page_title: Proton therapy
section_title: Description
hierarchical_section_title: Proton therapy / Description
caption_attribution_description: English: Thematic diagram showing dose as a function of depth for overlay of proton radiotherapy and x-ray radiotherapy to facilitate a comparison of the two radiotherapy methods.
context_page_description: In the field of medical treatment, proton therapy, or proton radiotherapy, is a type of particle therapy that uses a beam of protons to irradiate diseased tissue, most often to treat cancer. The chief advantage of proton therapy over other types of external beam radiotherapy is that the dose of protons is deposited over a narrow range of depth, which results in minimal entry, exit, or scattered radiation dose to healthy nearby tissues.
  When evaluating whether to treat a tumor with photon or proton therapy, physicians may choose proton therapy if it is important to deliver a higher radiation dose to targeted tissues while significantly decreasing radiation to nearby organs at risk. The American Society for Radiation Oncology Model Policy for Proton Beam therapy states that proton therapy is considered reasonable in instances where sparing the surrounding normal tissue “cannot be adequately achieved with photon-based radiotherapy” and can benefit the patient. Like photon radiation therapy, proton therapy is often used in conjunction with surgery and/or chemotherapy to most effectively treat cancer.
context_section_description: Proton therapy is a type of external beam radiotherapy that uses ionizing radiation. In proton therapy, medical personnel use a particle accelerator to target a tumor with a beam of protons. These charged particles damage the DNA of cells, ultimately killing them by stopping their reproduction and thereby eliminating the tumor. Cancerous cells are particularly vulnerable to attacks on DNA because of their high rate of division and their limited abilities to repair DNA damage. Some cancers with specific defects in DNA repair may be more sensitive to proton radiation.
  Proton therapy offers physicians the “superb ability” to deliver a highly conformal beam, i.e., delivering radiation that conforms to the shape and depth of the tumor and completely sparing much of the surrounding, normal tissue. For example, when comparing proton therapy to the most advanced types of photon therapy—intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT)—proton therapy can deliver similar or higher radiation doses to the tumor with a 50%-60% lower total body radiation dose.  
  Protons have the ability to focus energy delivery to conform to the tumor shape, delivering only low-dose radiation to surrounding tissue. As a result, the patient can experience fewer side effects. All protons of a given energy have a certain penetration range; very few protons penetrate beyond that distance. Furthermore, the dose delivered to tissue is maximized only over the last few millimeters of the particle's range; this maximum is called the spread out Bragg peak, often referred to as the SOBP (see visual).
  To treat tumors at greater depths, the proton accelerator must produce a beam with higher energy, typically given in eV (electron volts). Accelerators used for proton therapy typically produce protons with energies in the range of 70 to 250 MeV. Adjusting proton energy during the treatment maximizes the cell damage the proton beam causes within the tumor. Tissue closer to the surface of the body than the tumor receives reduced radiation, and therefore reduced damage. Tissues deeper in the body receive very few protons, so the dosage becomes immeasurably small.
  In most treatments, protons of different energies with Bragg peaks at different depths are applied to treat the entire tumor. These Bragg peaks are shown as thin blue lines in the figure in this section. It is important to understand that, while tissues behind (or deeper than) the tumor receive almost no radiation from proton therapy, the tissues in front of (shallower than) the tumor receive radiation dosage based on the SOBP.
caption: In a typical treatment plan for proton therapy, the spread out Bragg peak (SOBP, dashed blue line) shows how the radiation is distributed. The SOBP is the sum of several individual Bragg peaks (thin blue lines) at staggered depths. Note that the vast majority of the proton radiation is delivered to the tumor, not to the skin and shallow tissues in front of the tumor or to the deep tissues behind the tumor. The red line shows the depth-dose plot of an X-ray beam (photon or conventional radiation therapy) for comparison. The pink area represents additional doses of X-ray radiotherapy in front and behind the tumor – which can damage normal tissues and cause secondary cancers, especially of the skin.[2]
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