Thiotepa-Based Regimens Are Valid Alternatives to Total Body Irradiation-Based Reduced-Intensity Conditioning Regimens in Patients with Acute Lymphoblastic Leukemia: A Retrospective Study on Behalf of the Acute Leukemia Working Party of the European Society for Blood and Marrow Transplantation

Total body irradiation (TBI) at myeloablative doses is superior to chemotherapy-based regimens in young patients with acute lymphoblastic leukemia (ALL) undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT). However, in elderly and unfit patients, where reduced-intensity conditioning (RIC) regimens are preferred, whether a TBI- or chemotherapy-based approach is better, is unexplored. Thiotepa can be used as part of ALL conditioning regimens. The aim of the current study is to compare transplant outcomes after RIC with TBI- or thiotepa-based regimens in ALL. Included were patients aged ≥40 years undergoing allo-HSCT for ALL in first complete remission between 2000-2020, receiving a RIC regimen containing either TBI- (4-6 Gray, Gy) or thiotepa. We identified a total of 265 patients, including 117 receiving TBI- and 148 receiving a thiotepa-based RIC regimen. In univariate analysis, no differences were observed in transplant outcomes (for TBI versus thiotepa: relapse, 23% versus 28%, p=0.24; non-relapse mortality, 20% versus 26%, p=0.61; leukemia-free survival, 57% versus 46%, p=0.12; overall survival, 67% versus 56%, p=0.18; graft-versus-host disease [GVHD]/relapse-free survival, 45% versus 38%, p=0.21; grade II-IV acute GVHD, 30% in both groups, p=0.84; grade III-IV acute GVHD, 9% versus 10%, p=0.89) except for chronic GVHD which was higher for TBI-based regimens (43% versus 29%, p=0.03). However, on multivariate analysis no differences in transplant outcomes were observed. In patients ≥40 years receiving a RIC regimen, use of a thiotepa-based regimen may represent a valid alternative to TBI-based regimens as no differences were observed in the main transplant outcomes.