Radiotherapy is a hospital treatment that uses radiation — a kind of strong energy you cannot see — to fight illness, most often cancer. The energy is aimed at the sick cells to kill them or stop them growing. A course can last many days; each visit may be short, but the tiredness can last. Though actually, people mix this up with chemotherapy, which uses drugs, not rays, and with a single X-ray photo, which only looks inside you for a moment. Patients often say they are "having radiation"; radiotherapy is the more formal name. The radio- here means rays, not the radio in the kitchen. You are not turned into a glowing piece of metal, though the hospital will still give you rules about skin and rest.