Introduction
Bronchiolitis obliterans syndrome (BOS) is a life-threatening complication of allogeneic hematopoietic cell transplant (HSCT)1. It is characterized by an irreversible small airway obstruction caused by epithelial injury, subepithelial inflammation, and fibrosis of small airways 2. The prevalence of BOS was reported 2%-6.5% in allogeneic HSCT recipients in adult cohort3-5, and 2.0-2.7% in pediatric cohorts6,7. The prognosis of children with BOS after HSCT is poor; the five-year survival rate is 45-59% 8,9. Since the number of patients receiving allogeneic HSCT is increasing, BOS in children will likely increase in the future.
A BOS diagnosis is based on declining FEV1, which is measured serially after HSCT 10, and confirmed with pathology of constrictive bronchiolitis or high-resolution computed tomography (CT) showing air trapping, small airway thickening, or bronchiectasis 11. After BOS diagnosis, pulmonary functions are measured regularly to monitor treatment efficacy and prognosis 12. Systemic steroids have been the backbone of BOS therapy 13-17 and inhaled fluticasone, azithromycin, and montelukast (FAM) have been recently introduced18. However, the prognosis of BOS is still poor.
A study enlisting subjects with BOS in a multicenter prospective trial and a retrospective cohort concluded that FEV1 declined rapidly in the 6 months prior to BOS diagnosis 19. The current study is one of few studies that describe the longitudinal change of lung function after BOS diagnosis in children and clarify the clinical implications of lung function changes on BOS prognosis.