Late pulmonary complications after allogeneic hematopoietic stem cell transplantation: diagnosis, monitoring, prevention, and treatment.

Bronchiolitis obliterans syndrome (BOS) is a life-threatening complication that occurs among recipients of allogeneic lung and hematopoietic stem cell transplantation (allo-HSCT). BOS usually occurs within the first 2 years but may develop as late as 5 years after allo-HSCT. Recent prevalence estimates suggest that BOS is likely underdiagnosed in the clinical setting and that 14% of all long-term survivors with chronic graft-versus-host disease (GVHD) may develop BOS. It is difficult to diagnose and once respiratory symptoms appear, most allo-HSCT recipients show severe airflow obstruction. This may be due, at least in part, to the low sensitivity of standard spirometry in detecting small airways obstruction and lack of formal recommendations for screening for this complication. The prognosis of BOS is poor with reported 5-year survival of about 15%. A key obstacle in advancing clinical research in BOS is the lack of diagnostic and therapeutic response standards, making interpretation of survival and treatment results between studies difficult. This situation has significantly improved due to the introduction of the National Institutes of Health (NIH) criteria, which provide investigators with common definitions for studying BOS and for assessing the effects of therapeutic interventions. Future advances in the therapy of BOS may need to include development of better early intervention strategies based on identification of reliable early biological markers of the disease. It would be also important to improve understanding of the biological heterogeneity of this devastating complication after allo-HSCT.