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What is proton beam therapy and for what is it used?
Proton beam therapy is a type of radiation therapy that uses protons instead of X-rays (photons) and can be used for the treatment of various solid tumors, including liver cancer (hepatocellular carcinoma or HCC). Protons are positively charged particles that, produced with high energy, can destroy cancer cells. Proton beam therapy is able to deliver targeted high doses of protons to a defined local area, minimizing damage to surrounding tissue and reducing side effects. The ideal patient with HCC for this treatment has only a small (less than 5 centimeters in diameter) solitary lesion (tumor) though benefit in a few patients with larger tumors has been described.
Proton therapy may be used by itself, or along with other treatments including standard radiation therapy, surgery, chemotherapy, and/or immunotherapy. It also may be used when surgery is contraindicated.
Proton beam therapy is used for patients who have tumors that are solid with well-defined borders that have not spread (metastasized) to other parts of the body.
Why is proton beam therapy used?
Proton beam therapy is used to treat several types of solid tumors including HCC. Proton beam therapy allows physicians to better control what tissues are affected by the radiation and, at the same, reduce damage to healthy surrounding tissue and vital organs.
X-ray therapy or traditional radiation causes more damage to the healthy tissues surrounding a targeted tumor while proton beam therapy radiation is able to more precisely target cancerous tumors.
Proton beam therapy may be useful in treating cancer that is located in a critical area where it is important to preserve the surrounding tissue as much as possible (for example, tumors in the brain, spinal cord, or in the eye). It may also be quite beneficial in children because standard radiation can damage developing organs and have long-term effects in children. Proton beam therapy minimizes the damage and long-term effects.
How is proton beam therapy done?
Proton beam therapy is typically done in an outpatient setting. Prior to the start of proton beam therapy, the patient undergoes a CT or MRI scan to localize the exact site of the cancer. The scan is performed with the patient sitting or lying in a device that immobilizes the part of the body that will undergo therapy. Based on the scan, plans are made to deliver the proton beam to the cancer. For each session of therapy, the patient is placed again in the immobilization device. In addition, X-rays or scans are taken before every session to make sure the patient is in the exact same position so the protons hit the tumor and not the surrounding healthy tissue.
The proton beam is invisible and painless. The number of treatment sessions depends on the type and stage of the cancer being treated. Therapy usually is conducted 5 days per week, usually for 15 days. Proton beam therapy sessions last about 15 to 30 minutes.
How effective is this therapy in treating liver cancer?
Proton beam therapy is a relatively new treatment modality. There is not yet much data regarding the efficacy of this treatment for liver cancer. Preliminary data from the U.S. suggest similar effectiveness as seen with transarterial chemoembolization (TACE) or ablation (using radiofrequency or alcohol) for the treatment of HCC. It is not known whether this type of radiation treatment can consistently prolong the life of the patient.
A study from Loma Linda University Medical Center showed proton beam therapy was effective in treating cancerous lesions in the liver up to 10 centimeters. Patients with one lesion smaller than 5 centimeters or up to three lesions smaller than 3 centimeters (this size definition is called the Milan criteria) have better survival rates than patients with larger tumors. The same study found that patients who had tumors within the Milan criteria that were treated with proton beam therapy had a 48% survival rate over 3 years. Those whose tumors fell outside the definition of the Milan criteria had 3-year survival rates of 12%.
Another study at the University of Tsukuba in Japan looked at the safety and efficacy of using proton beam therapy to treat metastatic liver cancer (cancer that had spread to the liver from elsewhere in the body), and it found the treatment to be both safe and effective. Those who had cancer only in the liver had better outcomes than those whose cancer had spread more widely.
What are the potential risks and benefits of proton beam therapy?
There are some benefits to proton beam therapy versus standard radiation therapy including:
- The treatment delivers up to 60% less radiation to the tissues surrounding the tumor, reducing damage to healthy tissues.
- It can allow for a higher dose of radiation to the tumor, which can increase the chance of destroying all of the targeted tumor cells.
- Side effects during and after treatment may be less severe, including a reduced incidence of low blood counts, fatigue, and nausea.
- Faster recovery occurs following treatment.
Proton beam therapy does have some drawbacks:
- Proton beam therapy requires expensive and specialized equipment that few medical centers have. As of 2015, there are only 16 proton therapy centers in the U.S. with 10 more under construction, according to The National Association for Proton Therapy location map.
- Proton beam therapy may cost more than standard radiation therapy, and some insurance providers may not cover it. The National Association for Proton Therapy states Preferred Provider Organizations (PPOs) are more likely to cover proton beam therapy than Health Maintenance Organizations (HMOs).
- Proton beam therapy works best on cancer that is localized to one area with well-defined borders and has not spread (metastasized) to other parts of the body. It may not be effective in treating all types of cancer.
- Proton beam therapy use still remains controversial. Many studies have not proven it more effective or beneficial than conventional radiation therapy.
What are the potential side effects of proton beam therapy?
The potential side effects from proton beam therapy treatment are the same as conventional radiation therapy. Side effects may vary depending on the part of the body being treated and the size of the tumor. If patients also are receiving chemotherapy along with proton beam therapy they may have additional side effects from the chemotherapy.
The proton beam therapy procedure is painless, but patients may experience side effects including fatigue and skin irritation (such as swelling, redness, dryness, peeling, or blistering). There also still may be some damage to healthy tissue.
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What research is being done on proton beam therapy?
A study is underway at Loma Linda University looking at proton beam therapy for HCC and other tumors. The study compares proton beam therapy outcomes with results in patients treated with transarterial chemoembolization (TACE), a more standard treatment for cancer, particularly HCC.
There are some studies being done comparing X-ray radiotherapy treatments to proton treatments, but so far no studies have shown proton beam therapy to be more effective at controlling cancer than traditional radiation therapy.
Other studies are aimed at developing a scanning proton beam for treating large and irregular shaped tumors.
There are a number of clinical trials looking at how proton beam therapy might affect certain specific types of cancers, including breast, central nervous system, gastrointestinal, genitourinary, head and neck, thoracic, and pediatric cancers.