April 28, 2026
Results from a recently published phase 3 randomized clinical trial demonstrate that intensity-modulated proton therapy (IMPT) is not inferior to intensity-modulated radiation therapy (IMRT) for progression-free survival in patients with oropharyngeal cancer. The findings also showed that IMPT is associated with reduced high-grade toxicity.
Mayo Clinic was one of 18 institutions nationwide to enroll patients in the first-ever phase 3 randomized trial directly comparing IMPT with IMRT. The study represents a landmark effort to rigorously evaluate whether the dosimetric advantages of proton therapy translate into meaningful clinical benefit.
The findings showed comparable disease control between treatment arms, along with improvements in overall survival and toxicity outcomes favoring IMPT.
"This has long-lasting ramifications for patients, both in terms of quality of life and projected lifespan," says Samir H. Patel, M.D., a radiation oncologist at Mayo Clinic Comprehensive Cancer Center in Arizona who helped lead Mayo Clinic's involvement in the study.
Trial design and clinical context
Historically, IMRT has been an effective and widely accepted standard-of-care treatment for oropharyngeal cancer. However, treatment-related toxicity, particularly in patients with favorable prognoses and long expected survivorship, has remained a significant concern.
Because proton beams can be delivered to stop at a defined depth, IMPT allows radiation oncologists to better spare surrounding critical structures, including the oral cavity, larynx and swallowing musculature. While prior planning studies suggested these advantages could reduce collateral damage, prospective randomized data were needed to support broader clinical adoption.
"Even though planning studies consistently showed we could reduce dose to normal tissues, we needed phase 3 evidence to demonstrate that these differences matter clinically," Dr. Patel says.
Reduced high-grade toxicity with proton therapy
The physical properties of proton beams allow a radiation dose to be delivered with greater precision than photon-based approaches. By limiting exit dose beyond the tumor target, IMPT reduces radiation exposure to adjacent organs and tissues that play a critical role in speech, swallowing and immune function.
The trial demonstrated significantly lower rates of treatment-related toxicity among patients treated with IMPT. Compared with photon-based therapy, proton therapy was associated with a 15% relative reduction in treatment-related immune suppression, as well as a 13% lower risk of severe swallowing complications. Patients receiving IMPT also were more than 13% less likely to require feeding tube placement.
These findings are particularly important in a disease population in which many patients survive long term.
"Our goal is to continue pushing the field forward. This trial represents a major step, but it's not the endpoint."
"A lot of patients with oropharyngeal cancer go on to live long, healthy lives after treatment," Dr. Patel says. "Chronic swallowing dysfunction or loss of taste is not just a physical burden. It also carries a significant long-term psychological impact."
Unexpected survival benefit warrants further study
In addition to reduced toxicity, the trial was the first to demonstrate a survival benefit associated with proton therapy in this setting. At five years, 90.9% of patients treated with proton therapy were alive, compared with 81% of patients who received IMRT.
While the biological mechanism underlying this survival advantage remains under investigation, the results were consistent across analyses.
"The survival benefit is not yet fully explained," Dr. Patel says. "There will need to be a deeper dive into potential contributing factors, but the finding is quite exciting."
Researchers emphasize that the survival signal is hypothesis-generating and underscores the need for continued investigation, rather than definitive mechanistic conclusions.
Implications for clinical practice and referral
The trial's findings strengthen the case for proton therapy as a standard-of-care option for appropriately selected patients with oropharyngeal cancer. They also have practical implications for referral patterns and access to care.
"We've faced significant hurdles, not only in generating high-level evidence, but also in obtaining insurance authorization for proton therapy in this patient population," Dr. Patel says. "These data will allow us to offer proton therapy more broadly, without having to be as selective or navigate extensive authorization processes."
Mayo Clinic's role and next research directions
Mayo Clinic physicians were involved in the study from its earliest stages, helping to establish primary endpoints and participating in ongoing data monitoring to ensure consistency in treatment planning across sites.
Looking ahead, Mayo Clinic researchers are continuing to explore strategies to further reduce treatment-related side effects. One area of active investigation is the use of circulating tumor DNA as a biomarker to guide radiation dose de-escalation while maintaining oncologic outcomes.
"Our goal is to continue pushing the field forward," Dr. Patel says. "This trial represents a major step, but it's not the endpoint."
For more information
Patel SH, et al. Proton versus photon radiotherapy for patients with oropharyngeal cancer in the USA: A multicentre, randomised, open-label, non-inferiority phase 3 trial. The Lancet. 2025; 47:174.
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