Background Pulmonary complications are a major cause of morbidity after allogeneic hematopoietic stem cell transplantation (HSCT). Late-onset non-infectious pulmonary complications (LONIPCs), especially bronchiolitis obliterans syndrome (BOS), are difficult to diagnose, particularly in paediatric patients.

Methods In this retrospective single-center study, 14 of 325 paediatric HSCT recipients (4.3%) who developed severe pulmonary symptoms between 1999 and 2016 were analyzed. Lung biopsies were correlated with high-resolution computed tomography (HRCT) and pulmonary function tests (PFTs). Fourteen postmortem biopsies from HSCT patients without pulmonary symptoms served as controls.

Results Histology showed BOS in eight patients, cryptogenic organizing pneumonia (COP) in three, and interstitial fibrosis in three. None of the controls had findings suggestive of LONIPCs. All patients with BOS exhibited obstructive spirometry results, while restrictive changes occurred in COP and fibrosis. HRCT findings, including bronchial wall thickening and dilation, were frequent but non-specific. The incidence of LONIPCs and BOS was 4% and 2%, respectively.

Conclusions BOS was the most common late-onset pulmonary complication after paediatric HSCT. Obstructive PFT changes correlated well with histological BOS, whereas HRCT findings lacked specificity. Regular pulmonary function monitoring appears more reliable than imaging for early detection and may help prevent progression to irreversible lung disease.

Impact
This study highlights the diagnostic value of combining functional and histological assessment in children after HSCT.

Underscores the limitations of HRCT in detecting early BOS.

Supports routine pulmonary function surveillance as a non-invasive strategy to improve long-term outcomes.