Malignant pleural mesothelioma (MPM) is a type of cancer that starts in the lining around the lungs. After diagnosis, many patients are closely monitored rather than treated straight away. Treatment is usually started only when the benefits outweigh the risks and when it is likely to help maintain quality of life.

Proton beam therapy (PBT) is a newer type of radiotherapy. It can target the cancer more precisely, meaning less radiation reaches the surrounding healthy tissues. This may reduce side effects compared with standard radiotherapy. If PBT can limit these side effects, it may help patients live longer while also maintaining their quality of life.

This study is designed to see whether PBT could be a useful early treatment option for patients who would normally be placed on “watch‑and‑wait” (initial surveillance) after diagnosis. The study will look at:

• How long patients live without their cancer worsening (progression‑free survival)
• Overall survival
• Safety and side effects
• Quality of life
• Impact on NHS costs and resources

How the trial works:

• Patients will be randomly assigned to one of two groups:
  • PBT group: Patients receive proton beam therapy
  • Standard care group: Patients continue with the usual surveillance approach
• PBT is given every weekday for 5 weeks at specialist centres in London or Manchester.
• The trial plans to recruit 148 patients across the UK.

There is also a small sub‑study, run with Mesothelioma UK and the University of Sheffield, involving 8–10 patients receiving PBT. This part of the research will look at patients’ experiences and expectations of the treatment and the trial.

All patients will be followed up at their local hospital for two years, including CT scans and blood tests to monitor the cancer and support additional research.

The results from this national trial will help update UK guidance on the best available treatments for mesothelioma, potentially improving care for future patients.