A rare type of cancer that is made up of smooth muscle cells, connective tissue cells, and certain types of immune cells. It can occur anywhere in the body, but it usually occurs in the lung, abdomen, pelvis, or back of the abdomen. Inflammatory myofibroblastic tumors rarely spread to other parts of the body but they are likely to recur (come back) after treatment. The tumor cells may have a certain genetic change called a translocation (part of one chromosome switches places with part of another chromosome). Inflammatory myofibroblastic tumors usually occur in children and young adults. They are a type of soft tissue sarcoma.