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BACKGROUND: Allogeneic hematopoietic cell transplantation (allo-HCT) is a standard of care for adults with high-risk acute lymphoblastic leukemia (ALL) in first or subsequent complete remission (CR). Myeloablative total body irradiation (TBI) combined with cyclophosphamide (Cy) is the most frequently used conditioning regimen. In order to reduce toxicity Cy may be replaced by fludarabine (Flu). The goal of this registry-based, retrospective study was to compare outcomes of allo-HCT following TBI/Cy vs. TBI/Flu conditioning. PATIENTS AND METHODS: Included in the analysis were 2255 patients aged 18-65 years, treated with allo-HCT from either a matched sibling (43%) or unrelated (57%) donor in CR1 (83%) or CR2 (17%), between the years 2010-2020. Patients with Ph(-) B-ALL, Ph(+) B-ALL, and T-ALL were represented in equal proportions. TBI 12Gy + Cy was used in 2105 cases while TBI 12Gy + Flu was administered to 150 patients. Patients treated with TBI/Flu were significantly older (median 35 years vs. 33 years, p=0.006), with poorer Karnofsky performance score ( RESULTS: Engraftment rate was 99% for both TBI/Cy and TBI/Flu patients. In a univariate analysis the use of TBI/Cy as compared to TBI/Flu was associated with a tendency to reduced incidence of relapse (24% vs. 29% at 2 years, p=0.1), increased incidence of grade 2-4 acute graft versus host disease (GVHD, 35.5% vs. 28%, p=0.08) and improved leukemia-free survival (LFS, 62% vs. 57%, p=0.18). The rates and causes of non-relapse mortality (NRM) did not differ significantly between the two conditioning groups. In a multivariate model adjusted for other prognostic factors, TBI/Cy conditioning was associated with reduced risk of relapse (HR=0.69, p=0.049) and increased risk of grade 2-4 acute GVHD (HR=1.57, p=0.03) without significant effect on other transplantation outcomes. An additional analysis was performed with TBI/Cy treated patients (n=132) matched strictly to those treated with TBI/Flu (n=132) in terms of disease subtype, disease status and donor type with the nearest neighbor for patient age, patient and donor sex, in vivo T-cell depletion, Karnofsky score and source of stem cells; the use of TBI/Cy as compared to TBI/Flu was associated with significantly reduced rate of relapse (18% vs. 30% at 2 years, p=0.015) and a tendency to an improved LFS (65% vs. 59%, p=0.07) and overall survival (OS, 73% vs. 68%, p=0.16) without effect on NRM and GVHD. CONCLUSIONS: The use of myeloablative TBI/CY as conditioning prior to allo-HCT for adult patients with ALL in CR1 or CR2 is associated with stronger anti-leukemic effect leading to significantly lower relapse rate compared to TBI/Flu and therefore should be likely considered a preferable regimen. Disclosures Giebel: Janssen: Honoraria, Speakers Bureau; Pfizer: Consultancy, Honoraria, Speakers Bureau; Novartis: Consultancy, Honoraria, Speakers Bureau; Amgen: Consultancy, Honoraria, Speakers Bureau. Labopin: Jazz Pharmaceuticals: Honoraria. Socie: Alexion: Research Funding. Yakoub-Agha: Jazz Pharmaceuticals: Honoraria. Kröger: Novartis: Research Funding; Riemser: Honoraria, Research Funding; Sanofi: Honoraria; Neovii: Honoraria, Research Funding; Jazz: Honoraria, Research Funding; Gilead/Kite: Honoraria; Celgene: Honoraria, Research Funding; AOP Pharma: Honoraria. Forcade: Novartis: Consultancy, Other: Travel Support, Speakers Bureau; Gilead: Other: Travel Support, Speakers Bureau; Jazz: Other: Travel Support, Speakers Bureau; MSD: Other: Travel Support. Huynh: Jazz Pharmaceuticals: Honoraria. Spyridonidis: Menarini: Current Employment. Perić: therakos: Honoraria; servier: Honoraria; MSD: Honoraria; Astellas: Honoraria; NOVARTIS: Honoraria; Abbvie: Honoraria; Pfizer: Honoraria. Mohty: Pfizer: Honoraria; Celgene: Honoraria, Research Funding; Amgen: Honoraria; Astellas: Honoraria; Gilead: Honoraria; Jazz: Honoraria, Research Funding; Janssen: Honoraria, Research Funding; Novartis: Honoraria; Bristol Myers Squibb: Honoraria; Takeda: Honoraria; Sanofi: Honoraria, Research Funding; Adaptive Biotechnologies: Honoraria.