The work behind this word is the treatment of illness — most often cancer — with a carefully aimed beam of radiation. That treatment is called radiotherapy. A radiotherapist is the person whose job that treatment is. Though actually, hospitals do not all use the title the same way. In some places the radiotherapist is the doctor who plans the treatment (in American English that doctor is usually a radiation oncologist). In others, especially around British hospital staff, the person who sees you each day and runs the machine may be called a radiotherapist or a therapeutic radiographer, while a doctor oversees the plan. They are not the radiologist who reads your scan, and they are not using a radio. If a dictionary only says “the treatment of illnesses using radiation,” it has named the work; this word, with therapist at the end, is the person.

放射治療師（實際做放射治療的人。若是規劃治療的醫師，臺灣多稱放射腫瘤科醫師）