Letermovir Prophylaxis in Children With EBV-Positive T/NK-Cell Lymphoproliferative Disease and Refractory/Relapsed EBV-Associated Hemophagocytic Lymphohistiocytosis

Inclusion Criteria: * Diagnosed with EBV-positive T/NK lymphoproliferative disease (EBV-T/NK LPD) according to ICC 2022 criteria, or diagnosed with refractory/relapsed EBV-associated hemophagocytic lymphohistiocytosis (EBV-HLH) according to the 2004-HLH diagnostic criteria; * Undergoing first allogeneic hematopoietic stem cell transplantation (allo-HSCT) at the study center; * Age \< 18 years; * CMV seropositive (IgG+) prior to transplantation; * Presence of at least one high-risk factor for CMV infection: haploidentical transplantation, HLA-mismatched transplantation, receipt of ATG (including ATLG/ALG) in conditioning, sustained corticosteroid use post-conditioning, donor/recipient CMV serostatus mismatch, or positive NGS result pre-transplant. Exclusion Criteria: * History of CMV end-organ disease within 6 months prior to enrollment; * Severe hepatic dysfunction (defined as Child-Pugh Class C); * End-stage renal impairment with creatinine clearance \< 10 mL/min (calculated by Cockcroft-Gault equation); * Prior allogeneic hematopoietic stem cell transplantation; * Expected survival ≤ 3 months; * Received radiation therapy during conditioning; * Initiation of letermovir prophylaxis after day 28 post-transplant; * Letermovir dosage or administration not in accordance with the prescribing information; * Lack of signed informed consent.