medwireNews: Ultra-hypofractionated stereotactic body radiotherapy (SBRT) has a low rate of side effects, similar to a standard course of conventionally or moderately hypofractionated radiotherapy for low- or intermediate-risk prostate cancer, indicate 2-year toxicity findings from the phase 3 PACE-B trial.
Writing in The Lancet Oncology, Alison Tree (The Royal Marsden Hospital, Sutton, UK) and co-investigators describe the SBRT protocol as being “well tolerated” and say the results are now awaited for the overall primary outcome of biochemical or clinical failure for men with early prostate cancer.
The trial included 844 men with either low-risk prostate adenocarcinoma (clinical or MRI stage T1c–T2a, N0, M0/X; Gleason score of ≤6; prostate-specific antigen [PSA] <10 ng/mL) or prostate cancer with one or more intermediate-risk characteristics (clinical stage T2b or T2c; Gleason score 3+4; PSA 10–20 ng/mL).
The participants were randomly assigned to receive 36.25 Gy of SBRT given in five fractions over 1–2 weeks or to receive standard radiotherapy consisting of either 78 Gy in 39 fractions over 7.8 weeks or the later amended regimen of 62 Gy in 20 fractions over 4 weeks.
For the current prespecified analysis of late toxicity, the co-primary endpoints were grade 2 or more severe physician-assessed events on the Radiation Therapy Oncology Group (RTOG) scales of genitourinary and gastrointestinal toxicity.
Assessment of 384 patients in the SBRT arm and 381 patients given control radiation showed no significant differences between the groups for both the genitourinary (3 vs 2%) and gastrointestinal (2 vs 3%) measures.
Nor were there any cases of RTOG grade 4 or worse adverse events or treatment-related deaths reported over the 24 months.
But the researchers did detect a significant 5.7 percentage point increase after 2 years in the secondary endpoint of genitourinary toxicity at grade 2 or more severe using the Common Terminology Criteria for Adverse Events (CTCAE) criteria, albeit no significant differences were found for CTCAE grade 2 or worse gastrointestinal toxicity.
Furthermore, toxicity grades at different times over the 2 years of follow-up suggested a “flare” of a higher rate of grade 2 or worse genitourinary toxicity with SBRT than standard radiation for both the RTOG and CTCAE measures between 12 and 15 months of follow-up.
“These findings [suggest] that, although overall toxicity is low regardless of fractionation, using SBRT techniques could increase the risk of moderate, but not severe, genitourinary side-effects,” Tree et al say.
Median Expanded Prostate Cancer Index Composite Short Form (EPIC-26) scores at 2 years were comparable between the treatment arms for urinary incontinence, irritative or obstructive urinary symptoms, and the bowel, sexual and hormone composite scales.
“The low toxicity rates seen in PACE-B encourage further study of SBRT,” say the authors who now look forward to findings from the PACE-C study of intermediate- and high-risk prostate cancer and the PACE-NODES trial of five-fraction nodal irradiation.
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Lancet Oncol; doi:10.1016/S1470-2045(22)00517-4