A Clinical Reality: Advances in Photon Beam Therapy

Jason Li
March 22, 2018

Submitted as coursework for PH241, Stanford University, Winter 2018


Fig. 1: A cyclotron at the University of Washington used to produce proton beams (Source: Wikimedia Commons)

Although proton beam therapies were first proposed in 1946, there are currently only 28 proton therapy centers in the United States. [1] In the last few decades, they were considered too expensive to become a realistic option to fight cancers, with proton therapies costing 2.4 times more than standard X-ray therapies in 2003. [2] In the last few years, costs have come down rapidly, and the cost of receiving proton therapy is now similar to conventional X-ray radiation. [3] Now, photon therapies are becoming more commonplace, and 24 more centers are currently under development. [1]

Mechanism of Treatment

Radiation treatments work by damaging the DNA of rapidly dividing cancer cells, traditionally with photon beams, also called X-ray waves. [1] However, proton therapies use beams of protons, which can be more precisely controlled with magnets to minimize the harmful radiation to healthy parts of the body. [1] And because the beam is also more concentrated, a lesser dose is required than traditional radiation therapies. [1]

A proton beam therapy system is typically composed of an accelerator, beam transport, beam delivery, and patient positioning systems. [4] An accelerator spins and accelerates the protons with alternating electric fields, aiming them into a beam with magnets. [1] A cyclotron, a type of accelerator, is shown in Fig. 1. While small amounts of radiation are deposited into the body on the way, the majority of the radiation directly targets the tumor, irradiating it layer-by-layer with each pulse. [1]

Clinical Outcomes

Literature has documented less toxicity and better health outcomes for proton therapy, although their costs and benefits vary in the diversity of cancers. Indeed, after 6 months of treatment, proton therapy has been found to reduce the odds of toxicity by 40% in prostate cancer compared to traditional radiotherapy. [5] In non-small cell lung cancer, proton therapy was associated with better 5-year survival rates, with 22% of patients surviving compared to 16% of matched patients receiving non-proton radiation therapy. [6] And in pediatric brain tumors called medulloblastomas, proton therapies were found to be the most effective in sparing healthy tissue compared to two conventional X-ray therapies, achieving lower long-term toxicity. [7] Indeed, children who have cancer are an especially important population given that more of their normal cells are still developing, so background radiation damage and toxicity to their healthy, developing cells are especially consequential. [1]

Current Accessibility

Proton therapy centers can treat over 70 types of cancers. [3] However, their availability to cancer patients are limited not just by the few number of proton therapy centers, but also by current insurance guidelines and cost-benefit considerations. [3] Given the vulnerability of children and the elderly, insurance companies often only reimburse treatments for those under 18 or over 65. [3] However, costs are coming down, with some speculating that technology may cut costs by half in the next five or six years. [1] Thus, given the rapid expansion of proton therapy centers and drop in costs, proton therapies may very well become commonplace clinical options in the next several years, a reality unimaginable just a few decades ago.

© Jason Li. 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.


[1] B. Berkowitz and A. Steckelberg, "How Proton Therapy Attacks Cancer," Washington Post, 5 Mar 18.

[2] M. Goitein and M. Jermann, "The Relative Costs of Proton and X-ray Radiation Therapy," Clin. Oncol. 15, S37 (2003).

[3] E. Sauber, "First Proton Therapy Center in Middle TN to Open in Franklin," Tennessean, 1 Mar 18.

[4] P. Ditiangkin, "Photon Beam Radiation Therapy," Physics 241, Stanford University, Winter 2013

[5] J. Yu et al, "Photon Versus Intensity-Modulated Radiotherapy For Prostate Cancer: Patterns of Care and Early Toxicity," J. Natl. Cancer Inst. 105, 25 (2013).

[6] K. Higgins et al, "National Cancer Database Analysis of Proton Versus Photon Radiation Therapy in Non-Small Cell Lung Cancer," Int. J. Radiat. Oncol. Biol. Phys. 97, 128 (2017).

[7] W. St. Clair et al, "Advantage of Protons Compared to Conventional X-ray or IMRT in the Treatment of a Pediatric Patient with Medulloblastoma," Int. J. Radiat. Oncol. Biol. Phys. 58, 727 (2004).