A radiation therapy procedure that uses specialized equipment to position the patient and precisely deliver a large radiation dose to a tumor and not to healthy tissue. This is not a traditional surgery where an incision is made and tissue removed. It is used to treat brain tumors and other brain disorders by delivering ablative (obliterative) radiation doses. It is also being studied in the treatment of other types of cancer, such as lung cancer. Also called radiation surgery, radiosurgery, stereotactic external beam radiation, stereotactic radiation therapy and stereotactic radiosurgery.