IMRT vs Proton Therapy for Early Prostate Cancer?

For patients with intermediate- or low-risk localized prostate cancer, intensity-modulated radiation therapy (IMRT) and proton beam therapy are both safe and effective options, according to results of the phase 3 randomized controlled PARTIQoL trial.

For patients with intermediate- or low-risk localized prostate cancer, intensity-modulated radiation therapy (IMRT) and proton beam therapy are both safe and effective options, according to results of the phase 3 randomized controlled PARTIQoL trial.

With both techniques, disease control rates were over 90%, with virtually no difference in bowel function or other quality-of-life ratings after 2 years, reported Jason Efstathiou, MD, PhD, with Massachusetts General Hospital, Boston, at the American Society for Radiation Oncology (ASTRO) 2024 annual meeting.

“This is a tremendous study [that] really shows us we have two great options, with equal results across the board for both control rates and toxicity rates,” said Sameer Keole, MD, incoming ASTRO president, during a press briefing.

“These control rates are phenomenal, and the complication rates were very low,” continued Keole, with Mayo Clinic, Phoenix, Arizona. “I think men can go and seek definitive treatment when it’s appropriate with a radiation oncologist and know that whether it’s proton therapy or IMRT; it’s an excellent treatment option.”

Overall, about 70% of new cases of prostate cancer each year are localized disease, which represents about 200,000 patients in the United States each year, Efstathiou explained. These patients have several treatment options, including different choices for external beam radiation therapy.

“Because many of these patients are going to survive their cancer and live many years after treatment, quality of life becomes paramount because they’re at risk for long-term post-treatment morbidity,” Efstathiou explained. “Quality of life will inform their decision-making.”

Efstathiou noted that proton beam therapy comes with certain dosimetric advantages with the potential to reduce morbidity and improve cancer outcomes, but it is generally more resource intensive and costly than IMRT.

The PARTIQoL multicenter phase 3 randomized trial compared patient-reported quality of life after external beam radiation using either IMRT or proton beam therapy to determine whether one performs better on the local control and toxicity fronts.

After stratifying by institution, age (< 65 years vs ≥ 65 years), rectal spacer use (no vs yes) and moderate hypofractionation (no vs yes), participants were randomized to either proton beam therapy or IMRT.

Patients were followed longitudinally for 60 months after completing radiotherapy. The primary endpoint was bowel function at 24 months using the Expanded Prostate Cancer Index Composite (EPIC) instrument. Secondary outcomes included urinary and erectile function, sexual function, toxicity and efficacy, or disease control endpoints.

Of the 450 patients randomized, 221 of 226 (97.8%) randomized to proton beam therapy and 216 of 224 (96.4%) randomized to IMRT started on their respective treatments, and 167 and 162, respectively, completed the EPIC at 24 months. This represents about a 27% rate of missing data, which “was much better than anticipated,” Efstathiou noted.

For the primary endpoint, there was no difference between proton beam therapy and IMRT in mean change of the EPIC bowel score at 24 months, with both treatment groups showing only a small, clinically nonrelevant decline from baseline. There was only about a 2% decrease on a 100-point scale in bowel quality of life after 2 years, Efstathiou reported.

Similarly, the team noted no difference in bowel function at earlier or later time points. “We see some small fluctuations, but at no time point did these reach statistical significance,” he noted.

There were also no differences observed in the other domains at any point, including urinary incontinence, urinary irritation, or sexual function.

Turning to disease control, Efstathiou and colleagues found no difference between the two groups in progression-free survival. The progression-free survival rate was 99% at 24 months and 93.7% at 60 months with IMRT, compared with 98.1% at 24 months and 93.4% at 60 months with proton beam therapy.

When looking at key subgroups or factors, the team reported no sustained difference in any quality-of-life domain or in cancer control.

Patient monitoring over a longer follow-up period is ongoing. Efstathiou noted that the PARTIQoL trial was limited to localized, low- and intermediate-risk prostate cancer patients receiving either conventionally or moderately hypofractionated therapy. The trial also did not address the full range of disease scope, including higher risk disease, nodal therapy, concurrent use of hormonal therapy or other systemic therapy, local recurrent situations, or retreatment situations.

Efstathiou noted that because both proton therapy and IMRT continue to evolve, there is ongoing work to optimize the delivery of both.

Overall, the PARTIQoL trial results demonstrate “equivalent outcomes, with superb cancer control rates and extremely low toxicity from both treatments,” Jessica Karen Wong, MD, MEng, who wasn’t involved in the study, told Medscape Medical News. 

“Both are excellent treatments for low- and intermediate-risk prostate cancer patients,” said Wong, Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia. “This study corroborates prior single and multi-institutional experiences with the statistical power and rigorous methods of a clinical trial. Dr Efstathiou and authors should be commended for this comprehensive and well-run trial.”

Discussant for the study, Curtiland Deville, MD, of Johns Hopkins University School of Medicine, Baltimore, agreed that patients in the trial did “exceedingly well,” regardless of whether patients received IMRT or proton therapy.

Deville said the “fundamental question regarding the use of proton therapy for prostate cancer remains — is there a clinical benefit to protons that justifies their increased costs in this setting? In a cost-neutral setting, it may still be considered very reasonable to deliver proton therapy for prostate cancer.”

In his view, this study is “practice-informing” but not yet “practice-changing as we await the imminent findings of the COMPARE trial,” which uses a pragmatic design powered to assess the co-primary patient-reported outcome endpoints of EPIC bowel summary, urinary function, and sexual function scores at 2 years and which enrolled over 2500 patients.

The study has no commercial funding. Efstathiou disclosed various relationships with IBA Proton Therapy, Blue Earth Diagnostics, Boston Scientific, AstraZeneca, Genentech, Lantheus/Progenics, Astellas/Pfizer, Elekta, Uptodate, Merck, Roivant Pharma, Myovant Sciences, EMD Serono, Bayer Healthcare, Janssen, Pfizer, Progenics Pharmaceuticals, Gilead, Angiodynamics, and Clarity Pharmaceuticals. Keole and Wong had no relevant disclosures. Deville is deputy editor of the ASTRO Red Journal.