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What is Proton Therapy?

Patient in CT Machine

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 treatment 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 it 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. Watch this video as Dr. Mahajan, director of international development at the MD Anderson Proton Therapy Center, explains intensity modulated proton therapy and what's involved in delivering this innovative treatment option.

Is Proton Therapy Right for You?

Proton therapy benefits patients whose tumors are solid with defined borders, meaning the cancer has not spread to other parts of the body. The noninvasive treatment does not require surgery to remove the cancer, making it ideal for inoperable tumors. 


© 2014 The University of Texas MD Anderson Cancer Center