‘Compelling’ results: SBRT may be efficacious for low-, intermediate-risk prostate cancer
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Key takeaways:
- Five-fraction SBRT appeared noninferior to conventional radiotherapy.
- Grade 2 or higher gastrointestinal adverse effects occurred at similar rates between the two study cohorts.
Five-fraction stereotactic body radiotherapy proved noninferior to conventional radiotherapy with regard to biochemical or clinical failure for men with localized prostate cancer, according to randomized phase 3 study results.
Researchers reported more grade 2 or higher genitourinary toxic effects among men assigned five-fraction SBRT; however, the difference in grade 2 or higher gastrointestinal adverse effects appeared comparable.
“When we designed the trial, we expected the control rates to be in the [range] of 88% to 90% biochemical control at 5 years,” Nicholas van As, MD, medical director of The Royal Marsden NHS Foundation and professor at The Institute of Cancer Research, told Healio. “We hoped and expected that SBRT would be noninferior.
“We were pleasantly surprised by the control rates of both arms of the study being in the [range] of 95%,” van As added. “We believe this is due to the evolution of radiotherapy technology and delivery techniques over the last 10 to 15 years.”
Researchers conducted the randomized phase 3 PACE-B trial to evaluate whether freedom from biochemical or clinical failure among men with localized prostate cancer varied if they received SBRT instead of conventionally or moderately hypofractionated regimens.
Eligible participants had stage T1 or T2 prostate cancer, a Gleason score of 3+4 or less and a PSA of 20 ng/mL or less. Study protocol did not allow for androgen-deprivation therapy.
The study included 874 men (median age, 69.8 years; median PSA, 8 ng/mL).
Researchers randomly assigned 433 men to SBRT, which consisted of 36.25 Gy in five fractions administered across 1 to 2 weeks.
They assigned the other 441 men to a control radiotherapy group. They received either 78 Gy in 39 fractions over 7.5 weeks or 62 Gy in 20 fractions over 4 weeks.
Freedom from biochemical or clinical failure served as the primary endpoint, with a critical HR for noninferiority of 1.45.
Median follow-up was 74 months.
Results showed comparable 5-year freedom from biochemical or clinical failure in the SBRT group and control radiotherapy group (95.8% vs. 94.6%; unadjusted HR = 0.73; 90% CI, 0.48-1.12).
Five-year cumulative incidence of late Radiation Therapy Oncology Group (RTOG) grade 2 or higher genitourinary toxic effects occurred at a significantly higher rate in the SBRT group (26.9% vs. 18.3%; P < .001). However, cumulative incidence of late RTOG grade 2 or higher GI toxic effects did not differ significantly between groups (10.7% vs. 10.2%).
Researchers acknowledged study limitations, including the fact that recommendations for five-fraction SBRT currently are limited to men with risk features similar to those included in the trial.
“Further research in this area [involves] exploring if treatment can be delivered in even [fewer] than five fractions ... and, importantly, whether we can further reduce toxicity of treatment,” van As told Healio.
For more information:
Nicholas van As, MD, can be reached at nicholas.vanas@rmh.nhs.uk.
Perspective
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Eric Horwitz, MD
PACE-B is one of a series of three prospective clinical trials comparing men with localized prostate cancer randomly assigned to surgery and different radiation techniques. In PACE-B, researchers randomly assigned men with low- or intermediate-risk prostate cancer to SBRT or conventionally fractionated intensity-modulated radiation therapy (IMRT). This study was designed to show noninferiority between SBRT (36.5 Gy in five fractions) and IMRT (78 Gy in 39 fractions).
The results demonstrated no biochemical or clinical difference between the two radiation techniques. Results showed a statistically significant difference in grade 2 or higher genitourinary adverse effects but no significant difference in late grade 2 or higher gastrointestinal adverse effects.
ASTRO, ASCO and American Urological Association published a joint consensus statement in 2018 stating that hypofractionated radiation should be the standard of care for men with prostate cancer. The caveat in the statement was that, although the data for moderately hypofractionated IMRT was robust, the data for SBRT was still limited.
Since that time, more data has been published supporting the use of SBRT, and the PACE-B study adds substantially to the evidence that SBRT should be the standard of care for men with low- or intermediate-risk prostate cancer treated with primary radiation therapy.
This is a well-designed, large randomized prospective study with a large number of patients and the results are compelling.
Lewis Katz School of Medicine
Temple University
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