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by John Leifer 9/2018 updated by Dr. C.H. Weaver 9/2019

There are few things more mind-numbing than nuclear physics. Fortunately, you don’t have to be a physicist to understand proton therapy. Your goal, after all, is not to understand sub-atomic particles, but to cut through the hype surrounding proton therapy so you can make an educated decision regarding potential treatment.

Scientists have known for decades that radiation can control and cure, as well as create cancers. The goal of radiation oncologists has been to focus radiation as precisely as possible on the intended target – sparing unaffected or healthy tissue whenever possible. Because of the limitations of technology, there is always some degree of collateral damage – resulting in what are referred to as “toxicities” or side-effects of treatment in the common vernacular.

The dominant technology has been a linear accelerator that produces photons or x-rays, which are then directed to the targeted area through an external beam (IMRT, or intensity modulated radiation therapy, is one of the most frequently used types of radiation). But just as a flashlight’s beam widens with distance, so, too, do x-rays – resulting in areas of the body receiving unintended doses of radiation. To add insult to injury, x-rays don’t stop when they reach the intended target, but proceed through the body at a decreased dose.

The resulting side-effects can be short-term or long-term, minor or truly debilitating. So the promise of an emerging technology to deliver precisely focused energy to a tumor has obvious appeal, particularly when the areas being treated are adjacent to structures that are highly sensitive to radiation.

Proton therapy appears to deliver on that promise by impacting a much tighter area surrounding the target. The particles stop and deposit their energy where planned – particularly when the target is relatively close to the surface of the body, such as the eye, spine, or base of the skull.

Such precise dosing should translate into lower toxicities in patients treated with protons…a theory being borne out by research with pediatric patients: “New patient outcome data are emerging showing…a lower incidence of vision or hearing loss and improved neurocognitive function in pediatric cancers…” But what about adults suffering from deeper-seated tumors, such as prostate cancer? That’s when the picture begins to blur. (1)

Whereas pediatric tumors are rare, prostate cancers are common. For a proton center that has sunk $150-200 million in facilities and equipment, patient volume is its lifeblood. And, since “between 3 and 5 prostate patients can be treated in the time it takes to treat a single, complex pediatric case,” (2) it’s easy to understand why proton centers would want to recruit prostate cancer patients. But financial return is a not a medical indication for the use of a treatment, and research justifying such use has been sorely lacking: “Although radiation oncologists have been eager to adopt proton beam for prostate cancer, they have been slow to perform clinical studies.”(3)

Research to-date suggests that “despite the theoretical physical advantages of proton therapy, studies have yet to show any clear clinical benefit to proton beam over IMRT in terms of morbidity in the treatment of prostate cancer.” One thing that is unequivocal, however, is the financial “toxicity” of proton treatment. (3)

Proton therapy is expensive. Medicare often must pay fifty percent more to providers of this treatment than for IMRT treatment. There are hopes that treatment costs will drop as smaller, more affordable treatment centers are constructed. That’s still several years away.

Both consumers and physicians are often captivated by the latest technology. There’s no better example than the surgical robot: Little evidence suggests any significant clinical benefit to the patient, yet there’s tremendous demand for it despite its hefty price tag.

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And though proton therapy has existed for many years, it has only recently been hyped to a stratospheric level, suggesting that it is the gold-standard of care. Based upon price alone, it may be the platinum standard—but until there is more evidence of its benefits when used in the treatment of commonly occurring cancers, such as prostate cancer, one must exercise caution before embracing this modality of care.

What do studies show?

No Difference in Toxicity Between Proton Therapy and IMRT for Prostate Cancer

Proton therapy does not reduce toxicity compared with intensity-modulated radiation therapy (IMRT) in men with prostate cancer, according to the results of a study published in the Journal of the National Cancer Institute.

To compare IMRT and PRT, researchers performed a retrospective study of all Medicare beneficiaries age 66 or older who received PRT or IMRT for prostate cancer during 2008 and/or 2009. The analysis included 27,647 men—553 (2%) received PRT and 27,094 (98%) received IMRT. In order to assess toxicity, each PRT patient was matched with two IMRT patients with similar clinical and sociodemographic characteristics.

The results indicated that PRT was associated with a statistically significant reduction in genitourinary toxicity at 6 months compared with IMRT (5.9% vs. 9.5%); however, there was no difference at 12 months (18.8% vs. 17.5%). In terms of cumulative rates of all toxicity—there was no significant difference between the two methods at 6 or 12 months. (4)

The researchers concluded that PRT is substantially more costly than IMRT and does not produce a difference in toxicity 12 months after treatment.


  1. J. A. Efstathiou, P. J. Gray, and A. L. Zietman, “Proton Beam Therapy and Localised Prostate Cancer: Current Status and Controversies,” British Journal of Cancer 108 (2013): 1225, doi: 10.1038/bjc2013.100.
  2. Zietman, Anthony. “Proton Beam and Prostate Cancer: An Evolving Debate,” Reports of Practical Oncology and Radiotherapy 18 (2013): 338.
  3. J. A. Efstathiou, P. J. Gray, and A. L. Zietman, “Proton Beam Therapy,” p. 1227.
  4. Yu JB, Soulos PR, Herrin J, et al. Proton versus intensity-modulated radiotherapy for prostate cancer: Patterns of care and early toxicity. Journal of the National Cancer Institute. Published early online December 14, 2012. doi: 10.1093/jnci/djs463

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