Radiation therapy is a treatment that uses ionizing radiation primarily to treat cancer. It delivers radiation from an external machine or from radioactive sources placed inside the body, damaging malignant cells while limiting exposure of surrounding healthy tissue. Treatment may aim to cure disease, prevent recurrence, control tumor growth, or relieve symptoms. Unlike diagnostic radiology, which uses imaging to investigate disease, radiation therapy delivers doses intended to produce a therapeutic biological effect. (cancer.gov)
Physical and biological basis
Therapeutic radiation includes X-rays, gamma rays, and beams of charged particles. Radiation transfers energy to tissue and produces ionization, the removal of electrons from atoms or molecules. Biological damage may occur directly when radiation deposits energy in essential cellular molecules, or indirectly through reactive products formed when radiation interacts with water. These products can damage DNA and other cellular components. (training.seer.cancer.gov)
DNA damage that exceeds a cell’s capacity for repair can prevent further division or cause cell death. The response is not necessarily immediate: cancer cells may continue dying for weeks or months after treatment ends. Healthy cells can also be affected, making the balance between tumor control and normal-tissue injury central to treatment design. (cancer.gov)
The fundamental physical quantity is absorbed dose, the energy deposited per unit mass. Its unit is the gray (Gy), equal to one joule per kilogram. Absorbed dose alone does not fully describe biological effects, which also depend on radiation type and the conditions under which exposure occurs. (www-pub.iaea.org)
Delivery methods
External beam radiation therapy directs radiation toward a target from outside the body. Most treatments use photons, while electron beams are useful for relatively superficial targets. Proton beams can reach deep tumors and have a finite range in tissue, allowing different dose distributions from photon beams. These properties do not establish that one beam type is preferable for every cancer. (cancer.gov)
Brachytherapy places radioactive sources in or near the tumor. Sources may take the form of seeds, wires, ribbons, or capsules. Depending on the technique, they remain temporarily or permanently. Brachytherapy concentrates radiation near the source and may be used alone or combined with external irradiation. Its applications include selected cancers of the cervix, prostate, breast, and other sites. (training.seer.cancer.gov)
Radionuclide therapy administers radioactive substances by swallowing or injection. Their distribution determines which tissues receive radiation. Radioactive iodine is used for certain thyroid cancers; targeted radiopharmaceuticals combine a radioactive isotope with a molecule that directs it toward particular cancer cells. Radioimmunotherapy uses a monoclonal antibody as the targeting component. (cancer.gov)
Planning and precision techniques
External-beam treatment begins with simulation: imaging and positioning procedures that establish the treatment geometry. Computed tomography and, where appropriate, magnetic resonance imaging provide information about the tumor and nearby tissues. Reproducible positioning may involve molded supports or immobilization masks. Computerized planning determines beam directions and dose distributions. (cancer.gov)
Three-dimensional conformal treatment shapes beams to the target. Intensity-modulated radiation therapy varies beam intensity to create more complex dose distributions. Image-guided radiation therapy uses imaging around treatment delivery to check positioning and account for anatomical changes. Stereotactic techniques deliver highly focused radiation to selected targets: radiosurgery commonly concerns intracranial lesions, whereas stereotactic body radiation therapy treats targets outside the brain and spinal cord. Despite its name, radiosurgery does not involve a surgical incision. (cancer.gov)
Dose schedules and clinical roles
A course commonly divides the prescribed dose into separate treatments, or fractions. This permits healthy tissue to recover between exposures while maintaining the intended treatment effect. Hypofractionation uses larger doses per fraction and fewer treatments; hyperfractionation uses smaller fractions delivered more than once daily. The appropriate schedule depends on the disease, treatment goal, dose, and tissues involved, rather than a universal number of sessions. (cancer.gov)
Radiation may be the principal treatment or part of a combined approach with surgery, chemotherapy, or immunotherapy. Before surgery, it can reduce tumor size; afterward, it can treat microscopic disease remaining in the treatment region. In palliative care, radiation can relieve pain or other problems caused by tumors, including painful spread to bone. Treatment intent and expected outcomes therefore differ substantially between clinical settings. (training.seer.cancer.gov)
Adverse effects and radiation safety
Adverse effects depend on the treated region, dose, schedule, and other therapies. Fatigue is common. Local effects may include skin reactions, hair loss within the irradiated area, swallowing difficulties, diarrhea, or urinary symptoms. Some effects resolve after treatment; others persist or first appear months or years later. Late effects can include tissue hardening, organ dysfunction, fertility impairment, and, rarely, a second cancer. These are possible outcomes, not effects experienced by every patient. (cancer.gov)
External-beam treatment does not make the patient radioactive. Internal sources can emit radiation while present, and systemic radioactive treatments can temporarily make body fluids radioactive. Radiation-safety arrangements therefore differ by delivery method and source, including whether radioactive material remains in the body. (cancer.gov)