The study, which was published in the International Journal of Radiation Oncology Biology Physics, was undertaken specifically to see if the results from three benchmark clinical trials could be replicated in this larger population of unselected consecutive patients treated in a similar fashion — and for the most part, they were.

In terms of tumor control, the low and high risk rates of recurrence were each within 2 percent of the benchmark, however the intermediate risk recurrence rate was 5 percent lower in the benchmark. The reason for that disparity is unclear, according to the researchers, though it may have something to do with a broader sample of patients in the larger study.

The researchers also found that the risk of developing a serious urologic toxicity appears to be higher in men who have a large prostate volume, pre-treatment use of alpha blockers, pre-treatment prostate reductive procedures, diabetes, a higher volume of bladder tissue receiving a dose of 30 Gy, or a combination of those factors.

Quality of life outcomes were good for urinary and bowel function but median sexual function summary scores fell from 75 to 55.

And how do these outcomes compare with those obtained through intensity modulated radiotherapy (IMRT)? The researchers say there is still not sufficient reliable evidence to address that. "Prospective comparative studies are needed for definitive comparison of proton therapy with IMRT," said Bryant.

He and his colleagues plan to undertake just such a study. "The goal will be to determine the relative value of proton therapy compared to the other radiation therapy modalities in terms of minimizing the risk for toxicity, preserving patient-reported quality of life, and providing tumor control," said Bryant.

"We are working out details which we hope to share soon," he added.

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