Synovial sarcoma is a rare type of cancer that tends to arise near large joints, particularly the knee, in young adults. Despite its name, it typically doesn't affect the interior of joints, where synovial tissue and fluid are located.

The first sign of trouble is usually a deep-seated lump that may be tender or painful. Synovial sarcoma generally grows slowly. While these tumors can occur in young children, they generally develop in people between the ages of 15 and 40.

While synovial sarcoma can occur almost anywhere in the body, the most common locations are in the legs, arms and throat.

Synovial sarcoma is usually slow-growing, so it can be years before a definitive diagnosis is made. In some cases, synovial sarcoma initially is diagnosed incorrectly as arthritis or bursitis.

Imaging methods that may be helpful include:

• Plain X-ray. In some cases, large areas of the tumor are marked or even outlined by calcifications. However, many synovial sarcomas don't show up on X-rays.
• Computerized tomography. CT scans often are useful in determining the extent of the tumor.
• Magnetic resonance imaging. An MRI can show the degree to which the tumor is affecting adjacent soft tissue, such as blood vessels and nerves.

A sample of the tumor can be removed with a needle and then studied under a microscope to help determine the best treatment. Synovial sarcoma can be mistaken for other types of sarcomas, so a correct diagnosis depends on experienced pathologists.

Surgery is the mainstay of treatment for synovial sarcoma. The goal is to remove the cancer and a margin of healthy tissue around it. This can sometimes mean the removal of an entire muscle or muscle group, or even amputation.

To decrease the chances of recurrence, your doctor might suggest a regimen of radiation therapy or chemotherapy in addition to surgery.