Inhaled Budesonide for REcurrence Prevention and Adjuvant THerapy in Checkpoint Inhibitor Pneumonitis

Inclusion Criteria: 1. Patients must be 18 years of age, or older on the day of signing informed consent and be willing and able to provide written informed consent/assent and, in the opinion of the Investigator, comply with protocol tests and procedures 2. Patients require histologically confirmed solid tumour undergoing immune checkpoint inhibitor (ICI) therapy 3. Diagnosis of first documented diagnosis of Checkpoint Inhibitor Pneumonitis (CIP) made per European Society for Medical Oncology (ESMO)/American Society for Medical Oncology (ASCO) guidelines with severity \>/grade 2 by Common Terminology Criteria for Adverse Events (CTCAE)v5.0 a. Per ASCO/ESMO consensus guidelines, workup must include a compatible clinical picture, plus/minus supporting radiographic evidence (chest x-ray or preferably computed tomography (CT)), combined with clinical and/or microbiologic ruling out of alternative etiologies including infections or pulmonary disease progression. This includes a negative COVID test. Bronchoscopic sampling is not required, but can be considered. 4. Be able to effectively operate and use budesonide delivery method (Turbuhaler®), either independently or with aid of caregiver who anticipates being able to do so throughout trial period 5. Have adequate organ function, as judged by enrolling clinician 6. Females of childbearing potential have a negative urine or serum pregnancy test prior to study day 1. Patients of childbearing potential are those who have not been surgically sterilized or have not been free of menses for at least 1 year 7. Females of childbearing potential are willing to use contraception or abstain from heterosexual sexual contact for the course of the study Exclusion Criteria: 1. Diagnosis of interstitial lung disease (ILD) active (clinically and radiologically evident) within last year prior to diagnosis of CIP 2. Clinically suspected significant lung disease for which inhaled steroid would be standard of care (moderate asthma or severe chronic obstructive pulmonary disease (COPD)) and for which randomization to no inhaled steroids (control arm) would not represent optimal medical care. Based on enrolling physicians judgement; spirometry/pulmonary function tests are NOT required to assess this exclusion criteria. 3. Current (within last two weeks), active (not medically able or unwilling to discontinue prior to treatment start) and regular (2 or more times per week) use of inhaled steroids (for any indication) or systemic (\>10mg prednisone equivalent) corticosteroids (for indication other than CIP) at time of randomization 4. Receiving systemic, non-chemotherapy immunosuppressive agent at time of randomization (hydroxychloroquine is acceptable) 5. Use of a medication with significant interaction with inhaled budesonide (HIV protease inhibitors, ketoconazole or other potent CYP3A4 inhibitors), unless deemed required and safe by co-investigator. 6. Known poorly controlled diabetes, defined as A1c \>10, prior to initiation of steroids for CIP 7. History of active and unstable systemic disease, including heart failure New York Heart Association (NYHA) III or IV, cirrhosis with Child Pugh B or C, Renal Failure with creatinine clearance (CrCl) \<30 per Cockcroft-Gault formula, or other unstable life limiting condition as determined by trial investigators 8. Current or prior participation in a study of an investigational agent or device within 4 weeks of randomization 9. History or current evidence of any condition, therapy, or laboratory abnormalities which might confound trial results, interfere with the patient's participation for the full duration of the trial, or otherwise causing it to be not in the best interest of the patient to participate in the trial, in the opinion of the treating investigator. 10. Is or has an immediate family member (e.g., spouse, parent/legal guardian, sibling or child) who is directly involved with this trial, unless prospective ethics board approval (by chair or designee) is given allowing exception to this criterion for a specific patient 11. Breastfeeding is not permitted during the duration of trial participation.