Natella 2023) The UK sarcoma guideline notes that older UK criteria based on size over 5 cm, depth or pain sent many lipomas and cysts to specialists and found few sarcomas. (UK sarcoma 2025)
- What size counts as large. The 2023 review uses over 10 cm under the skin or over 5 cm when deep; the 2024 review found 110 mm or more most often linked to an atypical lipomatous tumour. (Natella 2023)(Muhib 2024)
- How common lipomas are. The sources agree lipomas are very common, but no exact figure is given, because many cause no symptoms and are never reported. (NHS lipoma)(Natella 2023)
Your doctor decides what your result means for you.
Questions people ask
Is a lipoma cancer?
No. A lipoma is a harmless lump of fat cells. The NHS says lipomas are common, soft, slow growing and not usually painful. Scan reports sometimes mention atypical features, which means the lump does not look exactly like plain fat and may need a closer look. (NHS lipoma) (Natella 2023)
How common are lipomas?
Very common. A UK sarcoma guideline says a benign lipoma is by far the most common soft tissue lump of the arms, legs and body seen by family doctors. Lumps of fat tissue are seen most often between the ages of about 40 and 70. (NHS lipoma) (UK sarcoma 2025) (Natella 2023)
What does "atypical lipomatous tumour" mean on a report?
It is a fatty growth with some features that differ from a plain lipoma, such as thick strands or lumps of non-fat tissue inside it. It is much less common than a lipoma and tends to be larger, deeper and in the leg. Scans cannot always tell the two apart, so a specialist may arrange more tests. (UK sarcoma 2025) (Natella 2023) (Muhib 2024)
What signs mean a lump should be checked?
The NHS says to see a GP for any lump, and especially a lump that is painful, red or hot, hard and fixed, or getting bigger. NICE says doctors should consider an urgent ultrasound for an adult with an unexplained lump that is increasing in size. Your doctor decides what your result means for you. (NHS lipoma) (NICE NG12)