Inclusion criteria:

• Patients ≥18 years of age, with biopsy confirmed MPM and histological subtyping (epithelioid or non-epithelioid)

• N0 or N1 and M0 disease

• Written informed consent

• Patient and local/regional MDT opt for active surveillance and deferral of SACT until clinical or radiological progression

• WHO Performance Status 0-1

• Disease confined to one hemithorax based on CT assessment

• Adequate pulmonary function

  • ≥ 40% predicted post-FEV1;
  • ≥ 40% predicted DLCO/TLCO

• Agreement to travel to either proton beam therapy centres (i.e. UCLH or The Christie) if randomised to arm 2

• Agreement to be followed up at a local HIT-Meso trial site

• Patient likely able to complete PBT planning based on local assessment

Exclusion criteria:

• Presence of metastatic or contralateral disease
• Cytological diagnosis and/or undetermined histological subtype
• Prior thoracic radiotherapy, chemotherapy, immunotherapy for MPM
• Prior radical surgery for MPM (extrapleural pneumonectomy or extended pleurectomy decortication or pleurectomy decortication)
• Initial systemic therapy or surgery is required and the patient and local/regional MDT do not opt for active surveillance
• Involvement of contralateral or supraclavicular lymph nodes
• T4 disease with invasion of the myocardium
• N2 and/or M1 disease
• Presence of new effusion that is not amenable to drainage
• WHO Performance Status ≥ 2
• Women who are pregnant or breast feeding
• Current or previous malignant disease which may impact on the patient's life expectancy
• Patient fitted with a pacemaker or implantable cardioverter-defibrillator (ICD)\
• Diagnosis of clinically significant interstitial lung disease (ILD), excluding mild fibrosis or incidental findings
• Chronic non-malignant disease with an estimated three-year survival rate of less than 20%
• Patient with prior thoracic / abdominal radiotherapy for malignancy who was not discussed with sponsor before recruitment