Presented during ESTRO 2026 by Dr. Sian Cooper, the HERMES study showed that treating localized prostate cancer with only two sessions of MRI-guided radiotherapy over eight days caused no additional side effects compared with the standard five-session schedule. The findings bring the field closer to a shorter, more convenient radiotherapy approach for selected patients with prostate cancer.
Background
Radiotherapy is one of the main curative treatment options for localized prostate cancer. In many countries, the current standard short-course approach consists of five radiotherapy sessions delivered over approximately two weeks.
In recent years, prostate cancer radiotherapy has moved toward shorter treatment schedules, using fewer sessions with a higher dose per session. This approach can reduce the number of hospital visits while maintaining effective tumor treatment.
A two-session radiotherapy schedule could be especially important for patients who live far from treatment centers, have work or family responsibilities, or face travel difficulties. For hospitals and radiotherapy departments, fewer sessions could also improve treatment capacity and allow patients to receive care more efficiently.
The HERMES study was designed to evaluate whether delivering an equivalent radiotherapy dose in only two sessions is feasible and safe, particularly in terms of urinary, bowel, and quality-of-life outcomes.
Methods
HERMES was a randomized phase 2 study that included 46 patients with prostate cancer.
Patients were assigned to one of two treatment groups. Twenty-four patients received the standard five-session radiotherapy schedule over two weeks. Twenty-two patients received the same equivalent dose in only two sessions delivered over eight days.
Treatment was delivered using MRI-guided adaptive radiotherapy. This technology combines an MRI scanner with a radiotherapy machine, allowing clinicians to visualize the prostate during treatment and target the tumor with high precision.
The aim was to deliver radiotherapy accurately to the prostate while reducing radiation exposure to surrounding healthy tissues, including the bladder and bowel.
Results
The study found that two-session MRI-guided radiotherapy was feasible and did not increase side effects compared with the standard five-session schedule.
Between six months and two years after treatment, around one in four patients in both groups experienced moderate urinary side effects, such as increased urinary frequency or urgency.
Importantly, no severe urinary or bowel side effects were reported in either group. Bowel side effects were very low overall, and no patients in the two-session group reported bowel side effects.
Quality-of-life outcomes were also encouraging. After two years, patients reported minimal changes in quality of life, with no meaningful difference between the two-session and five-session treatment groups.
Safety
The safety profile of the two-session approach was reassuring. The shorter schedule did not appear to increase urinary or bowel toxicity.
The absence of severe urinary or bowel side effects is clinically important because these are among the main concerns when radiotherapy doses are condensed into fewer sessions. The findings suggest that, when delivered with MRI-guided adaptive technology, ultra-short-course radiotherapy can be delivered safely in selected patients.
Interpretation
The HERMES study suggests that prostate cancer radiotherapy may be safely reduced to only two outpatient sessions in selected patients, without increasing side effects compared with the current five-session approach.
For patients, this could make treatment less disruptive to daily life, work, travel, and family responsibilities. For healthcare systems, shorter radiotherapy schedules may improve workflow and increase treatment availability.
However, the study included a small number of patients, and the results were presented as a non-peer-reviewed experimental study in people. Larger studies with longer follow-up are needed before two-session MRI-guided radiotherapy can be considered a new standard of care.
Clinical Relevance
These ESTRO 2026 data support further investigation of two-session MRI-guided radiotherapy for localized prostate cancer.
The key message is clear: with advanced MRI-guided technology, prostate cancer radiotherapy may be delivered in a much shorter schedule while maintaining a favorable safety profile. This approach could offer a more convenient and patient-centered treatment option if confirmed in larger trials.
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