radiation therapy
Meaning of radiation therapy: Treatment can come from a machine outside the body, from a source placed inside it, or from a radioactive drug carried in the blood.
Definition of radiation therapy
Radiation therapy is a cancer treatment that uses high energy radiation, such as x-rays, gamma rays, or beams of particles, to damage the DNA inside cancer cells so they can no longer divide. Healthy cells in the path of the beam are damaged as well, but they generally repair that damage more successfully than cancer cells do.
How does the radiation reach the tumor?
| Form | Where the radiation comes from | Typical use |
|---|---|---|
| External beam | A machine outside the body aims beams from several angles | The most common approach, used for tumors almost anywhere |
| Brachytherapy | A sealed radioactive source placed inside or beside the tumor | Concentrated dose to a small area, as in cervix or prostate |
| Systemic | A radioactive drug swallowed or injected, carried in the blood | Disease in several places at once, or tissue that takes up the drug |
Systemic treatment relies on the body sending the radioactive substance where it is needed. Sometimes the tissue itself does the work, as thyroid tissue does with radioactive iodine. Sometimes the radioactive atom is attached to a molecule that seeks out a marker on the tumor surface, so the dose travels with it.
Why is treatment split into many small sessions?
Dividing the total dose into daily fractions is deliberate. Normal tissue repairs radiation damage between sessions more reliably than tumor tissue does, so spreading the dose out widens the gap between the harm done to the cancer and the harm done to everything around it. That is why a course often runs on weekdays over several weeks. Some situations reverse the logic: a small, well defined target away from delicate structures can be treated with a few large, tightly focused doses instead. Beams of protons or other charged particles are used for the same reason, since they deposit most of their energy at a set depth and drop off sharply beyond it, as in proton beam radiation therapy.
What decides which approach is used?
The choice follows the cancer type, where it sits, how far it has spread, and what the treatment is meant to achieve. Radiation given to cure works to a different plan from radiation given to relieve pain or pressure, which is often shorter. Proximity to structures that tolerate radiation poorly, such as the spinal cord, bowel or lens of the eye, shapes both the technique and the dose. Radiation is also timed around other treatment, given before surgery to shrink a tumor, after surgery to clear cells left behind, or alongside chemotherapy that makes cells more sensitive to it.
Also known as
Used in a sentence
External beam radiation therapy was planned as part of the patient's treatment following surgery.
Good to know
External beam and internal radiation therapy differ in how the radiation reaches the tumor, a distinction that shapes both the side effects and the way a course of treatment is scheduled.
Word origin
Where “radiation therapy” comes from
Formed from therapy, from Greek therapeia, meaning treatment or healing, combined with radiation, from Latin radius, meaning a ray or spoke.