Overview
Undifferentiated pleomorphic sarcoma is a rare type of cancer. It starts mostly in the body's soft tissues. Soft tissues connect, support and surround other body structures. Soft tissues include muscle, fat, blood vessels, nerves, tendons and linings of the joints.
Undifferentiated pleomorphic sarcoma typically forms in a leg or an arm. Less often it can happen in the area behind the organs in the belly. That area is called the retroperitoneum.
This cancer gets its name from the way the cancer cells look under a microscope. Undifferentiated means the cells don't look like the healthy cells around them. Pleomorphic (plee-o-MOR-fik) means the cells grow in many shapes and sizes.
Treatment depends on where the cancer is located. Treatment often involves surgery, radiation therapy and medicines.
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Symptoms
Symptoms of undifferentiated pleomorphic sarcoma depend on where the cancer is located. It most commonly forms in a leg or an arm. But it can happen anywhere in the body.
Signs and symptoms may include:
- A painless slow-growing lump or area of swelling.
- A feeling of pain or pressure.
- Loss of appetite.
- Weight loss.
When to see a doctor
Make an appointment with a doctor or other healthcare professional if you have ongoing symptoms that worry you.
Causes
The exact cause of undifferentiated pleomorphic sarcoma isn't known.
Cancer happens when cells get changes in their DNA. A cell's DNA holds the instructions that tell the cell what to do. In healthy cells, the DNA tells the cells to grow and multiply at a set rate. The DNA also tells the cells to die at a set time.
In cancer cells, the DNA changes give different instructions. The changes tell the cancer cells to grow and multiply quickly. Cancer cells can keep living when healthy cells would die. This causes too many cells.
The cancer cells might form a mass called a tumor. The tumor can grow to invade and destroy healthy body tissue. In time, cancer cells can break away and spread to other parts of the body. When cancer spreads, it's called metastatic cancer.
Risk factors
Factors that may raise the risk of undifferentiated pleomorphic sarcoma include:
- Older age. This cancer is most common in people 50 to 70 years old. But it can happen at any age.
- Previous radiation therapy. Rarely, this cancer can form in an area of the body that was previously treated with radiation therapy.
Most people who develop undifferentiated pleomorphic sarcoma have no known risk factors. Many people who have risk factors never develop this cancer.
Aug. 12, 2026