SAN DIEGO – delivered at higher doses in 20-39 fractions, according to new data from the phase 3 randomized PACE-B trial.
Overall, the 5-year event-free survival rates were 95.8% among patients who received SBRT and 94.6% among those who had conventional radiation. The incidence of adverse events was also low in both groups, with no significant differences observed between the trial arms.
The similar event-free survival and toxicity profiles in both groups provide more support for SBRT, which treats prostate cancer with larger radiation fractions over a shorter time period.
“I think we can also say now with a high level of confidence that SBRT can be considered a new standard of care for low and favorable intermediate-risk prostate cancer,” said Nicholas van As, MD, MB, from the Royal Marsden NHS Foundation Trust and Institute of Cancer Research in London, who presented efficacy and safety results from the noninferiority trial at the American Society for Radiation Oncology (ASTRO) annual meeting. SBRT is more convenient for patients and more cost-effective for health care providers, Dr. Van As added.
Invited discussant Alejandro Berlin, MD, MSc, from Princess Margaret Cancer Centre and the University of Toronto, agreed “that this should be a standard of care for low and favorable intermediate-risk prostate cancer,” an option already endorsed by relevant guidelines.
But, Dr. Berlin noted, SBRT requires careful attention to technique to achieve the desired results. Further research will be needed to identify and potentially reduce variability among radiation oncology practice regarding margins, dosimetry goals, dose heterogeneity, treatment schedules, and other factors, he said.
An international trial
PACE-B is one of three branches of a multi-center collaboration among 37 radiation therapy centers in the United Kingdom, Ireland, and Canada.
In the trial, investigators enrolled 874 patients with T1c or T2c prostate cancer, Gleason score of 3+4 or less, prostate-specific antigen (PSA) level no higher than 20 ng/mL, MRI staging, and no prior androgen deprivation therapy. Investigators then randomly assigned them on a 1:1 basis to receive either conventional radiation (n = 441) or SBRT (n = 433).
At the start of the trial, patients who were assigned to the conventional radiation group received 78 Gy in 39 fractions over 4-8 weeks. However, after results from the CHHiP trial, which showed that a 60-Gy, 20-fraction regimen was not inferior to a 74-Gy, 37-fraction regimen, the PACE-B investigators modified the protocol to 62 Gy delivered in 20 fractions.
Patients assigned to SBRT received 36.25 Gy divided into give fractions delivered over 1-2 weeks, with 40 Gy to the clinical target volume.
The primary outcome was noninferiority of SBRT, measured as whether patients remained free of biochemical clinical failure. Biochemical clinical failure was defined as evidence that the cancer was returning, such as an increase in PSA levels or distant metastases or death from prostate cancer.
At a median follow-up of 73.1 months, 5-year event-free survival rates were 94.6% for patients who received conventional radiation therapy and 95.8% for patients who received SBRT, meeting the prespecified criteria for noninferiority of SBRT (P = .007).
Freedom from biochemical and clinical failure, the trial’s primary endpoint, “was significantly better on both arms than our original power calculation, where we expected control rates of approximately 85%,” Dr. Van As said in an ASTRO plenary session.
Toxicity rates were also low in both study arms. The rate of grade 2 or greater urogenital side effects at 5 years was 5.5% in the SBRT arm and 3.2% in the conventional therapy arm. Grade 2 or greater gastrointestinal side effects occurred in only two patients, one in each study arm.
Given the findings, “I think it’s now imperative that our surgeons discuss this data with their patients before they perform prostatectomies,” Dr. Van As said.
Neha Vapiwala, MD, president-elect of ASTRO who moderated a media briefing where Dr. Van As summarized the PACE-B data, commented that “this study was conducted very rigorously, with excellent quality assurance.”
The study also highlights that clinicians in the United States have considerable catching up to do, said Dr. Vapiwala, from the Hospital of the University of Pennsylvania, Philadelphia.
In the United States, “we are way behind our colleagues on the other side of the pond,” she said. “We are way behind in our uptake of ultra-hypofractionated radiation [such as SBRT], and I do believe that some of that comes from the lack of feeling comfortable with the techniques that are needed and the expertise that is needed.”
PACE-B was funded by Accuray. Dr. Van As disclosed research grants from the company and consulting fees from Varian. Dr. Berlin reported no conflict of interest relevant to the study. Dr. Vapiwala has disclosed a consulting or advisory role with Bayer.
A version of this article first appeared on Medscape.com.