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SAN ANTONIO -- A shortened radiation therapy (RT) protocol for high-risk prostate cancer led to similar outcomes with no increase in toxicity compared with the standard treatment schedule, a randomized trial showed.

Overall survival (OS) at 7 years was 82%, with no difference between standard RT and hypofractionation that reduced treatment duration from 8 to 5 weeks. Prostate cancer-specific mortality (PCSM), biochemical failure-free survival (BFFS), and distant metastasis-free survival (DMFS) were all similar between treatment arms.

The study is the first randomized trial to demonstrate noninferiority for hypofractionated RT in high-risk prostate cancer, reported Tamim M. Niazi, MD, of McGill University in Montreal, at the American Society for Radiation Oncology (ASTRO) annual meeting.

"The sample size was based on the co-primary endpoints of acute and delayed toxicity, and survival outcomes were analyzed as secondary endpoints," Niazi said during an ASTRO press briefing. "Although the sample size was based on toxicity data, the survival outcomes are pretty much identical and superimposable. Therefore, [we] conclude that hypofractionated radiation therapy ... is as effective as conventional fractionation ... with similar and acceptable toxicity.

"Moderate hypofractionated radiation therapy should be considered as a new standard of care for high-risk prostate cancer patients who are candidates for external-beam radiation therapy and long-term androgen deprivation therapy [ADT]."

Hypofractionated RT has an established role in the treatment of low- and intermediate-risk prostate cancer, but trials establishing that role have generally excluded patients with high-risk disease characteristics. Results of the Canada-based extend the supporting evidence for hypofractionation to high-risk prostate cancer and should have a practice-changing impact on management of high-risk prostate cancer, said press briefing moderator Jeff Michalski, MD, of Washington University in St. Louis.

"One of the reasons for [excluding high-risk patients] is that commonly the volume of radiation therapy delivered for men with high-risk prostate cancer is larger and might often include the pelvic lymph nodes, for example, and therefore, there was concern that shortening the overall course through the use of hypofractionation or larger doses of radiation per treatment might lead to unacceptable toxicity," said Michalski. "So adoption of these shorter courses of radiation has been very slow and not yet considered by many [radiation oncologists] to be standard of care in men with higher-risk disease.

"That's why this trial from Dr. Niazi and his collaborators in Canada is so important," Michalski said. "Now, I believe their trial has shown that reducing the number of treatments for men with high-risk cancer, which included many patients who also received elective lymph node irradiation, is tolerable."

Hypofractionation offers several potential advantages over standard RT protocols, including lower treatment cost and reduced treatment burden and improved access and convenience, he added.

The randomized phase III PCS5 trial included 329 men with newly diagnosed high-risk prostate cancer, defined as stage T3a+, PSA level ≥20 ng/dL, and/or Gleason score 8-10. All patients received 28 weeks of ADT and were randomized to receive standard RT (76 Gy in 38 fractions, including sequential boost) or hypofractionated RT (68 Gy in 25 fractions, including concomitant boost). The co-primary endpoints were acute and delayed toxicity.

Acute toxicity differed only with respect to grade 1/2 gastrointestinal (GI) effects, which occurred more often with hypofractionation. Rates of grade ≥3 GI toxicity and all grades of genitourinary toxicity did not differ significantly between treatment groups, said Niazi. The two treatment strategies did not differ with respect to grade or type of delayed toxicity.

Survival and disease-related endpoints, all secondary outcomes, also did not differ significantly between hypofractionation and standard treatment after 7 years of follow-up, including:

• OS: 81.7% vs 82%

• PCSM: 94.9% vs 96.4%

• BFFS: 87.43% vs 85.1%

• DMFS: 91.5% vs 91.8%

"I think the next challenge that we face is can we do even shorter courses of radiation?" Michalski asked. "I'm happy to say that there are studies that are ongoing right now to test that hypothesis. I do think that with this presentation by Dr. Niazi, we will see shorter courses of radiation embraced by the radiation oncology community and make this treatment more available for our patients."

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