1. Department of Pediatric Oncology and Hematology, University of Cologne, Germany
2. Department of Pediatric Oncology and Hematology, University of Minsk, Belarus
3. Department of Pediatric Oncology and Hematology, University of Brno, Czech Republic
4. Department of Pediatric Oncology and Hematology, University of Cologne, St. Petersburg, Russia
5. Institute of Medical Statistics, Informatics, and Epidemiology, University of Cologne, Germany