|Fig. 1: A model of the Mevision S250 proton accelerator, which creates a proton beam used for cancer therapy. (Source: Wikimedia Commons)|
Proton beam therapy is a mode of cancer treatment that penetrates tumors via targeted radiation, which uses protons instead of electrons. As of 2015, there were 10 proton therapy facilities in the U.S., 38 centers around the world, and more under construction, indicating that hospitals are willing to take a risk on the new technology.  The surge in facility production is contributed to the ever- growing need for improved cancer therapy that is as potent to tumors as current radiation treatment, without causing as much damage to existing healthy tissue. With this objective in mind, proton beam therapy may be the way forward for targeted cancer therapy.
Before entering a patient's body, protons must acquire a great deal of energy and sustain it through the proton delivery system. A proton's journey starts at the ion source, where hydrogen atoms are split into negatively charged particles (electrons) and positively charged particles (protons). From there, the protons are transported via a vacuum tube into a linear accelerator, where it only takes a matter of microseconds for the protons to attain an energy of 7.0 MeV.  Once these protons reach such a high energy, the proton beam remains within the vacuum tube as it is transported to the proton accelerator, the synchrotron, and the following steps are taken: 
The synchrotron increases the total energy of the proton beam to 70 MeV - 250 MeV, depending on how deeply the radiologist would like the beam to penetrate the patient's body.
Protons are then transported via a beam-transport system, which makes use of magnets that focus the direction of the beam toward the correct patient room for treatment.
Once arriving to the correct patient room, the beam is shaped by an aperture, and a compensator shapes the protons themselves into three dimensions for optimal depth penetration in the patient.
The protons themselves, at their maximum energy, can result in the beam traveling up to 70% of the speed of light.  Despite the speed and high energy nature of these particles, proton cancer therapy has been hailed by radiologists for its extreme precision. Proton beam therapy is therefore not only a remarkable achievement on behalf of engineers and physicists for even making such technology possible, but of tremendous importance to physicians and cancer patients in an era of targeted, personalized medicine.
There are several advantages that proton beam therapy has over traditional cancer therapies.
Primarily, proton beam therapy provides targeted, high efficacy radiation without damaging as much peripheral healthy tissue as standard radiation therapy. In a study done by scientists at Loma Linda University, esearchers found that the use of proton beam radiation could effectively be used to treat men with prostate cancer, ith minimal biochemical indications of relapse. Operating at lower, more targeted doses, the facility recorded a 73% disease-free survival rate while the long-term outcomes for patient survival were comparable to those for other modalities intended for a cure.  The conservative treatment method offered by proton cancer therapy is highly desirable to minimize excess potent treatment to patients and to preserve hospital resources.
Second, a gantry system within the device allows the beam to rotate 360°. This mechanical advantage allows physicians to move the beam around the patient and project the beam in an optimal location for eradicating the cancer. Fig. 1 shows what the machine looks like within one of of the therapy rooms. Traditional X-radiation does not provide such a specific, targeted method; and as a result, many patients suffer from adverse effects, such as relapse, severely damaged healthy tissue, and other equally challenging maladies.
Finally, physicians can control where the bulk dosage of protons are deposited, improving treatment efficacy and efficiency. As the protons move through the body and toward the cancer, they start to interact with electrons and release their energy.  The point at which the protons release the most energy is referred to as the Bragg's Peak; and using proton beam technology, physicians can control where the peak occurs, which allows them to deposit the beam's highest energy particles to targeted tumor cells (see Fig. 2). At a tissue depth between 18-24 cm, the proton beam can effectively target and treat tumors.  Ultimately, this focused beam minimizes damage to surrounding healthy tissue, which in turn maximizes recovery potential.
|Fig. 2: Illustration of how protons release the most energy at the Bragg Peak. While the native proton beam typically has a sharp Bragg Peak, a modified proton beam extends the duration of the peak energy level so the entire target receives maximal dosage. (Source: Wikimedia Commons)|
The greatest disadvantage to proton beam therapy is the high cost of building a facility. Large investments are needed to build the infrastructure required for accelerators, gantries, and beam transport systems.  Building a proton beam facility costs more than $77 million based on the few data points available from the group of running treatment facilities in the U.S. While analysts should take into account that much of this data is based on a very small pool, as a result of the relatively new nature of proton beam therapy, researchers suggest that the construction and use of proton beam facilities may be cost-effective investments. So long as the appropriate cancer risk groups are chosen as targeted patients for proton beam therapy, and that the therapy itself is cost- effective compared to traditional radiation treatments, then medical institutions stand to benefit from the construction of such breakthrough technology.
Another concern scientists and investors have in proton therapy is about the amount of research that has yet to be done. While proton therapy institutions have accomplished a great deal in their years of operation, there is still some uncertainty when it comes to treating pediatric cancers or those that require less invasive treatment. In these cases, many physicians will default to using X-Rays, simply because the technology has more well-documented effects than that of proton beam therapy. As with any new medical device, designers of proton beam therapy systems have the challenge of first matching, then beating the pace of current, steadfast medical radiation tools in order to have a chance at getting into a clinic.
Importantly, such investments are only possible for institutions that have the backing, funding, and space to support such facilities. So while the technology is breakthrough, and even if the data continues to demonstrate promising results, such treatment may not be available to those in rural or impoverished parts of the world. Ultimately, the hope is that, as time passes by, the actual cost of building the device itself may drop, thereby opening the doors to more hospitals and institutions to install proton beam therapy facilities.
As proton beam therapy centers are built at medical institutions across the U.S., physicians and engineers are actively working to collect data on what other indications such therapy may be used for or how it can be tailored to provide more services to patients. Doctors at the MD Anderson Cancer Center have created pencil beam proton therapy - which is highly effective in treating complex cancers, like those in the prostate, eye, and brain - as well as intensity modulated proton therapy - which delivers a focused beam to complex, concave-shaped tumors in the spine, neck, head.  Ultimately, by expanding the number of procedures possible with a proton beam, institutions increase their net earnings from the facility as well as provide more extensive, well-documented proton beam procedures to patients with widely varying tumor types.
© Persiana Saffari. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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.
 W. D. Newhauser and R. Zhang, "The Physics of Proton Therapy," Phys. Med. Biol. 60, R155 (2015).
 J. D. Slater et al., "Proton Therapy For Prostate Cancer: The Initial Loma Linda University Experience," J. Radiat. Oncol. 59, 348 (2004).
 E. S. Wisenbaugh et al., "Proton Beam Therapy for Localized Prostate Cancer 101: Basics, Controversies, and Facts," Rev. Urol. 16, 67 (2014).
 V. Marx, "Cancer Treatment: Sharp Shooters," Nature 508, 133 (2014).
 J. Lundkvist et al., "Proton Therapy of Cancer: Potential Clinical Advantages and Cost-Effectiveness," Acta Oncol. 44, 850 (2005).