What Is Proton Beam Therapy for Cancer?

Understanding the Benefits and Risks of Proton Beam Therapy

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If your doctor has recommended proton beam therapy for your cancer, you’re likely anxious and confused. Unlike some cancer treatments, proton beam therapy is relatively new on the scene and less information is available. Let’s take a look at how exactly this type of treatment works, the advantages and disadvantages relative to other cancer treatments, and the possible side effects you may experience.

While proton beam therapy has only recently “taken off” with only a few handfuls of facilities providing this service, the technique has been around in some hospitals in the United States since 1990.

How Does Proton Beam Therapy Work?

The short explanation of how proton beam therapy works is that—similar to conventional radiation therapy—it uses energy to damage or destroy cancer cells.

For those who wish to understand the mechanism of this treatment in greater depth, proton beam therapy works by accelerating protons (positive particles) to a high energy state via a particle accelerator. This high energy state allows the protons to travel through tissue directly to a tumor; resulting in a very localized bombardment of protons on the tumor.

Protons are positively charged and therefore attract negative charges. When a proton is launched near a molecule such as DNA, negatively charged regions of the molecule will be attracted to the proton, thus interfering with that molecules normal orientation and function. The result of this process is ultimately the death of cancer cells.

Differences Between “Normal” Radiation Therapy and Proton Therapy

Proton therapy and conventional radiation therapy are both prescribed by radiation oncologists. One of the most obvious differences is the locality of treatment. Proton therapy accurately targets very specific locations, which may result in less damage to surrounding tissues. Conventional radiation therapy is less targeted, and more "normal" cells in the region of a tumor may be damaged.

Proton beam therapy is not one single type of treatment, but rather there are different types and methods. Newer types of proton beam therapy such as pencil beam scanning are thought to make this therapy even more tolerable.

What Cancers Are Treated With Proton Beam Therapy?

Proton beam therapy is usually used for localized tumors (stage I, II, or III) and is used infrequently for stage 4 tumors (tumors that have spread to other regions of the body). Some types of cancer in which proton beam therapy may be used include:

  • Lung cancer: With lung cancer, proton therapy may be used for early stage lung cancer which cannot be treated with surgery, or for a recurrence when conventional radiation therapy has already been given.
  • Brain cancer: The precise area of delivery offered by proton beam therapy may be beneficial for brain tumors in which even small amounts of damage to nearby normal tissues could have significant side effects.
  • Childhood cancers: Proton beam therapy, as discussed below, may do less damage to nearby normal tissues. Since DNA damage to normal cells may lead to secondary cancers, it's thought that this therapy may be a better option at times for children.
  • Head and neck cancers
  • Liver cancer
  • Gallbladder cancer
  • Esophageal cancer
  • Gastric cancer
  • Prostate cancer

Advantages and Disadvantages of Proton Beam Therapy

There are many new treatments for cancer that have arisen in the last decades, and this is wonderful. At the same time, however, when there are different treatment options, you and your oncologist will need to make a decision about what treatment is best for you as an individual.

Some people list out potential treatments and then weigh the options based on how effective the treatments may be as well as the possible side effects (and which side effects they would consider to be most and least annoying). Listing out the pros and cons of proton therapy may help with this process.

Pros of proton therapy include:

  • Proton beam therapy delivers damage to a very precise area of tissue. This can be particularly good for tumors which lie near vital structures, such as brain tumors, or lung tumors which lie near the heart.  Head and neck cancers, especially those related to human papillomavirus (HPV), have a greater survival rate than non-HPV related tumors, meaning that people will often need to cope with the long term side effects for many years. In this situation, proton beam therapy may be preferable to conventional radiation.
  • Due to the precise delivery of protons, a higher radiation dose can often be delivered to a tumor with proton beam therapy.
  • A precise area of radiation delivery with proton beam therapy may result in less damage to surrounding tissue. Not only does this possibly reduce the side effects (due to less damage to normal healthy tissues) but it may also reduce the likelihood of developing secondary cancer (cancers that are caused by cancer treatments such as radiation therapy and chemotherapy). It’s thought that the risk of secondary cancers will be lower with proton beam therapy than conventional radiation therapy, making it a consideration especially for young people with cancer.
  • Proton beams are easier to control than conventional radiation. In addition, once the beam reaches the tumor, it stops, rather than result in scattering and over penetration.
  • Proton beam therapy is usually well tolerated with fewer side effects than conventional radiation therapy.
  • Proton therapy (and stereotactic body radiotherapy or SBRT) may be used for early cancers that in theory should be operable, but are deemed inoperable due to their location near vital structures or a person is not a good candidate for surgery.

