Bronchiolitis Obliterans Syndrome After Allogeneic HSCT: Clinical Characteristics and Survival Outcomes in a Single Center Cohort

Background/Objectives: Chronic graft-versus-host disease (cGVHD) remains a significant complication after allogeneic stem cell transplantation. Bronchiolitis obliterans syndrome (BOS), the best characterized form of pulmonary cGVHD, represents its most severe and therapeutically challenging manifestation and is associated with poor long-term outcomes. However, data on treatment responses and survival remain limited. Methods: We performed a retrospective, single-center study to characterize the clinical features, treatment responses, and survival outcomes of patients with BOS following allo-HSCT. Results: Between January 2003 and December 2023, 28 (4.1%) of 678 transplanted patients met the National Institutes of Health (NIH) diagnostic criteria for BOS with lung scores of 2–3. The median age was 26 years (21–51), and 67.9% were male. The median time from transplantation to BOS diagnosis was 20.9 months (11.2–45.9). Median forced expiratory volume in one second (FEV1) declined from 97.5% at transplantation to 47.5% at BOS diagnosis, while median diffusing capacity for carbon monoxide (DLCO) decreased from 72% to 35% of predicted.The best observed response was partial response (PR). The overall first-line steroid-based response rate was 14.3%. In second- and subsequent-line settings, PR rates were 25% with ibrutinib, 25% with extracorporeal photopheresis, 33.3% with ruxolitinib, and 20% with mesenchymal stem cell therapy, while no responses were observed with imatinib. Median overall survival from transplantation was 90.9 months (48.4–133.5), whereas median survival from BOS diagnosis was 27.2 months (17.4–156.9). Survival after BOS diagnosis was significantly longer in patients with a prior history of acute GVHD (79.7 vs. 25.7 months; p = 0.026) and in those with concomitant extrapulmonary cGVHD compared with isolated BOS (79.7 vs. 7.6 months; p = 0.001). Conclusions: Treatment responses in stage 2–3 BOS were limited, and survival remained very poor, highlighting the importance of earlier recognition and more effective therapies before fibrotic progression.