What is Proton Therapy?
Imagine a 196-ton, cancer-killing machine that can target a patient’s tumor with sub-millimeter precision while sparing nearby healthy tissues and minimizing side effects. In its most simple terms, that’s proton therapy.
Standard radiation therapy has evolved and improved over the years and is effective in controlling many cancers. However, because X-ray beams are composed of primary photons and secondary electrons, they deposit their energy along the path of the beam, to the targeted tumor and beyond, and deliver radiation to healthy tissues before and after the tumor site. This radiation “exit dose” may cause health issues later because it can damage the normal tissue or organs near the tumor or area of concern.
The advantage of proton therapy (also called proton beam therapy) is that the physician can control where the proton releases the bulk of its cancer-fighting energy. As the protons move through the body, they slow down and interact with electrons, and release energy. The point where the highest energy release occurs is the “Bragg peak.” A physician can designate the Bragg peak’s location, causing the most damage to the targeted tumor cells. A proton beam conforms to the shape and depth of a tumor, while sparing healthy tissues and organs.
How does proton therapy work?
The best way to understand how proton therapy works is to take a look at the physics and engineering inside the proton accelerator, or the synchrotron, and the beam delivery system.
- The proton begins its journey at the ion source. Within fractions of a second, hydrogen atoms are separated into negatively charged electrons and positively charged protons.
- The protons are injected via a vacuum tube into a linear accelerator and in only a few microseconds, the protons’ energy reaches 7 million electron volts.
- Proton beams stay in the vacuum tube as they enter the synchrotron, where acceleration increases their energy to a total of 70 million to 250 million electron volts, enough to place them at any depth within the patient’s body.
- After leaving the synchrotron, the protons move through a beam-transport system comprised of a series of magnets that shape, focus and direct the proton beam to the appropriate treatment room.
- To ensure that each patient receives the prescribed treatment safely and efficiently, the facility is controlled by a network of computers and safety systems. The gantry can revolve 360 degrees, allowing the beam to be delivered at any angle.
- As protons come through the nozzle, a custom-made device (the aperture) shapes the beam of protons, and another custom-made device (the compensator) shapes the protons into three dimensions, delivering them to the depth of the tumor.
- At maximum energy, a proton beam travels 125,000 miles per second, which is equivalent to the two-thirds the speed of light.
- From the hydrogen canister to the patient, a proton typically travels 313,000 miles.
Pencil beam and intensity modulated proton therapy
The team at MD Anderson Proton Therapy Center continues to expand ways to use proton therapy to benefit patients. The team pioneered pencil beam proton therapy, also called scanning beam, and intensity modulated proton therapy (IMPT). We are one of the few centers worldwide offering these types of proton therapy to our patients.
Pencil beam technology and IMPT build on the benefits of proton therapy. With a proton beam just millimeters wide, these advanced forms of proton therapy combine precision and effectiveness, offering unmatched ability to treat a patient’s tumor and minimizing effect on a patient’s quality of life – during and after treatment. They rely on complex treatment planning systems and an intricate number of magnets to aim a narrow proton beam and essentially “paint” a radiation dose layer by layer.
Pencil beam is very effective in treating the most complex tumors, like those in the prostate, brain, eye, and cancers in children, while leaving healthy tissue and other critical areas unharmed. IMPT is best used to deliver a potent and precise dose of protons to complex or concave-shaped tumors that may be adjacent to the spinal cord or embedded head and neck or skull base, including nasal and sinus cavities, oral cavity, salivary gland, tongue, tonsils, and larynx.
Pencil beam scanning, also called scanning beam, is the most advanced type of proton therapy. Only a few millimeters wide (the width of a pencil), pencil beam scanning is used to treat complex cancers with unparalleled precision. It’s used to treat head and neck, gynecologic, lung, prostate, breast, brain, liver, lymphoma, sarcoma and tumors in children. Proton therapy may also be used for tumors that recur in areas that have previously been treated with standard radiation therapy.
