Can Radiotherapy for Prostate Cancer Be Reduced to 2 Sessions? Results of the ESTRO 2026 HERMES Study

Can Radiotherapy for Prostate Cancer Be Reduced to 2 Sessions? The HERMES Study Points to a New Milestone in MRI-Guided Treatment

Shorter Radiotherapy for Prostate Cancer Reduces the Burden on Patients

Prostate cancer is one of the most common cancers in men, and radiotherapy is one of the curative treatment options in localized disease. For many years, conventional radiotherapy was delivered in small doses over many fractions. This was because sensitive organs such as the bladder, rectum and urethra surround the prostate. Even a small error in the treatment area could lead to urinary and bowel side effects.

In recent years, a distinctive aspect of prostate cancer radiobiology has come to the fore: prostate cancer cells may be sensitive to higher doses delivered in fewer fractions. This biological characteristic enabled the development of hypofractionation and SBRT approaches. As a result, treatment lasting several weeks could be reduced to as few as 5 sessions for some patients.

The HERMES study is one of the boldest steps in this development. As prostate SBRT delivered in five sessions becomes an accepted approach at many centers, HERMES investigates whether delivering the same biological effect in just two sessions is safe. This question matters not only for patient comfort but also for healthcare system capacity, waiting times and more efficient use of radiotherapy equipment.

2 Fractions or 5 Fractions?

HERMES is a randomized, single-center, phase 2 / R-IDEAL 2b study evaluating whether MRI-guided adaptive SBRT delivered in 2 fractions is safe and feasible in prostate cancer. The study enrolled 46 patients with localized prostate cancer. Of these, 24 were assigned to standard radiotherapy delivered in 5 fractions, and 22 to ultra-short treatment delivered in 2 fractions.

Treatment was delivered using an MR-Linac system. This device combines MRI imaging and a linear accelerator on the same platform, allowing clearer monitoring of the prostate and surrounding organs during radiotherapy. On each treatment day, the plan can be adapted to the patient's anatomy that day.

No Additional Increase in Side Effects Was Observed With Two-Session Treatment

The most striking finding of HERMES is that reducing treatment from 5 sessions to 2 did not create an additional burden in terms of patient-reported and clinical side effects. Researchers reported moderate urinary complaints in approximately one in four patients in both groups during follow-up ranging from 6 months to 2 years. These complaints included symptoms such as frequent urination or a sudden feeling of urgency.

More importantly, no severe urinary or bowel side effects were observed in either arm. Bowel side effects were very low; no patient in the 2-session arm reported bowel side effects. During the two-year follow-up, patients reported minimal changes in their quality of life.

From 8 Weeks to 2 Sessions in Prostate Radiotherapy: A Major Journey Toward Shorter Treatment

The historical development of radiotherapy for prostate cancer is a good example of how technology and radiobiology together have shortened treatment duration. In the conventional era, prostate radiotherapy was delivered in daily sessions over 7–8 weeks. Later, modern imaging, intensity-modulated radiotherapy and better dose planning made moderate hypofractionation widespread; treatment duration fell to around 4 weeks.

The next major step was SBRT. The sensitivity of prostate cancer to high-dose fractions and the development of image-guided technologies made treatment regimens consisting of 5 fractions possible. HERMES is an early but important research step seeking to reduce this progression to 2 fractions.

PACE-B Strengthened the Evidence for 5-Session SBRT; HERMES Tests 2 Sessions

The clinical reliability of SBRT delivered in 5 sessions for prostate cancer has been strengthened by important studies in recent years. In particular, the PACE-B study showed that SBRT delivered in 5 fractions could provide disease control comparable to conventional or moderately hypofractionated radiotherapy in low- and intermediate-risk localized prostate cancer. Therefore, the 5-session approach now stands closer to clinical practice than to the research setting.

What distinguishes HERMES is that it further shortens SBRT delivered in 5 sessions by testing a regimen consisting of 2 sessions. However, the crucial distinction is this: HERMES is not yet a large efficacy study with long follow-up like PACE-B. Its main message is that the 2-session regimen appears feasible using MRI-guided adaptive technology and reassuring in terms of early/intermediate-term side effects.

For Now, Highly Selected Patients With Localized Prostate Cancer

It is not appropriate to generalize the HERMES results to all patients with prostate cancer. The study was conducted in selected patients with localized prostate cancer whose visible tumor volume could be identified on MRI. In the published HERMES paper, the patient group was described as a population including intermediate-risk and lower high-risk localized prostate cancer.

Therefore, treatment delivered in 2 fractions is currently an approach that can be discussed at centers with advanced technology within clinical studies or protocol-based evaluation, rather than a general standard. Caution is especially needed in cases of suspected spread outside the prostate, a very large prostate volume, pronounced urinary tract symptoms, serious rectal risks or when the target cannot be safely identified through imaging.

Shorter Treatment Is a Matter of Access as Well as Comfort

Shortening radiotherapy does not merely mean gaining a few days for patients. Particularly for patients who live outside the city, continue working, have a caregiving role or struggle to reach a radiotherapy center, the number of sessions directly affects access to treatment. While even five sessions offer considerable convenience compared with conventional treatment, a safe two-session approach could transform the patient experience even further.

From the healthcare system's perspective, fewer fractions may also enable more efficient use of equipment capacity, shorter waiting times and lower treatment costs. However, these advantages matter only if oncological safety and the side effect profile are maintained. The value of HERMES lies in providing an early signal that this convenience may be possible without the price of additional side effects.

The New Goal in Radiotherapy: Fewer Visits, the Same Safety, Greater Precision

Outstanding Questions About Radiotherapy Delivered in 2 Sessions

A Future of 2-Session Prostate Radiotherapy Appears Possible, but It Is Not Yet Standard

HERMES is an important early safety study showing that radiotherapy for localized prostate cancer can be reduced to as few as two outpatient sessions. In the randomized phase 2 study involving 46 patients, treatment delivered in 2 fractions using MRI-guided adaptive technology did not cause additional severe urinary or bowel side effects compared with standard SBRT delivered in 5 fractions. Approximately one in four patients experienced moderate urinary complaints, but no severe toxicity was observed and the change in quality of life over two years was minimal.

However, these results need to be interpreted correctly. HERMES does not prove that treatment delivered in 2 sessions is now the new standard for all patients with localized prostate cancer. The study is small, dependent on technology and not yet mature in terms of long-term oncological efficacy. Its main message is that a reassuring early signal has been obtained that prostate SBRT can be shortened even further with MRI-guided adaptive radiotherapy.

The future of prostate cancer treatment will be built not only on delivering higher doses, but also on visualizing the target more accurately, adapting treatment to that day's anatomy and offering patients the same safety with fewer hospital visits. HERMES should be viewed as a two-session rehearsal for that future.