1. Department of Otorhinolaryngology/Head and Neck Surgery University of Groningen University Medical Center Groningen Groningen The Netherlands
2. Department of Otorhinolaryngology/Head and Neck Surgery Medical Centre Leeuwarden Leeuwarden The Netherlands
3. Department of Radiology University of Groningen University Medical Center Groningen Groningen The Netherlands
4. Department of Otorhinolaryngology/Head and Neck Surgery Aintree University Hospital Liverpool England
5. Department of Radiology Aintree University Hospital Liverpool England
6. Department of Epidemiology University of Groningen University Medical Center Groningen Groningen The Netherlands