Myxofibrosarcoma (MFS) is a type of cancer that typically appears as a slow-growing, painless lump on one of your legs or arms. The disease occurs mainly in people between the ages of 50 and 70 and is slightly more common in men than in women.

MFS is a soft tissue sarcoma. Like other soft tissue sarcomas, it arises in connective tissue — specifically, the connective tissue that surrounds muscles and separates muscles from each other and from skin. An MFS may lie just below the skin or deeper in muscle tissue.

MFS is not as well-known or understood as many other types of cancer, partly because the features distinguishing it from another soft tissue sarcoma came into focus only recently. Also, an MFS is often mistaken for a benign tumor and treated as such, by removing only the visible growth. This delays accurate diagnosis and complicates further treatment.

Although MFS tends to recur in the same area where it started, a majority of patients live five or more years after treatment.

Diagnosing an MFS can be a challenge. The initial growth often represents just part of the underlying tumor, which may have spread extensively around the base of the growth and down into deeper tissue. Even with advanced imaging, it can be difficult to accurately assess the true extent of the cancer.

Signs that a growth on a leg or arm may be an MFS or a similar cancer include:

• Being golf ball-sized or larger
• Getting larger over time
• Lying deep in muscle instead of just under the skin

Tests to assess a suspicious-looking growth that may be an MFS include:

• Magnetic resonance imaging.
• Core needle biopsy

If these tests find an MFS or another type of soft tissue sarcoma, a chest X-ray is typically the next step. Its purpose is to determine whether the cancer has spread (metastasized) to your lungs, the most likely site for MFS metastasis.

Treatment depends on the characteristics of the tumor, including:

• Depth. Tumors between layers of muscle tend to be more aggressive.
• Size. Tumors measuring 1.9 inches (5 centimeters) or more in width, length or depth may require more extensive surgery, sometimes with additional treatment, such as radiation with or without chemotherapy.
• Grade. An MFS can be either low (composed mainly of inactive rather than dividing cells, with no accumulation of dead cells); intermediate (with a higher proportion of actively dividing cells); or high (containing large numbers of dividing cells, along with dead cells). Low-grade tumors are less aggressive than intermediate- and high-grade tumors.
• Previous treatment. When MFS recurs, the second tumor often has a higher grade than the first.

MFS always requires surgery to remove the cancer and a margin of healthy tissue around it. The extent of surgery varies according to tumor size and other characteristics. Radiation therapy usually supplements surgical treatment.

• Small, low-grade, superficial tumors. If it isn't right next to a joint, a network of blood vessels or another critical structure, a small MFS just below the skin may be managed with surgery alone.
• Intermediate- or high-grade tumors. For a tumor with aggressive characteristics, pre-surgical radiation therapy can make it easier to identify the borders of the tumor so that there's a better chance of taking it out with a margin of healthy tissue. Alternatively, radiation therapy may follow surgery, with the goal of destroying cancer cells that might not have been removed. Anti-cancer drugs (chemotherapy) play a smaller role in MFS treatment than in treatment for other types of cancer.

In the past, as many as half the MFS patients were initially treated with amputation of the affected limb. Today, surgical, medical and radiation oncologists make limb preservation a priority whenever possible. In many cases, plastic surgeons also are involved in the initial surgery, laying the groundwork for reconstructive procedures to restore limb function.