Canadian Cancer Society logo
You are here: 

How radiation therapy works

Radiation therapy uses high-energy rays or particles to damage and destroy cancer cells in a specific (local) area of the body. Radiation can be delivered as a beam or inside implants. Sometimes a radioactive material is given by mouth or as an injection. It travels throughout the body, where it is absorbed by cancer cells.

The goal of radiation therapy is to give a high enough dose of radiation to kill cancer cells, while limiting the amount of radiation exposure and damage to normal cells. The dose of radiation that can be given to a tumour is limited by the amount of radiation nearby normal tissues can tolerate. The total dose of radiation is usually divided into a number of smaller doses (called fractions) so that it kills cancer cells but causes less damage to normal tissues.

Effect of radiation on cells

Radiation therapy works by damaging a cancer cell’s DNADNAThe molecules inside the cell that program genetic information. DNA determines the structure, function and behaviour of a cell. (deoxyribonucleic acid) so that it stops growing and dividing. Cells can repair damage from some radiation exposure. If doses of radiation are high enough, cells can be permanently damaged beyond repair. Different cells and tissues in the body tolerate radiation differently. Each part of the body can tolerate different amounts of radiation. Many areas of the body can only tolerate a certain amount of radiation in a lifetime (maximum dose).

The following characteristics of cancer cells make them vulnerable to the effects of radiation. As a result, radiation therapy is an effective treatment for many types of cancer.

Rapid growth and division

Normal cells and cancer cells divide to produce more cells. This process is called the cell cycle. Radiation therapy is usually most effective on cells that grow and divide quickly.

Cancer cells tend to divide more quickly than most normal cells. This makes them more vulnerable to the effects of radiation (radiosensitive) than normal cells.

Radiation seems to be most effective just before cell division (the G2 phase) and during cell division (the mitosis or M phase).

Giving radiation on a daily basis disrupts the cell cycle of cancer cells, so that more of them are in the radiosensitive phase when the next dose is given.

Presence of oxygen

Radiation has a greater effect on cancer cells when oxygen is present. Well-oxygenated cells are more sensitive to radiation than cells with too little oxygen (hypoxic).

When a tumour outgrows its blood supply, the cells in the centre of the tumour do not get enough oxygen. As the tumour shrinks during radiation treatment, the hypoxic cancer cells in the centre of the tumour come closer to the blood supply. They then become oxygenated and more sensitive to the next dose of radiation.

Limited ability to repair

Cancer cells have a limited ability to repair damaged DNA. This makes them more sensitive to the effects of radiation than normal cells in the body.

If cancer cells are damaged at the right time and repeatedly, they cannot repair themselves and they will die.

Most normal cells can repair damage and reproduce themselves between daily radiation treatments. Cancer cells are less able to do so.

Back to top

Radiation dose

The gray (Gy) unit has replaced the rad (radiation absorbed dose) unit as the accepted term for radiation dosage.

  • 1 Gy is equal to 100 rads.
  • 1 centigray (cGy) equals 1 rad.

The dose of radiation is determined by several factors, including:

  • how vulnerable the tumour is to radiation (radiosensitivity)
  • how well normal tissue tolerates radiation
  • how much tissue will be treated with radiation

Maximum dose

Different body cells or tissues tolerate different amounts of radiation. The maximum dose of radiation will depend on the area of the body being treated. Doctors consider the maximum dose when planning treatment. Once an area has received the maximum dose, it may not tolerate any more radiation. Therefore, having radiation therapy in the past may limit the amount of radiation a person can safely receive in the future.

Back to top

Half-life of radioactive materials

The time it takes for half of a material’s radioactivity to disappear is called the half-life. Different radioactive materials have different half-lives. This information helps the radiation therapy team choose the type of material to use and to plan the treatment regimen. It also determines how long safety precautions must be taken following treatment with internal radiation.

Back to top


Holly Benson Thanks to good research, thanks to funding for research, I’m here and enjoying my life.

Read Holly's story

Access to services in your community

Illustration of houses side-by-side

The Canadian Cancer Society’s Community Services Locator helps cancer patients and their families find the services and programs they need in their community.

Learn more