Tumor

A lump you can feel: lipoma or sarcoma?

Hee Seung Nam, MD, Orthopedic Surgeon · October 31, 2026

“I felt a lump in my thigh in the shower. Could it be cancer?” It is a question I hear often in clinic. Let me start with the reassuring part. Lumps you can feel are mostly benign. Only very rarely a malignant tumor called a sarcoma is mixed in, and at first a sarcoma looks almost the same as a lipoma from the outside.

What troubled me most while caring for bone and soft tissue tumor patients during residency was that the size of the later operation can change greatly depending on the order in which a lump was first investigated and treated. This article lays out how to tell lumps you need not worry about from lumps you must check.

99 of 100 lumps are benign

According to a review in the BMJ, soft tissue tumors of the limbs are about 100 times more often benign than malignant. The most common is the lipoma, a lump of fat, followed by the ganglion cyst around joints, the epidermoid cyst under the skin and the schwannoma arising from nerves.

A sarcoma is a malignant tumor that arises in connective tissue other than bone, such as muscle, fat, nerve, blood vessel and fibrous tissue. It is rare, about 1% of all adult cancers, and there are dozens of subtypes. The problem is that most are painless. Because it does not hurt, people assume “it must be a lipoma,” and often come to the hospital only after it has grown large.

How deep it lies matters

Under the skin there is a layer of fat, and beneath it the tough membrane that wraps the muscle, the fascia. Whether a lump is above or below the fascia is the first fork in the road.

The ultrasound guidance of the British Sarcoma Group (BSG) explains that a fatty lump in the layer of fat just under the skin is rarely malignant even if it looks slightly unusual on ultrasound. Conversely, a lump deep below the fascia, or a lump that grew back after being removed, is more likely to be a sarcoma.

Lumps to worry about: four signs

The warning signs that UK and European sarcoma guidelines have in common are as follows. If even one applies, it is best to check with imaging.

The UK National Institute for Health and Care Excellence (NICE) recommends that adults with a growing lump of unknown cause have an ultrasound within two weeks. The European Society for Medical Oncology (ESMO) guideline also recommends that a superficial lump larger than 5 cm, or a deep lump, be checked with imaging and biopsy.

There are liposarcomas that look like lipomas

Not every lump made of fat is a lipoma. Among fatty tumors that grow large in deep places, such as between thigh muscles, some are atypical lipomatous tumors (well-differentiated liposarcoma). They almost never spread elsewhere, but if removed incompletely they grow back and, rarely, can change into a more aggressive form.

In a study by a German team that compared 113 fatty tumors with MRI and genetic testing, there was not a single atypical lipomatous tumor in the fat layer under the skin, and most lay between muscles. On MRI, septa thicker than 2 mm, areas that brighten with contrast and very large size were the suspicious findings, and the final confirmation is made by finding MDM2 gene amplification in tissue.

There is an order to the tests

• Ultrasound: the first test. It shows the depth and size of the lump, its internal appearance and blood flow. A typical shallow lipoma or ganglion cyst can mostly be judged on ultrasound alone.
• MRI: if there are warning signs or the ultrasound finding is unclear, MRI is used to check the extent and nature of the tumor and its relation to nearby nerves and vessels.
• Biopsy: a bit of tissue is taken with a thick needle to confirm the diagnosis. Because the biopsy tract must also be removed with the tumor later, the principle is that the team that will do the final surgery plans where the needle goes in.

In a study by the Musculoskeletal Tumor Society in the United States that analyzed 597 patients, when the biopsy was done somewhere other than a tumor treatment center, diagnostic errors and complications were 2 to 12 times more frequent. In 19% of all patients, treatment became more complicated because of biopsy problems, and there were 18 amputations that could have been avoided.

Why “let's just take it out first” is dangerous

Sometimes a small, soft lump is removed without imaging and the biopsy then shows a sarcoma. In a meta-analysis (2024) of such unplanned excisions, when a wide re-excision was done, about 60% still had tumor remaining, and the five-year local recurrence risk was higher than with planned surgery.

To be fair, in this study there was no meaningful difference in overall survival. But to get rid of the remaining tumor you have to operate again much more widely than the first time, and radiation treatment is added in some cases. If sarcoma had been kept in mind and planned for from the start, it could have been done in one operation. That is why the principle is to get imaging first, before removing a lump that has warning signs.

In closing

• Lumps you need not worry too much about: a lump just under the skin that is small and soft, has stayed the same size for years and does not hurt is mostly a lipoma. Even so, have it checked once with an examination and ultrasound.
• Lumps you must check: 5 cm or larger, deep below the fascia, getting bigger, painful or recurrent. If even one of these applies, check with ultrasound and MRI, and if needed have a biopsy by a team experienced in tumor treatment.
• The order matters: imaging, then biopsy, then surgery. Following this order alone can change the outcome of sarcoma treatment.

This article is general medical information. The tests and treatment needed can differ with the location and size of the lump and the imaging findings, so please confirm through an examination.