Focused energy can destroy cancer cells while sparing enough normal tissue for recovery. Radiotherapy is the use of ionizing radiation to kill or damage cancer cells as part of cancer treatment.

External-beam radiation is delivered by machines outside the body; brachytherapy places radioactive sources inside or beside the tumor. Planning uses detailed imaging to shape the dose to the target and limit exposure to organs at risk. Treatment is typically fractionated over days or weeks.

Side effects depend on the region treated and may be acute or delayed. Radiotherapy can be curative, adjuvant, or palliative. Advances in precision—intensity modulation, image guidance, proton beams—have improved the therapeutic ratio. Coordination with surgery and systemic therapy is routine.

• Cancer treatment using ionizing radiation
• Delivered externally or by implanted sources
• Planned to maximize tumor dose and spare normal tissue
• Used for cure, adjuvant therapy, or palliation