Limited clinical data are available to assess whether the sequencing of cyclophosphamide (Cy) and total body irradiation (TBI) changes outcomes. We evaluated the sequence in 1769 (CyTBI N=948, TBICy N=821) recipients of related or unrelated hematopoietic cell transplantation (HCT) who received TBI (1200-1500cGY) for acute leukemia from 2003 to 2010. The two cohorts were comparable for median age, performance score, type of leukemia, first complete remission, Ph+ ALL, HLA matched siblings, stem cell source, anti-thymocyte globulin use, TBI dose, and type of graft-versus-host disease (GVHD) prophylaxis. The sequence of TBI did not significantly affect TRM (24% vs. 23% at 3y, p=0.67; relative risk [RR] 1.01, p=0.91), leukemia relapse (27% vs. 29% at 3y, p=0.34; RR 0.89, p=0.18), leukemia-free survival (49% vs. 48% at3y, p=0.27; RR 0.93, p=0.29), chronic GVHD (45% vs. 47% at 1y, p=0.39; RR 0.9, p=0.11) or overall survival (53% vs. 52% at 3y, p=0.62; RR 0.96, p=0.57) for CyTBI and TBICy respectively. Corresponding cumulative incidences of sinusoidal obstruction syndrome were 4% and 6% at 100 days (p=0.08). This study demonstrates that the sequence of Cy and TBI does not impact transplant outcomes and complications in patients with acute leukemia undergoing HCT with myeloablative conditioning.
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