Proton Beam Therapy for Prostate Cancer: Is Billion-Dollar Technology Really Better?

What Is Proton Therapy? What Physics Promises

Radiotherapy has remained a cornerstone of cancer treatment for decades. However, the two basic forms of radiation differ fundamentally in how they deposit energy in a tumor. Conventional radiotherapy, intensity-modulated radiation therapy (IMRT), uses X-ray-based photon beams. Proton beam therapy (PBT), as its name suggests, uses proton particles as its weapon.

This physical difference is extremely important. Photon-based beams pass through the target tumor and continue depositing energy in surrounding tissues. Protons, through a property called the Bragg peak, deposit most of their energy at a specific depth, within the tumor, and then stop abruptly. This “stop and deposit” property raised expectations that, in theory, damage to surrounding healthy tissue would be reduced and the side-effect profile improved.

This physical advantage is highly valuable in situations where proton therapy is truly critical: tumors close to vital structures such as the heart, brainstem or spinal cord, and particularly pediatric cancers. Proton therapy offers a real advantage when protecting developing bone and tissue growth in children is vital. But is prostate cancer a setting in which this advantage applies?

The Largest Comparative Trial to Date: PARTIQoL

To answer this question, a multicenter, Phase III, randomized controlled trial involving 30 centers was designed: PARTIQoL (NCT01617161). Between June 2012 and November 2021, 450 men with low- or intermediate-risk localized prostate cancer were randomized equally to two arms: 226 received proton therapy and 224 received IMRT. The primary endpoint was bowel function measured with the EPIC (Expanded Prostate Index Composite) questionnaire 24 months after radiotherapy completion. Patients were followed for 60 months for bowel, urinary, sexual and hormonal side effects.

Median follow-up was 60.3 months (approximately 5 years). Baseline patient characteristics were balanced between the two arms: median age was 68, 59% had intermediate-risk disease, 51% received a hypofractionation protocol, and 48% had a rectal spacer. In the proton arm, 49% of patients were treated with pencil beam scanning.

The findings were exceptionally clear. No significant difference in bowel function was detected between the two arms at 24 months (p=0.836); both groups showed only a small, clinically unimportant decline from baseline. This absence of a difference was observed consistently not only at 24 months but at all assessments at 3, 6, 9, 12, 18, 36, 48 and 60 months. There was also no significant difference between the two arms in urinary, sexual or hormonal function at any time point. For tumor control, progression-free survival at 5 years was 93.7% with IMRT and 93.4% in the proton arm — this difference was not statistically significant either (p=0.706).

Why Is There No Difference? IMRT's Quiet Revolution

The theoretical physical advantage of proton therapy is real. Why did this advantage fail to translate into a clinical difference? The answer lies in the quiet evolution of IMRT technology over the past 20 years.

When proton therapy first entered practice at Loma Linda in 1990, the technical limitations of precisely targeting a prostate tumor embedded within the pelvis were highly consequential. At that time, proton therapy's physical advantage was much more clinically meaningful. Since then, however, IMRT has learned to deliver a much more precise dose to the target using multiple beams that conform exactly to the tumor's shape. Rectal spacers significantly reduced the bowel dose by placing a buffer between the rectum and prostate. Hypofractionation protocols increased effectiveness and shortened treatment duration by delivering higher doses in fewer sessions. Consequently, IMRT almost caught up with proton therapy in avoiding high doses to the rectum and bladder — and the clinical difference between them disappeared.

A 50-Million-Dollar Machine, 2 Times the Treatment Cost

With equivalent effectiveness, the question of cost inevitably comes to the forefront. The cost picture for proton therapy has several layers:

In 2014, the Institute for Clinical and Economic Review (ICER) concluded that proton therapy was not cost-effective for prostate cancer. Twelve years later, the 2026 NCCN guideline update includes a statement consistent with that assessment: “The higher cost of proton therapy should be considered when photon and proton beam therapy produce similar outcomes for tumor control and side effects.” NCCN explicitly emphasizes that this additional cost can cause direct financial toxicity for the patient.

Business Model or Evidence? The Economic Reality of Proton Centers

It is now openly discussed that the widespread recommendation of proton therapy for prostate cancer is not motivated solely by medicine. Proton centers need sufficient patient volume to recover their enormous infrastructure investments. Rare pediatric cancers or brainstem tumors alone cannot provide this volume. Prostate cancer, as the most common cancer type in the United States with over 300000 new diagnoses annually, has become the key to financial sustainability for proton centers.

ICER observed this dynamic firsthand during its work: clinicians from proton centers stated that a center could not remain financially viable by treating rare tumors alone. Patients with more common cancers, particularly prostate cancer, had to be accepted. This created the conditions for a process of generating demand that operated independently of medical evidence production, bringing debates about conflicts of interest with it.

What Do the Guidelines Say? The NCCN 2026 Update

The NCCN guideline published in January 2026 set out a framework consistent with the PARTIQoL findings: “There is no evidence supporting a clear superiority or inferiority of proton therapy compared with IMRT in treatment effectiveness or long-term toxicity.” This statement is an important step in bringing years of academic debate into the formal language of guidelines.

However, proton therapy is not without value across all cancers. Scientists and clinicians agree that proton therapy remains valuable in certain scenarios:

What Is on the Horizon? The COMPPARE Study

It is also true that PARTIQoL has not ended the debate. One reviewer of the study highlighted the larger COMPPARE study: this pragmatically designed study includes over 2500 patients, addresses three primary endpoints — bowel, urinary and sexual function — and is approaching maturity as of late 2024–2025. COMPPARE results will provide data complementary to PARTIQoL, particularly in subgroup analyses.

Researchers are also considering whether proton therapy might have synergistic effects in combinations — for example, with PARP inhibitors or immunotherapy. For now, however, the available evidence does not support routinely recommending proton therapy for low- and intermediate-risk prostate cancer.

The Situation in Turkey: Access, Costs and Decision-Making

Proton therapy is not yet a widely available clinical option in Turkey; a limited number of centers offer or plan to offer this technology. In this context, the message of PARTIQoL is especially valuable for Turkish practice: patients diagnosed with prostate cancer who are researching radiotherapy options may discover proton therapy, particularly through the internet or social media. At this point, clinicians need to inform patients from an evidence-based perspective — that IMRT offers excellent tumor control and quality of life in prostate cancer, while proton therapy provides no meaningful additional benefit according to the current evidence — and clearly explain that the extra cost cannot be justified in this setting. If proton capacity expands in Turkey, directing this resource toward pediatric cases and specific indications where proton therapy offers a real advantage is more defensible both medically and economically.

This article was created as part of the workflow using various editorial tools, including artificial intelligence. It was reviewed by human editors before publication.