Optimal conditioning intensity: favorable outcome of a 3-day busulfan based regimen in younger acute myeloid leukemia patients

Myeloablative conditioning is standard for younger acute myeloid leukemia (AML) patients undergoing allogeneic hematopoietic cell transplantation (HCT), but optimal busulfan dosing remains unclear. We retrospectively analyzed 686 AML patients aged <55 years who received busulfan-based conditioning (2, 3, or 4 days) for first HCT at Asan Medical Center between 2000 and 2020. A 3-day course of busulfan (Bu3) showed significantly better overall survival (OS, p < 0.001) and lower non-relapse mortality (p = 0.005) compared to Bu2 and Bu4. Although Bu4 had the lowest relapse rate (p < 0.001), event-free survival was similar to Bu3. In multivariate analysis, only Bu3 maintained an OS benefit (hazard ratio 0.513, p = 0.016). Graft-versus-host disease-free, relapse-free survival did not differ among groups. Among patients in complete remission, Bu3 also showed a significant OS advantage (p = 0.040), while higher busulfan doses did not improve survival in refractory cases. These findings support Bu3 as the optimal conditioning regimen in younger AML patients undergoing HCT.