Myeloproliferative neoplasms (MPNs) are uncommon in children/young adults. Here, we present data on unselected patients diagnosed before 25 years of age included from 38 centers in 15 countries. Sequential patients were included. We identified 444 patients, with median follow-up 9.7 years (0-47.8). Forty-nine (11.1%) had a history of thrombosis at diagnosis, 49 new thrombotic events were recorded (1.16% patient per year [pt/y]), perihepatic vein thromboses were most frequent (47.6% venous events), and logistic regression identified JAK2V617F mutation (P = .016) and hyperviscosity symptoms (visual disturbances, dizziness, vertigo, headache) as risk factors (P = .040). New hemorrhagic events occurred in 44 patients (9.9%, 1.04% pt/y). Disease transformation occurred in 48 patients (10.9%, 1.13% pt/y), usually to myelofibrosis (7.5%) with splenomegaly as a novel risk factor for transformation in essential thrombocythemia (ET) (P= .000) in logistical regression. Eight deaths (1.8%) were recorded, 3 after allogeneic stem cell transplantation. Concerning conventional risk scores: International Prognostic Score for Essential Thrombocythemia-Thrombosis and new International Prognostic Score for Essential Thrombocythemia-Thrombosis differentiated ET patients in terms of thrombotic risk. Both scores identified high-risk patients with the same median thrombosis-free survival of 28.5 years. No contemporary scores were able to predict survival for young ET or polycythemia vera patients. Our data represents the largest real-world study of MPN patients age < 25 years at diagnosis. Rates of thrombotic events and transformation were higher than expected compared with the previous literature. Our study provides new and reliable information as a basis for prospective studies, trials, and development of harmonized international guidelines for the specific management of young patients with MPN.

Biology of Cardiovascular Diseases University of Bordeaux INSERM UMR1034 Pessac France

Department of Biomedicine Experimental Hematology University Hospital and University Basel Switzerland

Department of Cellular Biology INSERM UMRS 1131 St Louis Hospital AP HP Paris France; and

Department of Hematology Amsterdam University Medical Centers Universiteit Amsterdam Cancer Center Amsterdam Netherlands

Department of Hematology and Bone Marrow Transplantation University of Medical Sciences Poznan Poland

Department of Hematology and Transplantation Medical University and Clinical Center Gdansk Poland

Department of Hematology Annecy Genevois Hospital Centre Pringy France

Department of Hematology Blood Neoplasms and Bone Marrow Transplantation Wroclaw Medical University Wroclaw Poland

Department of Hematology Collegium Medicum Jagiellonian University Krakow Poland

Department of Hematology Collegium Medicum Nicolaus Copernicus University of Torun Bydgoszcz Poland

Department of Hematology Guy's and St Thomas' National Health Service Foundation Trust London United Kingdom

Department of Hematology Holy Cross Oncology Center Kielce Poland

Department of Hematology Hospital del Mar Barcelona Spain

Department of Hematology Institute of Oncology and Hematology University Hospital Brest France

Department of Hematology Iuliu Hatieganu University of Medicine and Pharmacy Cluj Napoca Romania

Department of Hematology Jagiellonian University Hospital Krakow Poland

Department of Hematology Le Mans Hospital Le Mans France

Department of Hematology Medical University Lodz Poland

Department of Hematology Norfolk and Norwich University Hospitals National Health Service Trust Norwich United Kingdom

Department of Hematology Paul Brousse Hospital Paris France

Department of Hematology St Vincent De Paul Hospital Lille France

Department of Hematology State Hospital Opole Poland

Department of Hematology University Hospital Dubrava School of Medicine University of Zagreb Zagreb Croatia

Department of Hematology University Hospital Juan Canalejo Complejo Hospitalario Universitario a Coruña La Coruña Spain

Department of Hematology University Hospital Linkoping Sweden

Department of Hematology University Hospital Santiago de Compostela Spain

Department of Hematology University Hospitals Leuven and Department of Microbiology and Immunology Laboratory of Molecular Immunology Katholieke Universiteit Leuven Leuven Belgium

Department of Hematology University Medical Center Amsterdam Netherlands

Department of Internal Medicine Sumy State University Medical Institute Respiratory Medicine Center Sumy Ukraine

Division of Hematology Department of Oncology Geneva University Hospitals Geneva Switzerland

Division of Hematology Oncology Chang Gung Memorial Hospital Taipei Taiwan

Hematology Belfast City Hospital Queen's University Belfast Belfast United Kingdom

Institute of Hematology and Blood Transfusion Prague Czech Republic

Institute of Hematology and Transfusion Medicine Warsaw Poland

Laboratory of Biological Hematology University Hospital Dijon France

The Confluent Private Hospital Nantes France

Université de Paris Assistance Publique des Hôpitaux de Paris Hôpital Saint Louis Centre d'Investigations Cliniques 1427 INSERM Paris France

University Clinic for Hematology Oncology Hemostasis and Palliative Care Johannes Wesling Medical Center University of Bochum Minden Germany

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