1. Institute of Social and Preventive Medicine, University of Bern, Finkenhubelweg 11, CH-3012 Bern; Switzerland
2. Department of Oncology, University Children's Hospital of Zürich, Steinwiesstrasse 75, CH-8032 Züerich; Switzerland
3. Department of Paediatric Oncology and Haematology, University Children's Hospital Basel UKKB, Spitalstrasse 33, CH-4056 Basel; Switzerland