Cons of proton therapy may include:

  • Due to the precise area of radiation delivery, proton therapy may miss small areas of cancer cells and metastases which lie near but outside the radiation field.
  • At the current time, proton beam therapy is roughly twice as expensive as conventional radiation therapy due to overhead costs (proton therapy requires a large linear accelerator). It currently costs more than 200 million dollars to build a facility.
  • Challenges include motion management and changes in anatomy that take place before and during treatment.
  • Since only a handful or so of proton beam therapy centers are present in the United States, people may need to travel quite a distance to receive this treatment.

Alternatives to Proton Beam Therapy

While conventional radiation therapy is typically less precise and involves more “scatter” than proton beam therapy, newer radiation techniques are also providing much more precise delivery of radiation.

One example is stereotactic body radiotherapy (SBRT). With SBRT, higher doses of radiation are delivered to a precise area. SBRT may be used with a curative intent for tumors that cannot be surgically removed due to their location, or to treat “oligometastases” (single or only a few metastases to the brain, liver, or lung from solid tumors).

In a 2017 review of studies comparing proton beam therapy with SBRT, researchers did not find any appreciable differences in the effectiveness of these two approaches for the treatment of non-small cell lung cancer.

Possible Side Effects of Proton Beam Therapy

Like most cancer treatments, proton beam therapy can cause side effects as well. Even though proton beam therapy is delivered to a precise area, damage to normal cells near the tumor may occur. Many of the side effects are similar to the side effects of conventional radiation therapy, but due to the precise focus of damage, may be less severe.

The most common side effects include:

  • Fatigue: As with conventional radiation therapy, fatigue is one of the most commonly reported symptoms of treatment. This fatigue tends to be most severe the further along in treatment you are.
  • Skin redness (radiation dermatitis): Rarely, blistering and skin breakdown may occur with proton beam therapy.
  • Hair loss: Hair loss may occur in the region in which proton beam therapy is given, for example the head when used for brain cancer or the chest when used for lung cancer. Unlike the hair loss associated with chemotherapy, hair loss related to proton beam therapy may be permanent.
  • Inflammation of the lungs: Lung inflammation referred to as radiation pneumonitis may occur when proton beam therapy is used on the chest. The importance of radiation pneumonitis is that it must be recognized and treated in order to reduce the risk of developing pulmonary fibrosis (scarring of the lungs).

Where Is Proton Beam Therapy Available?

There are still a limited number of cancer centers in the United States and globally which provide the option of proton beam therapy. A map of proton therapy centers can show you whether this treatment is offered near your location.

Preparing for Proton Beam Therapy

Before beginning treatment with proton beam therapy, you will meet with a radiation oncologist who will discuss what you can expect, and the benefits as well as risks of the procedure. You will have a CT scan done to map out the region of your body to be treated and will be fitted with an immobilization device (as with conventional therapy). This mold will be placed over the part of your body receiving radiation during each treatment.

Similar to conventional radiation, proton beam therapy treatments are often done daily during the week for a total of 20 to 40 sessions. While the actual radiation portion of the visit only takes a minute or two, most appointments will last 30 to 45 minutes.

Proton therapy is not painful, and no anesthesia is needed (with the exception of children who may require sedation to be still during the treatment).

A Word From Verywell

Proton beam therapy offers yet another option for treating cancer. Due to its ability to precisely focus radiation on cancer, its primary advantages are not in eliminating cancer but reducing the risk of side effects related to treatment. As such, it may be a good option for tumors that are near vital structures and for younger people who may have a lower risk of secondary cancers with proton beam therapy versus conventional radiation.

It may also be used for tumors which had previously been treated with conventional radiation but recurred. The precise location of treatment may also allow radiation to be given at a higher dose than conventional radiation.

There is currently controversy about the rapid adoption and development of centers which provide these treatments as in some cases either conventional radiation or a procedure such as SBRT may be just as effective but less expensive. Since proton beam therapy is still relatively new, we simply do not have the studies yet to fully back up the theoretical advantages of proton therapy over other treatments. Discuss this option with your provider to see if it's right for you.

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