Optimizing GVHD Prophylaxis After Allogeneic Hematopoietic Cell Transplantation

Inclusion criteria: * Adults aged 60 years or older * Diagnosis of a hematological malignancy or other serious hematological disorder that requires an allogeneic hematopoietic cell transplantation * Planned to receive any reduced-intensity conditioning regimen (any graft source is acceptable) and availability of human leukocyte antigen (HLA)-matched donor at HLA loci A, B, C, and HLA-DR beta chain antigen (DRB1) * Karnofsky Performance Status (KPS) of 70% or higher. Exclusion criteria: * Previous history of one or more prior allogeneic stem cell transplants (i.e., second or third allogeneic transplant) * Planned use of high doses of cyclophosphamide (e.g., a total cyclophosphamide dose of approximately 50 mg/kg or more) as part of the conditioning regimen prior to allogeneic stem cell transplant. A lower dose of cyclophosphamide (e.g., fludarabine, cyclophosphamide, and low-dose total body irradiation regimen that uses 2 doses of cyclophosphamide at 14.5 mg/kg) is acceptable. * Known diagnosis of liver cirrhosis or other advanced liver disease that may impact cyclophosphamide metabolism. * Diagnosis of myelofibrosis * Creatinine clearance less than 40 mL/min/1.73 m², which may increase the risk of hemorrhagic cystitis with post-transplant cyclophosphamide (PTCy) * Systolic cardiac dysfunction with an ejection fraction of less than 45%. * Use of a haploidentical or mismatched donor. * Any other condition judged by the physician to increase the risk of toxicities associated with PTCy.