1. Department of Surgery; Medical Centre Leeuwarden; Leeuwarden The Netherlands
2. Department of Surgery; Hospital Group Twente; Almelo The Netherlands
3. Department of Epidemiology; University Medical Centre Groningen; Groningen The Netherlands
4. Department of Epidemiology; Leeuwarden Medical Center; Leeuwarden the Netherlands
5. Department of Surgery; University Medical Center Groningen; Groningen the Netherlands
6. Post Graduate School of Medicine; University Medical Center Groningen and University of Groningen; Groningen the Netherlands