Hematopoietic stem cell transplantation is an important therapeutic approach for hematologic malignancies, and the impact of donor age on transplant outcomes remains an active area of investigation. Older donors may be associated with impaired stem cell fitness, delayed immune reconstitution, and reduced T-cell function. However, randomized controlled trials directly comparing transplant outcomes by donor age are difficult to conduct because of ethical constraints, and previous retrospective studies have yielded inconsistent findings due to confounding bias and limited causal interpretability. Target trial emulation (TTE) is a methodological framework that uses observational data to emulate the design principles of a randomized trial, thereby reducing biases such as immortal time bias, time-varying confounding, and prevalent-user bias, and improving the validity of causal inference. Therefore, this study will use a large single-center retrospective clinical cohort to perform a TTE analysis, aiming to approximate the causal framework of an randomized trial and systematically evaluate the effect of donor age on clinical outcomes after allo-HSCT, thereby providing higher-quality evidence to optimize donor selection strategies.

Inclusion Criteria:

• Age 18 years or older.
• Diagnosis of hematologic malignancy.
• Underwent first allogeneic hematopoietic stem cell transplantation.
• HLA matching level of 0.5 or higher.
• Available donor age information.
• Available clinical follow-up and outcome data.

Exclusion Criteria:

• Previous autologous or allogeneic hematopoietic stem cell transplantation.
• HLA matching level below 0.5.
• Missing donor age information.
• Missing key exposure-defining or outcome-defining data required for the analysis.

This study includes adult patients with hematologic malignancies who underwent allogeneic hematopoietic stem cell transplantation at the study center. Eligible patients will be identified from retrospective clinical records and grouped according to donor age recorded at the time of transplantation.