CHICAGO, May 22 (Xinhua) -- Proton therapy results in fewer side effects than traditional X-ray radiation therapy for many cancer patients, according to a study released by the Washington University in St. Louis on Wednesday.

The researchers focused their study on what are called grade 3 adverse events, which are severe enough to require hospitalization, including pain, difficulty swallowing that might result in weight loss, difficulty breathing, and nausea and diarrhea severe enough to cause dehydration.

After controlling for differences between the groups, such as age and additional medical problems, the researchers found that patients receiving proton therapy experienced a two-thirds reduction in the relative risk of severe side effects within 90 days of treatment, compared with patients receiving X-ray radiation therapy.

Forty-five or 11.5 percent of 391 patients receiving proton therapy experienced a severe side effect in the 90-day time frame. In the X-ray radiation therapy group, 301 or 27.6 percent of 1,092 patients experienced a severe side effect in the same period.

Patient data on side effects were gathered as the trial was ongoing, rather than after the fact.

"Proton therapy was associated with a substantial reduction in the rates of severe acute side effects, compared with conventional photon, or X-ray, radiation for patients treated with concurrent radiation and chemotherapy," said Brian Baumann, an assistant professor of radiation oncology at Washington University and the Perelman School of Medicine at University of Pennsylvania. "While there have been other studies suggesting that proton therapy may have fewer side effects, we were somewhat surprised by the large magnitude of the benefit."

Patients in the proton group had fewer side effects despite the fact that they were older with an average age of 66 than patients in the X-ray radiation therapy group with an average age of 61, Baumann noted.

The researchers also found no differences between the two groups in survival, suggesting that proton therapy was just as effective in treating the cancer even as it caused fewer side effects. Overall survival at one year for the proton therapy group was 83 percent of patients as against 81 percent for the X-ray radiation therapy group. This difference was not statistically significant.

This study is the first large review of data across several cancer types to show a reduced side-effect profile for proton therapy compared with X-ray radiation therapy for patients receiving combined chemotherapy and radiation.

In the next step, the researchers will compare the costs of the two different therapies and their associated adverse events.