Improved Outcomes of Haploidentical Hematopoietic Cell Transplantation with Total Body Irradiation-Based Myeloablative Conditioning in Acute Lymphoblastic Leukemia

The optimal myeloablative conditioning (MAC) for patients undergoing haploidentical hematopoietic cell transplantation (haplo-HCT) is unknown. We studied the outcomes of total body irradiation (TBI)-based versus chemotherapy (CT)-based MAC regimens in patients with acute lymphoblastic leukemia (ALL). The study included 427 patients who underwent first haplo-HCT with post-transplantation cyclophosphamide (PTCy), following TBI-based (n = 188; 44%) or CT-based (n = 239; 56%) MAC. The median patient age was 32 years. Fludarabine-TBI (72%) and thiotepa-busulfan-fludarabine (65%) were the most frequently used TBI- and CT-based regimens, respectively. In the TBI and CT cohorts, 2-year leukemia-free survival (LFS) was 45% versus 37% (P = .05), overall survival (OS) was 51% versus 47% (P = .18), relapse incidence (RI) was 34% versus 32% (P = .44), and nonrelapse mortality (NRM) was 21% versus 31% (P < .01). In the multivariate analysis, TBI was associated with lower NRM (hazard ratio [HR], 0.53; 95% confidence interval [CI], 0.33 to 0.86; P = .01), better LFS (HR, 0.71; 95% CI, 0.52 to 0.98; P =.04), and increased risk for grade II-IV acute graft-versus-host disease (GVHD) (HR, 1.59; 95% CI, 1.08 to 2.34; P = .02) compared with CT-based MAC. The type of conditioning regimen did not impact RI, chronic GVHD, OS, or GVHD-free, relapse-free survival after adjusting for transplantation-related variables. TBI-based MAC was associated with lower NRM and better LFS compared with CT-based MAC in patients with ALL after haplo-HCT/PTCy.