What radiation therapy is and how it treats cancer

Radiation therapy uses high-energy beams—usually X-rays or particles—to damage cancer cells and stop them from growing. The radiation damages the DNA inside cancer cells, which prevents them from dividing and multiplying. Normal cells can often repair this damage, but cancer cells usually cannot, so they die.

Doctors use radiation therapy either as the main treatment for some cancers, or alongside surgery and chemotherapy for others. The goal is to destroy cancer cells while limiting harm to healthy tissue nearby. A radiation oncologist—a doctor who specializes in this treatment—plans exactly where the beams will go and how strong they need to be.

Key Takeaways

  • Radiation therapy uses focused energy beams to damage cancer cell DNA so the cells cannot divide and grow.
  • External beam radiation comes from a machine outside your body, while brachytherapy places radioactive material inside or near the tumor.
  • A radiation oncologist creates a treatment plan tailored to your specific cancer, including how many sessions you need and which areas to target.
  • Side effects depend on which part of your body receives radiation and how much treatment you get, and most fade after therapy ends.
  • Your treatment team will monitor you during and after radiation to watch for any problems and check whether the cancer is responding.

The two main types of radiation therapy

External beam radiation is the most common type. A large machine called a linear accelerator (or LINAC) directs radiation beams at your tumor from outside your body. You lie still on a table while the machine moves around you, aiming from different angles. Each session usually lasts 15 to 30 minutes, though most of that time is positioning—the actual radiation delivery takes only a few minutes.

Brachytherapy works differently: radioactive material is placed inside your body, either directly into the tumor or in a container positioned near it. This might happen during a procedure in an operating room or a clinic. Brachytherapy delivers a high dose of radiation to a small area, so it is often used for cancers of the cervix, prostate, or breast. The radioactive material stays in place for a set time, then is removed.

Some people receive both types. Others receive specialized forms of external beam radiation, such as intensity-modulated radiation therapy (IMRT), which shapes the beam to match the tumor's outline, or stereotactic body radiation therapy (SBRT), which delivers a very high dose in just a few sessions.

How your treatment plan is created

Before radiation begins, your radiation oncologist orders imaging scans—usually a CT scan—to see exactly where your tumor is and what healthy structures are nearby. The team uses computer software to map out the radiation beams, deciding the angle, strength, and number of sessions needed. This planning process typically takes one to two weeks.

You will also have a simulation appointment where you lie in the exact position you will be in during treatment. The team marks your skin with small dots or tattoos (usually just a few tiny marks) so they can position you the same way each time. These marks ensure the radiation hits the same spot every session.

Your radiation oncologist will tell you how many treatments you need. This varies widely depending on your cancer type and stage—some people receive 5 to 10 sessions, others receive 25 to 35 sessions over six to seven weeks. The schedule is usually five days a week, Monday through Friday, with weekends off to let normal cells recover.

What to expect during treatment

On treatment days, you will change into a gown and lie on the treatment table. The radiation therapist positions you using the marks from your simulation and takes X-ray images to confirm you are in the right spot. You will be alone in the room during radiation, but the therapist watches you on a camera and can hear you through a speaker if you need to stop.

The machine makes clicking and buzzing sounds as it moves and delivers radiation. You will not feel the radiation itself—it is painless. The hardest part for most people is staying still and coming back for multiple sessions over weeks.

After each session, you can go home and return to normal activities. Most people continue working or their regular routines during radiation therapy, though some feel tired as treatment goes on. This fatigue usually improves after treatment ends.

Side effects and what causes them

Side effects depend on which part of your body receives radiation and how much treatment you get. Radiation damages not only cancer cells but also some healthy cells in the treatment area. Common short-term side effects include skin irritation (similar to a sunburn), fatigue, and nausea. These usually start a week or two into treatment and fade within a few weeks after it ends.

If radiation targets the head or neck, you might have a sore throat or trouble swallowing. Radiation to the chest can cause a dry cough. Radiation to the abdomen or pelvis may cause diarrhea or bladder irritation. Your radiation oncologist will discuss which side effects are most likely for your specific treatment and what you can do to manage them.

Long-term side effects are less common but possible. They might include scarring of lung tissue, heart problems (if the heart was in the radiation field), or a small increased risk of a second cancer years later. Your doctor will discuss these risks with you before treatment starts and explain why the benefits of treating your current cancer outweigh these risks.

Monitoring your response to treatment

During radiation therapy, your radiation oncologist will see you weekly to check how you are tolerating treatment and manage any side effects. After treatment ends, you will have follow-up appointments to see whether the cancer has shrunk or disappeared. The timing of these visits depends on your cancer type—some require scans a few weeks after treatment, others wait several months.

Your doctor may order CT scans, MRI scans, or PET scans to check the treatment area. Blood tests or other imaging might also be used, depending on your cancer. These follow-up visits continue for years, because some cancers can return months or even years after successful treatment.

Radiation therapy combined with other treatments

Many people receive radiation alongside chemotherapy, surgery, or both. Chemotherapy can make cancer cells more sensitive to radiation, so the two treatments sometimes work better together than either alone. Surgery might remove the main tumor, and radiation then targets any remaining cancer cells in the area.

The order and timing of these treatments varies. Some people have surgery first, then radiation. Others receive chemotherapy and radiation at the same time, then surgery. Your oncology team will explain why they recommend a particular sequence for your situation.

Frequently Asked Questions

Is radiation therapy painful?

No. You will not feel the radiation itself during treatment. The machine may make noise and you need to stay still, but the process is painless. Some people experience discomfort from lying in one position for 15 to 30 minutes, but this is not caused by the radiation.

Will I be radioactive after treatment?

No, not with external beam radiation. The radiation stops as soon as the machine turns off. With brachytherapy, the radioactive material is removed after treatment, so you are not radioactive afterward. Your doctor will tell you if any precautions are needed while the material is in place.

Can radiation therapy cause cancer later?

There is a small risk of a second cancer developing years after radiation, but this risk is generally lower than the risk of your current cancer returning if you do not receive treatment. Your radiation oncologist will discuss this specific risk for your situation before you start.

What happens if I miss a treatment session?

Tell your radiation team right away if you need to miss a session. Missing one or two treatments usually does not significantly affect your outcome, but your doctor may adjust your schedule. Missing many sessions can reduce how well the treatment works, so it is important to attend as scheduled.

How long does it take to recover after radiation therapy?

Most short-term side effects fade within a few weeks after treatment ends. Fatigue may take longer to resolve. Long-term recovery depends on your specific cancer and treatment—your doctor will explain what to expect and when you can resume normal activities fully.