|Fig. 1: Image of a cyclotron that produces a proton beam. (Source: Wikimedia Commons)|
Proton beam therapy is used to treat tumor in cancer patients. An advantage of proton beam therapy over photon is that it deposits a precise yet lower integrated dose in the patient.  This results in reducing unnecessary dose to the affected tissue. This is possible because the dose distribution from a beam of protons can be described by a region of fairly flat dose followed by a sharp peak in the end, known as the Bragg peak.  The width of the peak can be controlled to generate desired dose distribution rates. The objective of this article is to provide an overview of proton beam radiation therapy.
Robert R. Wilson proposed the use of proton beams for radiation treatment in 1946 with the first treatment using proton beams occurring in the mid-1950s at the University of California at Berkeley.  Other U.S. institutions that have used proton beam therapy include Loma Linda University Medical Center, the Harvard Cyclotron Laboratory, and the University of California at Davis. Outside the U.S., countries that have used proton beam therapy include Russia, Switzerland, Japan, England, Belgium, and South Africa. 
Proton beam therapy uses ionizing radiation to target the tumor with a beam of protons.  The total integral dose deposited in the target tissue is lower when using protons compared to photon radiation treatments.  Protons slow down as they collide with electrons and nuclei during Coulomb collisions, by bremsstrahlung radiation loss, and by nuclear reactions.  The range for secondary electrons scattered by 250 MeV proton is about 2 mm.  Radiation loss is minimal for the energies used in proton therapy. The proton beam charged particles damage the DNA of cells, ultimately causing their death or interfering with their ability to proliferate. With proton beams, the end of the dose deposit is not directly detected due to the roton beam stopping within the treatment volume.  Gammas from nuclear reactions are detected to monitor the end of the range of the dose.
Proton beam therapy systems compose of the accelerator, beam transport, beam delivery, and patient support and positioning systems. The accelerator generates the proton beam of sufficient energy for desired penetration level. This must be done typically in less than 5 minutes.  Synchrotrons and cyclotrons are both used. Synchrotrons are precise and have very little spread in energy variation but have lower intensity compared to cyclotrons. Bending and focusing (beam transport) systems are needed because a single accelerator serves several treatment rooms. Proton loss is typically less than 5%.  Two types of beam spreading are passive beam spreading and dynamic beam spreading. Passive beam spreading scatters a proton beam through a high atomic number foil and dynamic beam systems work by scanning a proton beam magnetically in raster pattern or a pattern of concentric rings to produce uniform fields.  Dynamic beam delivery systems have improved control of dose distributions over passive beam systems.  Proton gantries are used to transport the beam and redirect it toward the tissue of concern. Proton therapy can take anywhere from one day to seven weeks depending on the tumor site. The length of treatment time will also decrease over time as heavier doses begin to increase.
© Paul Ditiangkin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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