Radiation Therapist Job Description. Radiation therapist assisting woman with a scan. Radiation Therapist Job Description: Treating cancer in the human body is the principal use of radiation therapy.As part of a medical radiation oncology team, radiation therapists use machines—called linear accelerators—to administer radiation treatment to patients.Linear accelerators, used in a procedure called external beam therapy, project high-energy x rays at targeted cancer cells.s part of the Radiation Therapist Job Description during the treatment phase, the radiation therapist monitors the patient’s physical condition to determine if any adverse side effects are taking place.