How Radiation Therapy Works Radiation therapy is the application of a variety of different types of radiation to treat tumors. It achieves the treatment by killing the cells. Therefore, the normal tissues irradiated during the treatment process will also be damaged, but because the ability of normal tissues to resist radiation damage and repair after being damaged is generally stronger than tumor tissues, so that when irradiated to a certain dose, tumor cells have died, while normal tissues can still repair despite the damage, which achieves the purpose of treatment, and that is why there are radiation reactions and side effects .