Proton Therapy

Written by: Stuart H. Burri, MD

Cancer remains the second leading cause of death around the world, reports the World Health Organization. It claimed an estimated 9.6 million lives in 2018, and about one in six deaths will be attributed to cancer. Although 70 percent of these deaths occur in low- and middle-income countries, the threat of cancer in the U.S. continues to affect countless lives. More than 400 people out of 100,000 in the U.S. are diagnosed with new cancers each year, notes the National Cancer Institute (NCI). For those with a cancer diagnosis, the threat of surgical intervention, chemotherapy or radiation can be overwhelming.

The idea of introducing radiation to the body is frightening, but a newer form of radiation therapy—proton therapy or proton beam therapy (PBT)—holds promising results for those with certain types of cancers, including cancers of the head. Before deciding what type of therapy is right for you, take a moment to consider the facts of proton therapy, when it’s useful as a treatment and more.

Proton therapy refers to a type of radiation used to treat cancer. The use of proton therapy is quickly becoming the go-to treatment for its less-damaging effects on bodily tissues and effectiveness at stopping them from growing in pediatric patients and some adults.

The basis behind proton therapy is simple. While traditional radiation treatments leverage intensified x-rays and other forms of radiation, proton therapy uses a beam of focused particles—protons. Protons are one of the three primary particles within an atom. Within a particle accelerator, also known as a cyclotron, atoms are accelerated, losing their electrons and neutrons, creating a stream of protons. These focused particles can be directed at a specific tumor to deliver a high dose to the tumor with less dose to surrounding normal tissues. This gives hope for a high chance of cure with less toxicity associated with treatment.

Proton therapy is not effective at treating certain cancers, such as those that have spread widely. Since the radiation focuses on a targeted area, it is impractical to use this treatment for cancers that have metastasized to more than a few places. Most importantly, proton therapy is most often used for cancers requiring radiation that are near extremely sensitive organs and tissues. In fact, proton therapy is used predominantly to treat the following types of cancer:

• Lung cancer
• Brain cancer
• Eye cancer
• Lymphoma
• Head and neck cancer
• Spine cancer
• Prostate cancer
• Cancers that have received previous high dose radiation, but have recurred
• Pediatric cancers

Treatment of these cancers using traditional radiation have a higher potential risk for both immediate and permanent damage to nearby tissues and cellular death.

It is important to understand proton therapy, which may reduce the risk of damage to tissues, still comes with some risk for side effects. These side effects, as with other forms of radiation, are related to adjacent tissues as radiation is a focal therapy, much more like surgery than chemotherapy.  With the special properties of protons the potential severity of side effects can be less in comparison to those of traditional radiation treatment.

Side effects also depend on the size and type of tumor, as well as the tumor’s location.

Radiation therapy in any area of the head and neck may result in dry mouth, thickened saliva, sore throat, or changes in appetite, tooth decay, mouth sores.  Treatment near the chest, as well as of the head, may contribute to difficulty swallowing. Treatment of lung cancers may lead to shortness of breath and cough. Abdominal treatments may result in nausea, vomiting, diarrhea, bladder irritation, frequent urination and even sexual dysfunction.

Although these side effects are unpleasant, proton therapy is associated with a decreased risk for radiation toxicity. In other words, the treatment is delivered at a high-intensity with a strong effect on the tumor target, but its effectiveness allows for the radiation to dissipate faster and reduce the risk of lingering side effects. However, fatigue is a common side effect of radiation therapy and can be noticeable after the proton therapy session.

Candidates for proton therapy may have received other radiation treatments in the past and face the prospect of treating a recurrence of cancer or cancer caused by damage from prior treatments. However, proton therapy preparation is not unlike other radiation treatments. Special pre-treatment tests and measures are necessary to ensure treatment reaches maximum effectiveness, including:

After positioning, the real work begins. Depending on the type of cancer and imaging, a magnetic resonance imaging (MRI) or computed tomography (CT) scan will provide an in-depth view of the tumor, assess the angles from which to treat the tumor, consider tissue density and further refine the treatment plan. Multiple scans may be necessary to gain enough information about how to best plan the treatment.

