The Effect of Hyperbaric OxygeN Therapy on brEast Cancer Patients With Late Radiation toxicitY - UMBRELLA HONEY Trial

Inclusion Criteria: * Self reported pain grade 3-4 (on a scale of 1-4) as assessed by the late radiation toxicity questionnaire; * Participation \>12 months in the UMBRELLA cohort; * Previous treatment with radiotherapy for breast cancer; * Completed surgery and (neo)adjuvant systemic therapy except adjuvant endocrine therapy, for breast cancer. Exclusion Criteria: * Poor responder to questionnaires (i.e. return of ≤ 2 UMBRELLA questionnaires); * Previous HBOT; * Contra-indications for HBOT (e.g. (severe) chronic obstructive pulmonary disease (COPD) or asthma, pacemaker, morbid obesity, epilepsy in medical history, severe heart failure); * Current metastatic disease or recurrent breast cancer. Additional exclusions criteria based on screening visit: * Inability to follow schedule of all consecutive HBO treatments (e.g. due to scheduled holidays \> 2 days); * Not meeting criteria for HBOT (e.g. due to complaints similar to late radiation toxicity, not caused by radiotherapy).