PACE B Study: SBRT Comparable With Conventional Radiation for Intermediate-Risk Prostate Cancer

Patients with intermediate-risk, localized prostate cancer can be treated as effectively with stereotactic body radiation therapy (SBRT) as they are with conventional radiation, according to 5-year results of the phase 3 Prostate Advances in Comparative Evidence (PACE B) study presented at the American Society for Radiation Oncology 2023 Annual Meeting.

External beam radiotherapy (EBRT) is considered a curative treatment for localized prostate cancer. Previous randomized, controlled trials have shown that moderately hypofractionated EBRT regimens are noninferior to conventionally fractionated regimens.

PACE B sought to demonstrate the noninferiority of SBRT compared with conventionally or moderately hypofractionated regimens for intermediate-risk, localized prostate cancer. To demonstrate noninferiority, patients had to remain free of biochemical clinical failure (BCF), defined as an increase in prostate-specific antigen levels, distant metastases, or death from prostate cancer.

A total of 874 patients from 38 locations were enrolled in the trial. Patients were randomly assigned to receive either SBRT (n=443) consisting of 5 fractions over 1 to 2 weeks (36.25 Gy total dose) or standard radiation (n=441) consisting of 39 fractions over 7.5 weeks (78 Gy) or 20 fractions over 4 weeks (62 Gy).

After a median follow-up of 73.1 months, researchers reported that patients in the SBRT arm had a BCF event-free rate of 95.7% compared with 94.6% for patients in the conventional radiation arm. This finding demonstrated that SBRT was noninferior to conventional radiation (90% CI, P=.007), they added.

Toxicity was similar between the patient groups; at 5 years post-treatment, 5.5% and 3.2%, respectively, experienced grade 2 or higher side effects related to the genital or urinary organs, and only 1 patient in each treatment arm experienced grade 2 or higher gastrointestinal side effects.

“Standard radiation treatment is already highly effective and is very well tolerated in people with localized prostate cancer,” said principal investigator for the trial Nicholas van As, MD, medical director of the Royal Marsden NHS Foundation Trust and a professor at the Institute of Cancer Research. “But for a health care system and for patients, to have this treatment delivered just as effectively in 5 days as opposed to 4 weeks has huge implications.”

Prof. van As and colleagues are further investigating the use of SBRT for patients with higher-risk, localized disease. PACE A compared quality of life following SBRT versus surgery, finding fewer urinary and sexual side effects related to SBRT but a higher risk of minor bowel complications. PACE C has completed accrual and is investigating SBRT in patients with intermediate-risk and high-risk prostate cancer who are also being treated with hormone therapy.

In the meantime, Prof. van As and colleagues believe SBRT should be considered as an alternative treatment option to longer courses of radiation or surgery for patients with intermediate-risk, localized prostate cancer.