“Pencil beam scanning is an intricate, personalized approach to proton therapy,” says Steven Frank, M.D., medical director of the MD Anderson Proton Therapy Center. “No two tumors are identical, so we customize the proton beam to target a patient’s unique tumor location, size and shape.”
Pencil beam scanning builds on proton therapy’s success
Pioneered at MD Anderson, pencil beam scanning uses a tumor’s location, shape and size to create a customized pattern of protons to precisely treat the tumor while avoiding nearby healthy tissue.
With standard radiation therapy, energized particles called photons are used to destroy cancer cells. But surrounding normal tissues are also exposed to the radiation, increasing the risk of side effects. By using a different type of energy called protons, proton therapy is able treat the tumor while minimizing radiation exposure to the rest of the body. “There’s a low entrance dose when the beam enters the body and a low exit dose once the radiation has been deposited into the tumor,” says Albert Koong, M.D., Ph.D.
However, pencil beam scanning takes the precision of proton therapy even further by shaping the area being treated, called the field, to mirror the tumor’s shape. “Although it’s possible to shape the field with traditional proton therapy, pencil beam technology can be shaped with an even greater degree of precision,” Koong says.
Pencil beam scanning allows for the manipulation of the beam to create a pattern of protons to more accurately administer the dose to the unique shape of the tumor. “We can increase the intensity of protons where there’s more volume of cancer and scale it back when there’s less,” Frank says.
Pencil beam scanning can treat a tumor with a single field or with multiple fields, sometimes up to five. When multiple scanning beam fields are used, it’s called intensity modulated proton therapy (IMPT). “The dose patterns of IMPT can overlap and intersect where the tumor has more bulk,” Koong says. This allows protons to treat even the most irregularly shaped tumors located around critical organs like the oral cavity, bowel or spinal cord.
Preparing for pencil beam scanning treatment
Developing a treatment plan that uses pencil beam scanning takes about a week. If your doctor has recommended pencil beam scanning, you’ll first undergo CT scans to capture images. Your radiation oncologist will use these images to identify the right targets and determine the beam arrangements.
A treatment course usually lasts six to seven weeks, but it varies based on the tumor and its sensitivity to radiation therapy. Patients can expect to be in the treatment room for about 30 minutes for set up, quality assurance and the actual treatment. “The delivery of the protons doesn’t take more than about a minute,” Frank says. “So it can be a total of three to four minutes, depending on the number of fields in the plan.”
Minimizing side effects, optimizing results
“Because we control the radiation dose and place it exactly where we want, we’re able to spare the surrounding tissues so patients experience fewer side effects,” Koong says.
Patients may still experience side effects, though, and they’re dependent on the tumor’s location. Gastrointestinal tumors like liver, colorectal and pancreatic cancers are close to the bowel. If the bowel is exposed to radiation, it can cause nausea, diarrhea or damage the bowel to the point that surgery is needed. Problems with sexual health and with bathroom use are possible for prostate cancer patients. Patients with head and neck cancers like tonsil, tongue, salivary gland, eye or skull base tumors can experience mouth ulcers, loss of taste, dry mouth and vision loss. Breast cancer patients can experience heart issues, and lung cancer patients can experience inflammation of the lung, causing shortness of breath.
With children, the concern of side effects is even greater because any amount of radiation can cause short-term side effects, but could also potentially lead to secondary cancers in the future. Proton therapy is ideal for treating younger patients because the overall radiation exposure is significantly less than with standard radiation therapy.
“We’re constantly monitoring patients throughout treatment to identify side effects and changes in the tumor,” Frank says. “As tumors shrink or anatomy shifts from treatment-related weight loss, we change the treatment plan to ensure it remains effective.”
“The patient is at the core of everything we do,” Koong adds. “Our approach focuses on the disease site, not our treatment specialty, so we’re always individualizing the treatment for each patient.”
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