Prostate Radiotherapy
Proton Therapy vs IMRT for Prostate Cancer: Early COMPPARE Results
Early results from COMPPARE, a prospective study of 2,343 analyzed patients, found no significant differences between proton therapy and IMRT in bowel quality of life, gastrointestinal toxicity, or early cancer control.
Short answer: In early results from COMPPARE, the largest prospective comparison of proton therapy and IMRT for prostate cancer to date, protons showed no significant advantage in patient-reported bowel symptoms, gastrointestinal toxicity, or early cancer control. Both treatments performed extremely well.
What is COMPPARE?
COMPPARE is a pragmatic, prospective comparative study designed to close a long-standing evidence gap: does proton therapy (PT) actually produce better outcomes than photon-based intensity-modulated radiation therapy (IMRT) for localized prostate cancer?
Between 2018 and 2022, 2,524 patients enrolled at 51 facilities — 28 proton centers and 23 IMRT centers. Treatment reflected real-world practice: both conventional fractionation (1.8–2.1 Gy per day) and moderate hypofractionation (2.4–3.1 Gy per day) were permitted, and rectal spacers and hormone therapy were used at physician discretion. Treatment-naïve patients with localized disease were eligible, excluding very high-risk cancer.
The analysis included 1,404 proton patients and 939 IMRT patients, with inverse probability of treatment weighting to balance the groups. Median follow-up was 4.0 years.
What did patients report?
The primary endpoint targeted the claim most often made for protons: less bowel injury. Using the validated EPIC quality-of-life tool at two years, the study found:
- no significant difference in bowel urgency (p = 0.324); and
- no significant difference in bowel frequency (p = 0.514).
These were tested against an alternative hypothesis of proton superiority — meaning the study was specifically designed to detect a proton advantage if one existed. It did not find one.
What about physician-graded toxicity?
Two-year cumulative incidence of grade 2 or higher gastrointestinal toxicity was:
- 5.2% with proton therapy; and
- 5.6% with IMRT.
The hazard ratio was 0.91 (95% CI 0.65–1.28, p = 0.6) — no significant difference.
Was cancer control different?
No. Three-year freedom from biochemical progression was 98.0% with protons and 97.9% with IMRT (hazard ratio 0.95, 95% CI 0.52–1.71, p = 0.90). Both approaches controlled localized prostate cancer exceptionally well at this early timepoint.
How does this fit with PARTIQoL?
The randomized PARTIQoL trial reported the same conclusion in 450 patients: no meaningful difference between protons and IMRT in bowel quality of life or five-year progression-free survival. COMPPARE now adds a much larger, multi-center, real-world cohort pointing the same direction.
Two complementary study designs — one randomized, one pragmatic and prospective — have now failed to demonstrate a clinical advantage for proton therapy in localized prostate cancer.
What are the limitations?
COMPPARE was not randomized; patients and physicians chose the treatment, and statistical weighting can only partially correct for that. These are also early endpoints: late toxicity, long-term disease control, and secondary malignancy rates need longer follow-up, which the study will continue to report.
What does this mean for patients?
Proton therapy is often marketed as a premium option, and for some cancers — pediatric tumors, some brain and skull-base tumors, and re-irradiation — its physics offers genuine clinical value. For localized prostate cancer, the accumulating evidence says the beam type is not what determines your outcome.
What does matter: daily image guidance, the quality of treatment planning, rectal spacing when appropriate, the experience of your treatment team, and choosing a fractionation schedule that fits your cancer and your life. If proton therapy would require significant travel, delay, or expense, current evidence does not support that trade-off for localized prostate cancer.
Questions patients often ask
Did COMPPARE show proton therapy is better than IMRT for prostate cancer?
No. The study was designed to test whether protons were superior for patient-reported bowel symptoms, and it found no significant difference in bowel urgency, bowel frequency, GI toxicity, or early cancer control.
How is COMPPARE different from PARTIQoL?
PARTIQoL was a randomized trial of 450 patients; COMPPARE was a larger, non-randomized prospective comparison of 2,343 analyzed patients treated in routine practice at 51 centers. Both reached the same conclusion: no meaningful clinical advantage for protons.
Should I travel or pay more for proton therapy for localized prostate cancer?
Current evidence does not show better outcomes with protons for localized prostate cancer. Treatment quality, image guidance, experience of the team, convenience, and cost are more meaningful differentiators than beam type.
Are these final results?
No. These are the pre-specified early endpoints at a median follow-up of 4 years. Long-term disease control, late toxicity, and secondary malignancy comparisons will require longer follow-up.
Sources and further reading
- Early results of COMPPARE, a prospective comparison of outcomes with proton and photon radiation in prostate cancer (LBA5012). Journal of Clinical Oncology (2026).
- COMPPARE Clinical Trial Record. ClinicalTrials.gov (2026).
