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SAN DIEGO -- Stereotactic body radiation therapy (SBRT) was as effective as conventional radiation therapy (CRT) in treating intermediate-risk prostate cancer, according to results from the randomized PACE-B trial.

Among 874 patients, those randomized to CRT had a biochemical/clinical failure-free rate of 94.6% versus 95.8% for those randomized to SBRT (HR 0.73, 90% CI 0.47-1.12, P=0.004), reported Nicholas van As, MD, of the Royal Marsden NHS Foundation Trust in London, during the American Society for Radiation Oncology annual meeting.

"In my view, SBRT should be considered a new standard of care in low- and favorable-intermediate-risk prostate cancer," van As said during a press briefing.

Neha Vapiwala, MD, of the University of Pennsylvania Perelman School of Medicine in Philadelphia, who served as the moderator at the press briefing, noted that a major limitation in U.S. practice has been the lack of level I evidence that can help patients with localized prostate cancer understand the available treatment options.

Studies like this have "really revolutionized our ability to conduct what we call shared decision making," she said. "We now have level I evidence for patients to understand, when they are in the position of having localized prostate cancer, what are the options and tradeoffs."

"The question of the safety, and now the efficacy, of stereotactic body radiation therapy ... is established when compared to longer courses of treatment," she added. "You can provide more convenient treatment for the patient, and it can be safe and it can be effective."

At 5 years, Radiation Therapy Oncology Group (RTOG) grade ≥2 genitourinary (GU) toxicity was seen in 3.2% of patients who received CRT and 5.5% of patients who received SBRT, while RTOG ≥2 gastrointestinal (GI) toxicity was seen in just one patient in each arm.

There was no significant difference in Common Terminology Criteria for Adverse Events (CTCAE) grade ≥2 GI toxicity between the two groups (1.7% in the CRT arm vs 2.5% in the SBRT arm). "However, with CTCAE GU toxicity, we did see a difference" -- 5.9% in the CRT arm versus 8.5% in the SBRT arm, with the biggest differences seen between months 9 and 18, van As noted. "It is important for patients to be aware of this."

"But then you see the lines come back together and at 5 years the toxicity is very low with no discernible difference," he pointed out.

van As also discussed quality-of-life results from , an international phase III randomized controlled trial comparing SBRT to surgery for localized prostate cancer. In this study, significantly fewer patients treated with SBRT reported an incidence of urinary incontinence (defined as any use of urinary pads) at 2 years compared with those treated with surgery (4.5% vs 46.9%). SBRT patients did report worse bowel bother subdomain scores, but better sexual bother subdomain scores.

Looking at PACE-A and -B together, "it is now imperative that our surgeons discuss this data with their patients before they perform prostatectomy," van As said.

He also noted the biochemical/clinical failure-free rates were significantly better in both arms of PACE-B than the rates of 85%-90% that the investigators expected. He suggested that the fact that patients were intermediate- or low-risk helped, and that there was rigorous quality assurance and very high-quality radiotherapy in both arms of the study.

"Simply, we were confident the radiation was getting to the prostate and minimizing the radiation to the surrounding tissue," he said. "These were probably the factors that made the difference."

Vapiwala pointed out that "it is important for our patients to know that this is absolutely reproducible in clinic."

However, she noted that "we [in the U.S.] are way behind our colleagues on the other side of the pond ... in our uptake of ultra-hypofractionated radiotherapy, and I do believe that some of that comes from a lack of feeling comfortable with the techniques that are needed and expertise that is needed."

"I want to point out that this study was conducted very rigorously," she added. "There is a way to do this and it is important for the practitioner in the clinic who wants to replicate these results and wants to offer it, that they have the training needed and the resources needed to provide this. It's a matter of time in the U.S. before this becomes increasingly adopted as clinicians become more comfortable in understanding the technique."

was conducted at 38 centers across the U.K. and Canada. The primary endpoint was freedom from biochemical/clinical failure. Biochemical failure was based on increases in prostate-specific antigen levels, commencement of androgen deprivation therapy, or date of orchiectomy, while clinical failure was based on local recurrence, nodal recurrence, distant metastases, and death from prostate cancer.

Patients were randomly assigned to receive SBRT consisting of five fractions over 1 to 2 weeks (36.25 Gy total dose) or CRT consisting of 39 fractions over 7.5 weeks (78 Gy) or 20 fractions over 4 weeks (62 Gy). None of the patients received hormonal therapy. Median follow-up was 73.1 months.

Median age of participants was 70, and the vast majority had intermediate-risk cancer (90% of those receiving CRT and 91% receiving SBRT).

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