1. Department of Pediatrics, Oulu University Hospital, Oulu, Finland.
2. Department of Pediatrics, Aarhus University Hospital, Aarhus, Denmark.
3. Department of Surgery, Oulu University Hospital, Oulu, Finland.
4. Danish Cancer Society, Institute of Cancer Epidemiology, Copenhagen, Denmark.
5. Finnish Cancer Registry, Institute for Statistical and Epidemiological Cancer Research, Helsinki, Finland.
6. Division of Hematology-Oncology and Stem Cell Transplantation, Department of Pediatrics, Helsinki University Central Hospital, Helsinki, Finland.