In allogeneic hematopoietic cell transplantation (allo-HCT) for adult T-cell leukemia-lymphoma (ATL), the optimal conditioning regimens have not yet been determined. We conducted a Japanese nationwide, retrospective study to investigate this issue. This study included 914 ATL patients who underwent allo-HCT between 1995 and 2015. In patients aged 55 years or younger, there was no statistically significant difference between reduced-intensity conditioning (RIC) regimens and myeloablative conditioning (MAC) regimens regarding risk of relapse (vs. RIC group: MAC group, hazard ratio (HR) 0.76, P = 0.071), non-relapse mortality (vs. RIC group: MAC group, HR 1.38, P = 0.115), or overall mortality (vs. RIC group: MAC group, HR 1.17, P = 0.255). Among RIC regimens, fludarabine plus melphalan-based (Flu/Mel) regimens were associated with a lower risk of relapse (Flu/Mel140 group, HR 0.59, P < 0.001; Flu/Mel80 group, HR 0.79, P = 0.021) than the Flu plus busulfan-based regimen (Flu/Bu2 group). Meanwhile, Flu/Mel140 group had a significantly higher risk of non-relapse mortality (vs. Flu/Bu2 group: HR 1.53, P = 0.025). In conclusion, it is acceptable to select a RIC regimen for younger patients. Moreover, it might be beneficial to select a Flu/Mel-based regimen for patients at high risk of relapse.
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