In this comparative study, the investigators aim to compare prospective pediatric patients who receive total lymphoid irradiation (TLI) using tomotherapy with age- and disease-matched controls who receive conventional total body irradiation (TBI) as part of conditioning for haploidentical hematopoietic stem cell transplantation (HSCT) for both malignant and non-malignant diseases. The investigators shall evaluate graft failure-free, graft-versus-host disease (GVHD)-free survival, overall survival, frequency of rejection, GVHD, relapse of malignancy, adverse effects and post-transplant immunoreconstitution.

Radiotherapy at a dose of 6Gy divided into 3 equal fractions (2Gy per fraction) to all lymphoid organs (TLI) over 2 days using tomotherapy will be given to participants as part of the conditioning treatment before haploidentical hematopoietic stem cell transplantation (HSCT). Retrospective patients who are age- and disease-matched with the recruited patients and had received radiotherapy to the whole body (TBI) with or without lung shielding at a total dose of 2-12 Gy delivered as 1-6 equal fractions by conventional opposed fields radiotherapy as part of conditioning will be compared for the outcomes.

Inclusion Criteria:

• Aged 0-18 years who requires allogeneic HSCT, has no human leukocyte antigen (HLA)-matched sibling donor but has an HLA-haploidentical donor.
• Adequate organ function to tolerate the conditioning chemotherapy and radiotherapy
• Karnofsky or Lansky performance status score ≥50

Exclusion Criteria:

• Pregnant or lactating woman
• HIV infection
• Patients for whom alternative treatment is deemed more appropriate by treating physician
• Patients who are unlikely to benefit from haploidentical hematopoietic stem cell transplantation, e.g., terminal malignancy with multiorgan failure