Positioning is among the most important parts of the pre-treatment and treatment phase of proton therapy. Due to the focused nature of the treatment, it is necessary for the body to be in the exact position during planning as treatment. As a result, it may be necessary to use custom cushions, supports and even restraints to prevent any movements. This ensures providers can determine the energy intensity, depth and angle of the proton beam. Ultimately, these preemptive steps reduce the risk of unnecessary radiation exposure to healthy cells.

Since proton therapy can be a complex process, these steps, as well as the consultation and review of treatment plans, will be included in a radiation simulation. The simulation helps recipients understand what to expect when it is time for actual treatment and why any movement could disrupt treatment progress. The plan is then developed and this behind the scenes process involves a highly trained staff of dosimetrists, physicists, and physicians can take a week or more.

Actual treatments will only take several minutes, but the time in the treatment room is longer with positioning of the patient and imaging that may be required for set-up. The patient does not sense the radiation being delivered (there is no shocking, burning or stinging).

After initial treatment, subsequent CT and MRI scans will be conducted to determine whether tumors shrink or otherwise change in size. Further planning and review of individual cases will help care team members determine the next best course of action. The treatment duration is variable based on the tumor type and location and can range from just a treatment or two to daily treatment for many weeks.

The results of proton therapy vary and depend on the type of cancer, risk of recurrence, personal histories, lifestyle factors and additional treatment measures used. The volume of data is significant and suggests the long-term benefits of proton-based radiation is safer than traditional radiation sources. While the goal of any radiation treatment is the same—avoid damaging nearby tissues—the unique characteristics for managing energy levels in proton therapy allow care providers to target smaller areas and avoid such damage.

Numerous studies have found results to support this claim. In one recent study, the recurrence of cancer following treatment with proton therapy appeared to have better results than traditional radiation. Mean dosage to the heart was reduced by 36.5 percent. Dosage to lung tissue was reduced by 33.5 percent, and dosage to the esophagus was reduced by 60 percent. These values may allude to a lower risk for future recurrence of cancer or other tissue damage that would have resulted from a traditional radiation source.

Another study sought to determine what long-term benefits versus risks existed after treating prostate cancer with proton therapy. The results found similar benefits in proton therapy treatment of prostate cancers that were high- or very high-risk for recurrence when treated with other available radiation options. More importantly, the study found a key benefit—significantly reduced the risk of damage to reproductive systems or radiation toxicity. Results from this study found decreased five-year, adverse biological effects from treatment.

A 2017 analysis and study of proton beam therapy as a suitable alternative to traditional radiation therapies found a significant potential for this form of treatment. It is the latest type of radiation treatment available, and its benefits hold greater potential for those with a high risk of long-term damage resulting from traditional treatments, with a clear benefit in pediatric patients. As noted by the analysis, proton treatment may be superior in curative effects, but it also poses great opportunities for those whose conventional radiation dose may be limited as a result of concerns of toxicity to normal tissues.

The analysis does suggest further research is necessary to determine the quality and usefulness of the treatment for many cancers, including some which are currently treatable within the parameters of proton treatment. Such research will determine which patients and cancers benefit most from this treatment, keeping the goal of minimizing damage to normal tissues, and the grouping of patients that most benefit from proton therapy. This study from 2022 asserts that promising results have been reported for many different cancers, but they are often based on small studies and cannot truly verify which cancers proton therapy is most effective against. At the same time, the costs of proton treatment remain high, but generally can be covered by cancer health insurance with the appropriate pre-certification process.

The applications of proton therapy for cancer are expanding, and the treatment is growing in popularity thanks to its successes in reducing radiation toxicity and long-term damage to normal tissues. As the treatment becomes more widely available, it is important to know the facts.