What is Radiation Therapy?

You may be wondering how radiation therapy kills cancer cells and keeps them from growing.

Radiation uses high energy x-rays to damage the cancer cells. When they cannot repair themselves, the cancer cells die. This helps healthy cells to grow back and the tissues to recover.

How does radiation therapy treat cancer?

  • Kills cancers cells so that they cannot spread
  • Destroys any cancer cells that remain after other cancer treatments
  • Lowers the risk of cancer coming back after treatment, at (or nearby) the original tumor site or in other parts of the body
  • Shrinks or slows the growth of cancerous tumors that cannot be removed with surgery

Radiation is considered a “local” treatment because it focuses on the tumor or cancerous area, not on other parts of the body. Radiation therapy given after surgery or another treatment, is called adjuvant therapy. Radiation is used in a safe and targeted way.

External Beam Radiation Therapy

  • External beam radiation therapy is the focus of this program. It uses a machine called a linear accelerator (LINAC) that moves around your body, but doesn’t touch you.
  • External beam radiation therapy is delivered in different ways. Your care team will choose the best approach for your particular case.

Click through the types of radiation therapy equipment at Stanford to learn more.

The machine that is used for your treatment may look slightly different than the pictures shown, but each machine provides the same quality treatment.

CT Machine

This noninvasive (does not involve inserting devices into the body) imaging device uses special x-ray equipment with sophisticated computers to produce multiple images or pictures of the inside of your body.

Linear Accelerator

A linear accelerator delivers prescribed doses of radiation to the body from outside the body.


noninvasive (does not involve inserting devices into the body) radiosurgery system that treats some cancerous and noncancerous tumors and certain other conditions – without the need for incisions.

What Does Radiation Therapy Feel Like?

  • Radiation is painless while you are receiving it. You won’t feel anything during your sessions.
  • Some patients may notice flashing lights during treatment, this is normal and not harmful. You may also hear some humming and whirling sounds from the machines.

If you are uncomfortable at any time during the treatment, let your team know. They can pause and restart the treatment when you are ready.

Visit the Stanford Health Library to Learn More

To find out more about the science of radiation therapy, visit Stanford’s Health Library.

  • The library can give you customized answers about radiation therapy as well as about other medical or health questions.
  • The library does not provide diagnoses, advice or recommendations.