Proton Therapy

Approved by the Cancer.Net Editorial Board, 10/2022

Proton therapy is a type of radiation therapy that uses protons rather than x-rays. It painlessly delivers radiation to treat some types of cancer. Proton therapy is a promising new type of cancer treatment, but its possible benefit over traditional radiation therapy is still being studied. Research is ongoing in clinical trials to see how it compares.

In general, proton therapy has fewer side effects than other types of radiation therapy. This is because it can be targeted more directly at the tumor and does less damage to other tissue.

Because proton therapy uses expensive technology, the equipment is only available at a limited number of cancer centers. More information is below.

How does proton therapy work?

Like other types of radiation therapy for cancer, proton therapy works by delivering radiation to a tumor. It uses protons, while other radiation therapy uses x-rays.

A proton is a positively charged particle. At high energy, protons can destroy cancer cells. Doctors may use proton therapy alone. They may also combine it with x-ray radiation therapy, surgery, chemotherapy, or immunotherapy.

A machine called a synchrotron or cyclotron speeds up protons. The high speed of the protons creates high energy. This energy makes the protons travel to the desired depth in the body. The protons then give a targeted radiation dose in the tumor.

Less radiation occurs outside the tumor than in regular radiation therapy where x-rays continue to emit radiation as they leave the body. This is why proton therapy can have fewer side effects than other radiation therapy.

What should I expect during proton therapy?

People usually receive proton therapy in an outpatient setting, meaning patients come into the center for each appointment and return home afterward. The number of treatment sessions depends on the type and stage of the cancer.

Sometimes, doctors deliver proton therapy in 1 to 5 proton beam treatments. They generally use larger daily radiation doses for a fewer number of treatments. This is typically called stereotactic body radiotherapy. If a person receives a single, large dose of radiation, it is often called radiosurgery.

How do I prepare for proton therapy?

Proton therapy requires planning before treatment begins, and sometimes it takes longer than the treatment itself. Doctors want to know exactly where to direct the protons.

Before treatment, you will have a specialized computed tomography (CT) or magnetic resonance imaging (MRI) scan. This scan is called a radiation planning scan. During this scan, you will be in the exact same position as during treatment.

Movement should be limited while having the scan. So you may be fitted with a device that helps you stay still, called an immobilization device. The type of device depends on where the tumor is in the body. For example, you may need to wear a custom-made mask if the tumor is in the eye, brain, or head.

The devices are designed to fit snugly so there is no motion during the radiation treatment. But your health care team wants you to be as comfortable as possible during treatment. It is important for you to talk with the team to find a comfortable position.

What happens during a radiation planning scan?

During a radiation planning scan, you will lie on a table and the doctor will figure out the exact places to direct radiation in your body. If you are fitted with a device to help you stay still, you should wear it during the planning scan. This helps make sure your position is accurate during each proton treatment.

Some people, particularly with treatment around the head and neck region, feel anxious when they need to lie still in one position with the device. It is important to talk with your medical team if this causes you anxiety. Your doctor can offer ways, including medication, to help you relax for the scans.

Your health care team will use the radiation planning scan to mark the location of tumors. They will also mark the location of your normal tissues are so those can be avoided. This process is similar to the radiation planning process with x-rays.

What should I expect during a proton therapy treatment?

People receive proton therapy in a special treatment room. For each treatment, a member of the health care team will place you into the device on a treatment table. For certain treatment areas around the head and neck such as the eye, patients will sit in a special chair, instead of lying on a table.

Your treatment team will make sure you are in the correct position before starting treatment. This involves using a laser to center on the marks that were placed on your body or the device during the radiation planning scan.

The team takes x-rays or CT scan pictures before every treatment. This helps them put you in the exact same position for every treatment. This is so that the protons hit the tumor and avoid healthy tissues nearby.

What is a gantry?

Some proton treatment rooms have a machine called a gantry. It rotates around the person. This way, the treatment is delivered to the tumor from the best angles.

Gantries can be very large, sometimes as tall as three stories. During treatment, the gantry rotates around you so that the machine's nozzle is in its proper position. The nozzle is where the protons come out of the machine.

Once you are in position, your team will leave the treatment room and go to the delivery controls outside the room. They will use these controls to deliver the proton treatment. Your team will be able to see and hear you through audio-visual equipment inside the treatment room.

