Allogeneic haematopoietic stem cell transplantation (HSCT) is an essential treatment component for high-risk paediatric acute myeloid leukaemia (AML), but the choice of the optimal conditioning regimen remains controversial. We compared outcomes of two widely used myeloablative regimens, BuCy (Busulfan, Cyclophosphamide) versus BuCyMel (Busulfan, Cyclophosphamide, Melphalan) in children aged under 2 years undergoing HSCT in 1st complete remission (CR1). This international, registry-based study was conducted by the Paediatric Diseases Working Party (PDWP) of the European Society for Blood and Marrow Transplant (EBMT) and included patients transplanted from 2000-2022. 335 patients, transplanted across 98 centres, were included, 185 (55.2%) patients received BuCy and 150 patients (44.8%) received BuCyMel. BuCyMel was associated with significantly better leukaemia-free survival (LFS) (4 y: 74.8% vs 58.8%, HR 0.52 (95%CI: 0.33-0.82), p = 0.004) and overall survival (OS) (4 y: 77.9% vs 66.2%, HR 0.55 (95%CI: 0.33-0.91), p = 0.02) compared with BuCy, and significantly lower relapse incidence (4 y: 18.1% BuCyMel vs 31.5% BuCy, HR 0.50 (95%CI 0.29-0.87), p = 0.01). Four-year non-relapse mortality (NRM) did not differ between groups. Our results suggest that BuCyMel is a better conditioning regimen compared with BuCy in children aged under 2 years undergoing HSCT for CR1 AML.

Azienda Ospedaliera Universitaria Pisa Pisa Italy

Bone Marrow Transplantation Unit Pediatric Department of Milano Bicocca University Fondazione Monza e Brianza per il Bambino e la sua Mamma Foundation Monza Italy

Catholic University of the Sacred Heart Rome Italy

Department of Paediatric Bone Marrow Transplant and Cellular Therapy Royal Manchester Children's Hospital Manchester University NHS Foundation Trust Manchester UK

Department of Paediatric Haematology and Oncology 2nd Faculty of Medicine Charles University and Motol University Hospital Prague Czechia

Department of Pediatric Hemato Immunology Hôpital Robert Debré and Université de Paris Paris France

Department of Pediatric Hematology and BMT IHOP and Claude Bernard University Lyon France

Department of Pediatric Hematology and Oncology and Research Unit EA 3279 Timone Enfants Hospital AP HM and Aix Marseille University Marseille France

Department of Pediatric Hematology Bordeaux Hospital Bordeaux France

Department of Pediatric Hematology Oncology and Stem Cell Transplantation University of Regensburg Regensburg Germany

Department of Pediatric Hematology Oncology King Faisal Specialist Hospital and Research Center Riyadh Saudi Arabia

Department of Pediatric Oncology and Hematology Regina Margherita Children Hospital University of Turin Turin Italy

Dipartimento di Onco Ematologia e Terapia Cellulare e Genica IRCCS Ospedale Pediatrico Bambino Gesù Roma Italy

Division of Pediatric Hematology and Oncology The Edmond and Lily Safra Children's Hospital Sheba Medical Center Tel Hashomer Tel Hashomer Israel

Division of Pediatric Hematology Oncology and Stem Cell Transplantation University of Padova Padova Italy

Division of Stem Cell Transplantation and Immunology Department of Pediatrics Goethe University Frankfurt Frankfurt Germany

EBMT Paris Study Unit Paris France

EBMT Statistical Unit Paris France

Geneva University Hospital Geneva Switzerland

Great Ormond Street Hospital for Children NHS Foundation Trust London UK

Hematopoetic Stem Cell Transplant Unit Hematology Oncology IRCSS Institute G Gaslini Genova Italy

Oncoematologia Pediatrica Fondazione IRCCS Policlinico San Matteo Pavia Italy

School of Women and Children's Health University of New South Wales Sydney NSW Australia

Service d'Oncologie HÉMATOLOGIE Pédiatrique Centre Hospitalier Universitaire Nantes France

St Anna Children's Hospital Department of Pediatrics Medical University of Vienna Vienna Austria

Stem Cell Transplant Unit Aghia Sophia Children's Hospital Thivon and Papadiamantopoulou Athens Greece

Transplant and Cellular Therapies Program Kids Cancer Centre Sydney Children's Hospital Randwick NSW Australia

University Hospitals Bristol NHS Foundation Trust Bristol UK

Wroclaw Medical University Department of Pediatric Hematology Oncology and Bone Marrow Transplantation Wroclaw Poland

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