Radiation therapy uses high-energy ionizing radiation to damage the DNA of rapidly dividing cancer cells, causing double-strand DNA breaks that prevent mitotic division and lead to cell death. Radiation affects both cancer and normal cells, but cancer cells generally have impaired DNA repair mechanisms, making them more susceptible. The 4 Rs of radiobiology explain tumor control vs. normal tissue sparing: Repair (normal cells repair better between fractions), Redistribution (radiation catches cells in sensitive G2/M phases), Reoxygenation (tumors reoxygenate between fractions improving response), and Repopulation (surviving cells repopulate). Side effects depend on the irradiated field: skin reactions in all fields, mucositis in head/neck, pneumonitis in chest, enteritis in abdomen/pelvis, and bone marrow suppression in large bone-containing fields.