For five years, clinicians at the University of Florida Proton Therapy Institute in Jacksonville, Florida, tracked the ongoing health of 211 men. Each had been diagnosed with low-, intermediate- or high-risk prostate cancer. And all had been treated with image-guided proton therapy between August 2006 and September 2007.

Among prostate cancer study participants, patients were deemed low risk with a Gleason score of 6, and a PSA less than 10. Intermediate-risk prostate cancer patients had a Gleason score of greater than 6 but less than 8, and/or a PSA more than 10, but less than 20. Patients with high-risk disease had a Gleason score greater than 8 and/or a PSA of 20 or higher. Other factors also helped determine prostate cancer severity.

The results, published earlier this year in the International Journal of Radiation Oncology Biology Physics, showed 99 percent of 82 patients with intermediate-risk prostate cancer remained disease-free five years after proton treatments.

According to Nancy P. Mendenhall, M.D., lead author of the peer-reviewed study and medical director at UF Proton Therapy Institute, the number of cancer-free men previously diagnosed with mid-level prostate cancer is of particular significance, as that classification of patients is fairly consistent across prostate cancer studies, whether using proton or photon radiation. “It’s important to highlight that intermediate group,” she emphasized. “Their disease is at a point where we know they all need treatment, otherwise it will progress. None of these were candidates for active surveillance.”

Disease control rates of 70 to 85 percent are typically seen among prostate cancer patients with intermediate-risk disease who are treated with photons using intensity modulated radiation therapy, Dr. Mendenhall noted.

“We had hoped for disease control outcomes of 85 percent for intermediate-risk patients, about the same as a recent Memorial Sloan Kettering study using intensity modulated radiation therapy,” Dr. Mendenhall said. “To have 99 percent was a pleasant surprise.”

The study also reported 99 percent of 89 patients who had low-risk prostate cancer and 76 percent of 40 patients with high-risk prostate cancer were free of their cancer five years after proton therapy.

“I think the most important takeaway for patients is that disease control outcomes for protons were really outstanding,” Dr. Mendenhall said.

While being cancer-free was one primary measure of the study, the other was the patient’s capacity to live a normal life, absent the troubling side effects frequently experienced by men whose healthy urinary and gastrointestinal tissues received unintentional radiation from photons.

Throughout the five-year study, Florida researchers asked members of the study group general questions about their daily health. “The patient was not guided or prompted in any way,” said Dr. Mendenhall. “So in terms of experiencing normal urinary and bowel function — which can be two very important things — the vast majority of patients reported no change from normal. The loss of erectile function did occur in a few patients.”

Ten years ago, radiation oncologists at Loma Linda University Medical Center Proton Treatment Center published the first long-term outcomes study of prostate cancer patients treated solely with protons. The 1,200 participants were treated with 3-D conformal proton therapy. “Our study confirms their excellent findings,” Dr. Mendenhall said.

This fall, Dr. Mendenhall and her associates will complete data collection for a larger outcomes study involving about 1,300 men treated for prostate cancer at UF Proton Therapy Institute from 2007 to 2010.