Atypical Teratoid/Rhabdoid Tumor (AT/RT) is a very rare, fast-growing tumor that commonly starts in the kidneys, the brain and spinal cord or other soft tissues of the body. While AT/RT can occur in adults and older children, it usually occurs in children ages three years and younger. About half of these tumors occur in the cerebellum (the part of the brain that controls movement and balance) or the brain stem (the part of the brain that controls breathing, heart rate and other vital life functions).