How does radiation therapy work?

Radiation therapy works by wrecking the DNA inside cancer cells. DNA is the instruction manual a cell needs in order to divide, and a cell whose DNA is damaged beyond repair either stops dividing or dies. Your body then breaks down and clears the dead cells.

The dose is what separates treatment from a picture. The same kind of radiation at low dose gives you the x-ray of a broken bone. At high dose it becomes a cancer treatment.

Why it takes weeks instead of minutes

Radiation does not kill cancer cells on the spot. It takes days or weeks of treatment before enough DNA damage accumulates for cells to die, and cancer cells keep dying for weeks or months after the last session.

That is why the schedule looks the way it does. Most people have external beam treatment once a day, Monday through Friday. Splitting the total dose into small daily pieces, called fractionation, gives healthy cells in the path time to repair themselves between sessions, and it makes the radiation more effective against the cancer. The stretch from your first session to your last is called a course of treatment.

Researchers are testing other rhythms. Accelerated fractionation gives larger daily or weekly doses to shorten the number of weeks. Hyperfractionation gives smaller doses more than once a day. Hypofractionation gives larger doses less often to cut the number of visits. The hope is fewer side effects or the same result with less disruption to your life.

Two ways to get the dose where it needs to go

External beam radiation comes from a machine outside your body that aims at the tumor. It is a local treatment, so cancer in the lung means radiation to the chest, not to your whole body. Beams are made of photons, protons, or electrons. Photons are used most, reach deep tumors, and continue past the tumor into tissue behind it. Protons stop when they reach the tumor, which may spare more normal tissue, but the machines are large and costly, so few centers have them. Electrons cannot travel far through tissue, so they are for tumors on or near the skin.

Internal radiation puts the source inside you. When the source is solid it is called brachytherapy: seeds, ribbons, or capsules placed in or next to the tumor, giving off radiation for a while. It is used most often for cancers of the head and neck, breast, cervix, prostate, and eye.

When the source is liquid, the treatment becomes systemic, meaning it travels in your blood throughout your body. You swallow it or receive it through a vein. Radioactive iodine, or I-131, is the familiar example and treats certain thyroid cancers. Targeted radionuclide therapy links a radioactive atom to a molecule that seeks out cancer cells, and is used in some advanced prostate cancers and neuroendocrine tumors. With systemic radiation, your urine, sweat, and saliva give off radiation for a period, so your team will give you precautions.

The lifetime limit nobody mentions until it matters

Any area of your body can absorb only so much radiation over a lifetime. If a region has already been treated to that safe limit, it may not be possible to radiate it again, even years later and even for a new cancer. A different area can often still be treated if it is far enough away.

Radiation is also used for two different goals. It can be given to cure a cancer, to lower the chance it returns, or to slow it. It can also be given purely to relieve symptoms, which is called palliative treatment, shrinking a tumor that is causing pain, breathlessness, or bowel and bladder problems. Radiopharmaceuticals are used this way for pain from cancer that has spread to bone.

Sources

https://www.cancer.gov/about-cancer/treatment/types/radiation-therapy

https://www.cancer.gov/about-cancer/treatment/types/radiation-therapy/external-beam