Rhabdoid tumor is a type of tumor that is made up of many large cells. Some rhabdoid tumors can grow in the brain, and these are called atypical teratoid rhabdoid tumors (ATRT). Most often, they grow in the kidneys and other soft tissues, like the muscles attached to the bones of the skeleton. Rhaboid tumors that grow outside of the brain are called extra-cranial malignant rhabdoid tumor, malignant rhabdoid tumor, or MRT. MRTs grow and spread to other parts of the body quickly. MRT is very rare. Some studies estimate that less than one person for every one million people has this type of rare tumor. It is most common in babies between 11 and 18 months old. MRTs sometimes look like other tumors. Doctors use a few different tests to diagnose MRT. Surgery is used to remove as much of the cancer as possible. Sometimes, doctors can remove the entire tumor in one piece.