Malignant pleural mesothelioma (MPM) is a cancer which begins in the tissues lining the lungs. Many patients are under initial surveillance after diagnosis and only receive treatment later when deemed suitable when considering the risk and benefits of the NHS approved treatment options and impact on quality of life. Proton beam therapy (PBT) is a type of radiotherapy which results in less radiation to healthy tissues surrounding the cancer compared to photon radiotherapy. Using PBT to limit these side effects would provide an invaluable way to improve outcomes and life expectancy for these patients whilst maintaining their quality of life. This trial aims to explore PBT as a treatment option for MPM patients who would routinely be put under initial surveillance in the standard of care pathway after diagnosis; to determine how PBT impacts progression free and overall survival, safety and toxicity, quality of life and health economics. Patients will be randomly allocated (1:1 ratio) to receive either PBT (the trial arm), or remain in the standard of care surveillance approach for MPM (the control arm). The PBT will be delivered daily (on weekdays) over 5 weeks at the proton centre in London or Manchester. The trial aims to recruit 148 patients in total. In collaboration with Mesothelioma UK and University of Sheffield, there is also a sub-study for 8-10 patients who are allocated to receive PBT to explore patient expectations and journey with the trial and proton therapy. All patients will be followed up locally at their recruitment centres for two years where tumour assessments with CT scans and blood samples will be collected for biomolecular research. The evidence from this national trial will support the national guidance on best available treatments for mesothelioma to be updated, and further patients to benefit from this in the future.