| Summary: | How conditioning intensity is related to outcomes of AML patients undergoing allografting in morphologic remission is an area of great ongoing interest. We studied 743 patients in morphologic remission and known pre-transplant measurable residual disease (MRD) status determined by multiparameter flow cytometry (MFC) who received a first allograft after myeloablative, reduced intensity, or nonmyeloablative conditioning (MAC, RIC, and NMA). Overall, relapse-free survival (RFS) and overall survival (OS) were longer after MAC than RIC or NMA conditioning, whereas relapse risks were not different. Among MRD<sup>pos</sup> patients, 3-year estimates of relapse risks and survival were similar across conditioning intensities. In contrast, among MRD<sup>neg</sup> patients, 3-year RFS and OS were longer for MAC (69% and 71%) than RIC (47% and 55%) and NMA conditioning (47% and 52%). Three-year relapse risks were lowest after MAC (18%) and highest after NMA conditioning (30%). Our data indicate an interaction between conditioning intensity, MFC-based pre-transplant MRD status, and outcome, with benefit of intensive conditioning primarily for patients transplanted in MRD<sup>neg</sup> remission. Differing from recent findings from other studies that indicated MAC is primarily beneficial for some or all patients with MRD<sup>pos</sup> pre-HCT status, our data suggest MAC should still be considered for MRD<sup>neg</sup> AML patients if tolerated.
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