The protons travel through the machine and then magnets direct them to the tumor. Sometimes the gantry will also be used. During treatment, you must stay still to avoid moving the tumor out of the focused proton beam.

How long does proton therapy take?

In general, a proton radiation treatment lasts about 15 to 30 minutes, starting from the time you enter the treatment room. The time will depend on the part of the body being treated and the number of treatments. It will also depend on how easily the team can see the tumor site with x-rays or CT scans during the positioning process.

Ask your health care team how long each treatment will take for you. Factors affecting treatment time include:

  • Delivering treatment from different gantry angles. Ask whether they will come in to move the gantry or if the gantry will rotate around you.

  • Targeting different areas that require other radiation “fields." For example, one treatment may target the tumor itself while another treatment may target your surrounding tissue or lymph nodes.

  • Waiting for the proton beam to move from one treatment room to the next. In centers that have more than one treatment room, the protons are magnetically steered from one room to the next so you may have to wait a few minutes.

What are side effects of proton therapy?

The treatment itself is painless. Afterwards, you may experience fatigue.

Other side effects include these problems on affected skin:

  • Redness

  • Irritation

  • Swelling

  • Dryness

  • Blistering and peeling

You may experience other side effects, especially if you are also receiving chemotherapy. The side effects of proton therapy depend on the part of your body being treated, the size of the tumor, and the types of healthy tissue near the tumor. Ask your health care team which side effects are most likely to affect you and how they can be relieved or managed.

Which cancers are treated with proton therapy?

Proton therapy is useful for treating tumors that have not spread and are near important parts of the body. For instance, this may include cancers near the brain and spinal cord.

It is also used for treating children because it lessens the chance of harming healthy, growing tissue. Children may receive proton therapy for certain cancers of the brain and spinal cord. It is also used for cancer of the eye, such as retinoblastoma and orbital rhabdomyosarcoma.

Proton therapy also may be used to treat these cancers:

  • Central nervous system cancers, including chordoma, chondrosarcoma, and malignant meningioma

  • Eye cancer, including uveal melanoma or choroidal melanoma

  • Head and neck cancers, including nasal cavity and paranasal sinus cancer and some nasopharyngeal cancers

  • Lung cancer

  • Liver cancer

  • Prostate cancer

  • Spinal and pelvic sarcomas, which are cancers that occur in the soft-tissue and bone

  • Noncancerous brain tumors

What are the risks and benefits of proton therapy?

Compared with x-ray radiation therapy, proton therapy has several benefits:

  • Lower risk of radiation damage to tissues

  • Higher radiation dose to the tumor, with better likelihood that all tumor cells are destroyed

  • Fewer and milder side effects such as low blood counts, fatigue, and nausea during and after treatment

There are also some drawbacks to proton therapy:

  • It requires highly specialized and costly equipment, so it is available at a limited number of leading medical centers in the United States. Find a list of centers that currently offer proton therapy on a separate, independent website.

  • It may cost more than x-ray radiation therapy. Insurance provider rules differ so call your insurance to find out what you will pay.

  • Not all cancers can be treated with proton therapy.

Questions to ask the health care team

Consider asking your health care team these questions about proton therapy:

  • What is the goal of having radiation therapy? Is it to eliminate the cancer, help me feel better, or both?

  • Is proton therapy a treatment option for the type of cancer that I have?

  • If proton therapy is recommended for me, is it available at my usual center of care? If not, where is the closest treatment center that offers proton therapy?

  • How long will it take to have each treatment? How often will I need to have it?

  • Will I need a support device to limit my movement during treatment? Can you describe what I can expect?

  • What side effects can I expect during proton therapy? Afterwards?

  • What can be done to relieve the side effects I experience?

  • If I'm very worried or nervous about cancer treatment, who can I talk with?

  • What is my insurance coverage for this treatment?

  • Will I receive other cancer treatments in addition to proton therapy?

  • When and how will we know if this treatment was successful?

Related Resources

Understanding Radiation Therapy

What to Expect When Having Radiation Therapy

More Information

National Cancer Institute: External Beam Radiation Therapy for Cancer

RadiologyInfo.Org: Proton Therapy