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Proton Therapy: The Cancer Treatment of the Future
If your doctor has recommended that proton therapy be part of your cancer treatment program, you probably have many questions and concerns about it.
Proton therapy, a noninvasive, painless treatment, is a type of radiation used to treat malignant tumors.
Proton therapy has been used to successfully treat pediatric cancers, head and neck cancers, brain cancer, lung cancer, lymphoma, prostate cancer, and sarcomas and malignancies in the gastrointestinal tract. Any cancer that appears as a solid mass could potentially be treated with proton therapy, and many types are currently being evaluated.
Although proton therapy has been used at specialized research facilities for the past 30 years, it was only available at one hospital-based center (Loma Linda) until the past decade. As of March 2012, 10 medical facilities in the United States offered proton therapy. Within the next 5 years, another 20 U.S. centers should either open or be under construction. To date, more than 70,000 people worldwide have received proton therapy at centers in the United States, Europe, and Asia.1
The most common type of radiation is called photon therapy, or conventional x-rays. Proton therapy works in the same way--it destroys cancer cells by preventing them from dividing and growing.
Patients who undergo proton therapy will have an MRI and/or CT scan before treatment to pinpoint the tumor's exact location and personalize treatment. During treatment planning, computer modeling will determine the best direction to aim the radiation beam to avoid damaging normal tissue.
Patients will be fitted for immobilization devices (usually a foam-type mold) to ensure that they don't move during treatment and are in the same position for every treatment. If they will receive treatment for their head, they will be fitted for a mask. These devices also help to move healthy tissue out of the path of the radiation beam.
Before treatment, the patient will lie on a table in a comfortable but stable position. Then the immobilization devices will be placed around him or her. Imaging will ensure that the patient is properly positioned. When treatment begins, a machine directs the proton beam toward the tumor from different angles for several minutes.
"Unlike photon therapy, which penetrates healthy tissue around the tumor as well as the tumor itself, proton therapy goes directly to the tumor," explains Jeffrey Bradley, MD, S. Lee Kling Professor of Radiation Oncology, Washington University School of Medicine, St. Louis, MO. This is because photons and protons deliver radiation differently due to differences in their physical properties. Photons are electromagnetic waves that have no mass or charge and penetrate completely through tissue. Protons are large, positively charged particles that penetrate matter to a finite depth.2
According to Steven Frank, MD, director of Advanced Technologies, Proton Therapy Center at The University of Texas MD Anderson Cancer Center, Houston, TX, "Our ability to deliver this technology in a pinpoint fashion makes it novel and extremely effective."
Typically, patients receive treatment 5 days a week for 6 to 8 weeks. That amounts to between 30 and 40 treatments.
Fewer protons than photons are needed when delivering therapy because protons deposit most of their radiation directly into the tumor and then stop. Consequently, patients can receive higher doses, which can be more effective.
In addition, the intensity of side effects and the risk of a second malignancy are reduced compared with photon treatment because healthy tissues are only minimally affected. "This enables patients to return to a normal life [sooner than with photon therapy] and have a better quality of life," says Eugen B. Hug, MD, professor of proton-radiotherapy, and medical director, ProCure Proton Therapy Centers, New York, NY. ProCure has partnered with the Seattle Cancer Care Alliance to build a proton center that will open in Spring 2013 in Seattle.
Side effects depend on the area being treated. If you're treated on your head or neck for example, you may experience loss of taste, difficulty swallowing, oral pain, nausea, and/or vomiting. You may require a feeding tube for nutrition and hydration. If you're treated for prostate cancer, you may experience more frequent urination, burning while urinating, or diarrhea.
After completing treatment, you may feel fatigued and experience acute side effects for up to 1 month. "Then the healing process progresses even faster," says Dr. Frank. Typical follow-up includes imaging every several months for 1 or 2 years, then every 6 months for 5 years, and then annually after that.
"We are at an exciting time of an exponential take off of this technology," says Dr. Hug. "Many studies are underway and the data look promising."
Already it has been shown that the ability to treat skull-based and paraspinal tumors has improved 15% to 20% with proton treatment. In addition, data comparing proton therapy with photon techniques in chordoma, chondrosarcoma, and uveal melanoma show that proton therapy can extend life even longer.
A major benefit of treating with proton therapy is the reduced amount of radiation dose delivered to large parts of the surrounding healthy body tissues and organs.
Proton therapy is a highly preferred radiation treatment for children because they are susceptible to injury from standard x-ray radiation because their tissues and organs grow rapidly.1
Fewer side effects may allow for better completion of treatment and tolerance of chemotherapy, and less toxicity later on, including secondary tumors and heart disease. "These benefits may allow patients to actually survive longer," Dr. Frank says.
One study showed that of the patients treated with photons, 12.8% of them developed secondary malignancies, whereas only 6.4% of the proton therapy patients developed secondary malignancies.3
Despite its promising benefits, hospitals that offer proton treatment are only slowly being built. This is because of multiple factors, including:
"I think this is the treatment of the future," Dr. Bradley says. "Using a proton beam (rather than photon therapy) makes fundamental sense because it causes fewer side effects, the cost is decreasing and its efficiency is improving. I think it will change cancer care for the better."
1. National Association for Proton Therapy Web site. http://www.proton-therapy.org/facts.htm. Accessed October 18, 2012.
2. Fowler JF. What can we expect from dose escalation using proton beams. Clin Oncol. 2003;15(1):S10-S15.
3. Chung CS, Keating N, Yock T, Tarbell N. Comparative analysis of second malignancy risk in patients treated with proton therapy versus conventional photon therapy. Int J Radiat Oncol Biol Phys. 2008;72(1):S8.
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