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.