Proton Therapy

Approved by the Cancer.Net Editorial Board, 12/2016

Proton therapy is a type of radiation treatment that uses protons to treat cancer. It’s also called proton beam therapy.

A proton is a positively charged particle. At high energy, protons can destroy cancer cells. Doctors may use proton therapy alone. Or, they may combine it with other treatments, such as standard radiation therapy, surgery, chemotherapy, and/or immunotherapy.

Proton therapy is a type of external-beam radiation therapy. It painlessly delivers radiation through the skin from a machine outside the body.

How proton therapy works

A machine called a synchrotron or cyclotron speeds up the protons. The protons’ speed determines the energy level. High-energy protons travel deeper in the body than low-energy ones.

The protons go to the targeted place in the body. There, they deposit the specific radiation dose in the tumor.

With proton therapy, radiation does not go beyond the tumor. In contrast, with photon-based external-beam radiation therapy, x-rays continue depositing radiation as they exit the body. This means that the radiation leaving the body may damage nearby healthy tissue. That damage can cause side effects. 

What to expect

You typically receive proton therapy in an outpatient setting. This means treatment does not require staying overnight in a hospital.

The number of treatment sessions depends on the type and stage of cancer. Sometimes, patients receive proton therapy in 1 to 5 treatments. Typically, this is called stereotactic body radiation therapy. If a person receives a single, large radiation dose, it is sometimes called radiosurgery.

Treatment planning

Like other external-beam radiation therapy technologies, proton treatment requires planning. Before treatment, you have a specialized computed tomography (CT) or magnetic resonance imaging (MRI) scan. During this scan, you are on a table in the same position as during treatment. This may require an immobilization device.

Immobilization devices

An immobilization device restricts movement. The type of device depends on the tumor’s location. For example, you may need to wear a custom-made mask for the treatment of a tumor in the eye, brain, or head. This ensures the radiation accurately targets the tumor during each treatment.

Comfort wearing an immobilization device

Immobilization devices are designed to fit snugly. This prevents motion during the radiation treatment. However, the radiation oncology team cares about your comfort. Talk with the team to find a comfortable, reproducible treatment position. Your doctor may prescribe medication to help you relax if you feel anxiety lying still in an immobilizing device.

Receiving treatment

Treatment is delivered in a special room. Members of the team will place you on the treatment table using an immobilization device, if needed.

The treatment team aligns lasers to aim radiation at the marked site. Before treatment, you receive another x-ray or CT scan. This ensures that you are in the correct position. These pretreatment images or target-tracking images are often critical for accurate radiation therapy. Talk with your doctors about whether the proton therapy you are receiving uses these advanced imaging technologies.

Some proton treatment rooms deliver proton beams through a gantry. A gantry can rotate and deliver radiation therapy from the angle prescribed by your doctor. Your team members will move the gantry to the desired position before treatment.

Then, the radiation oncology treatment team leaves the room. Team members see, hear, and communicate with you through audio/video equipment. They use controls outside the room to give you the proton therapy.

Protons leave the cyclotron or the synchrotron machine. Magnets then direct them to the tumor, sometimes using the gantry. During this time, you must stay still to avoid moving the tumor out of the proton beam.

Time needed for each treatment

Typically, treatment lasts about 15 to 30 minutes once you enter the treatment room. However, these times vary based on a number of factors:

  • The part(s) of the body receiving treatment

  • The number of treatment segments

  • The number of x-rays or CT scans done during the positioning process

Ask your treatment team about how long treatment will take. Sometimes, you may receive several segments from different gantry angles. If so, ask whether someone will enter the room between segments to reposition the gantry. In some cases, the team rotates the gantry remotely.

Proton beam availability may also affect timing. Most facilities have only one proton cyclotron or synchrotron. So you may need to wait a few minutes until another patient finishes treatment.

Side effects

Proton therapy is painless. Afterwards, you may experience fatigue and skin problems. These include redness, irritation, swelling, dryness, or blistering and peeling. You may experience other side effects, especially if you are also receiving chemotherapy.

The risk of developing side effects from proton therapy depends on:

  • The part of the body being treated

  • The size of the tumor

  • The types of healthy tissue next to the tumor

  • Whether you receive chemotherapy at the same time

Your radiation oncology team will explain which side effects you may experience.

Cancers treated with proton therapy

Proton therapy is useful in the following situations:

  • Tumors that are near important parts of the body. For example, tumors near the eye, brain, and spinal cord.

  • Childhood cancers of the eye, brain, and spinal cord. Proton therapy lessens the chance of harming healthy, developing tissue.

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 in the soft-tissue and bone

  • Noncancerous brain tumors

Risks and benefits

Compared with standard radiation treatment, proton therapy has several benefits:

  • It may deliver up to 60% less radiation to healthy tissue around the tumor, lowering the risk of damage to these tissues. However, more research is needed on these potential benefits to know for sure. Talk with your doctor about the benefits of proton therapy compared with other advanced external beam radiation therapy methods. These may include image-guided 3-dimensional conformal radiation therapy and intensity modulated radiation therapy (IMRT).   

  • It may allow for a higher radiation dose to the tumor. This increases chances that all tumor cells are destroyed. Talk with your doctor to find out if you might benefit from high-dose radiation therapy.

  • It may cause fewer and less severe side effects during and after treatment. Talk with your doctor about the differences between the side effects of proton therapy compared with other advanced external beam radiation therapy methods. 

Proton therapy also has drawbacks:

  • Proton therapy effectively treats only certain cancer types.

  • More research on the potential benefits of proton therapy is needed to fully understand how it compares with other advanced external beam radiation therapy methods.

  • Proton therapy requires highly specialized, expensive equipment. This means it is available at few medical centers in the United States.

  • It may cost more than conventional radiation therapy. Insurance provider rules about what is covered by insurance and how much patients need to pay vary. Talk with your insurance provider to learn more.

More Information

What is Radiation Therapy?

What to Expect When Having Radiation Therapy

Side Effects of Radiation Therapy

Additional Resource

RadiologyInfo: Proton Therapy