Two-Fraction SBRT Shows Acceptable Toxicity in Prostate Cancer Population

Two-fraction stereotactic body radiation therapy (SBRT) displayed feasibility and tolerability as a treatment for patients with intermediate/lower high-risk prostate cancer with visible gross tumor volume on multiparametric MRI compared with 5-fraction SBRT, according to findings from the phase 2 Hypofractionated Expedited Radiotherapy for Men With localisEd proState Cancer (HERMES)/R-IDEAL 2b trial (NCT04595019) published in the International Journal of Radiation Oncology, Biology, and Physics.

Among 47 patients, including 23 assigned to 2-fraction SBRT and 24 assigned to 5-fraction SBRT, no grade 3 genitourinary (GU) or gastrointestinal (GI) toxicities occurred. Among patients treated with 2-fraction vs 5-fraction SBRT, the rate of grade 2 or higher GU toxicities was 27.3% (n = 6/22; 95% CI, 11%-50%) and 29.2% (n = 7/24; 95% CI, 13%-51%) in the respective cohorts. A total of 24 grade 2 or higher GU events were observed across both arms.

Results showed that grade 2 or higher GU toxicities peaked in 4 weeks with the 2-fraction regimen vs 2 weeks with 5-fraction SBRT. Additionally, grade 2 or higher urinary frequency was observed in 4.5% of patients by week 2 and 13.6% of patients by week 4 in patients treated with 2-fraction SBRT. With 5-fraction SBRT, the 2-week and 4-week grade 2 or higher urinary frequency rates were 16.7% and 12.5%.

Furthermore, in the 2-fraction group, no patients experienced grade 2 or higher GI toxicities vs 6.8% (95% CI, 3%-32%) among patients treated with 5-fraction SBRT. No grade 2 or higher GU late adverse effects (AEs) were observed at 6 months; by 12 months, only 2 were reported, including 1 in each cohort. Additionally, no grade 3 or higher GU late toxicities occurred.

“The results [from the phase 2 trial] suggest comparable rates of acute grade 2 [or higher] GU toxicity in the 2-fraction and 5-fraction groups and exclude significantly higher toxicity than that seen in [the PACE-B trial (NCT01584258)],” Sian Cooper, FRCR, clinical research fellow at The Royal Marsden NHS Foundation Trust, wrote in the publication with study coinvestigators. “Acute urinary toxicity appears to peak earlier in the 5-fraction SBRT group in week 2 compared with week 4 after 2-fraction treatment.”

The single-center, phase 2 HERMES trial enrolled patients with intermediate or lower high-risk localized prostate cancer planned for radical radiation therapy and randomly assigned them 1:1 to receive 5-fraction SBRT over 10 days or 2-fraction SBRT over 8 days.

Patients assigned to either 5- or 2-fraction SBRT had median age of 73 years (range, 54-82) vs 74 years (range, 61-84), and most patients were White (87.5% vs 73.9%). The median PSA levels were 8 ng/mL (range, 3-23) vs 7 ng/mL (range, 2-21), and most patients had a Gleason score of 3+4 (75.0% vs 56.5%) as well as stage T2 disease (66.7% vs 78.3%). Furthermore, the majority of those treated had intermediate disease per NCCN risk group classification (79.2% vs 78.3%).

The primary end point of the trial was proportion of patients with CTCAE grade 2 or higher GU toxicity up to 12 weeks after treatment. Secondary end points included acute and late physician-reported toxicity per CTCAE and the incidence of grade 2 or higher GU toxicity at 6 and 12 months. The study also evaluated quality of life (QOL).

Regarding QOL, the baseline International Prostate Symptoms Score (IPSS) score in the 2- and 5-fraction groups, respectively, was 6.5 (IQR, 4-11) vs 4 (IQR, 2-6). The peak total score was 8 (IQR, 4-11) in the 2-fraction group at week 4; it was 13.5 (IQR,10-17) in the 5-fraction group at week 2. By week 12, the respective IPSS scores were 6 (IQR, 2-8) and 8 (IQR, 3-11) in each respective cohort.

Reference

Cooper S, Westley RL, Biscombe K, et al. HERMES: Randomized trial of 2-fraction or 5-fraction magnetic resonance imaging−guided adaptive prostate radiation therapy. Int J Radiat Oncol Biol Phys. 2025;123(3):773-782. doi:10.1016/j.ijrobp.2025.05.008