1. Department of Neurosurgery, Erasmus Medical Center , Rotterdam , the Netherlands
2. Department of Neurosurgery, Haaglanden Medical Center , The Hague , the Netherlands
3. Department of Neurosurgery, University Hospital Leuven , Leuven , Belgium
4. Department of Neurosurgery, University Hospital Bern , Bern , Switzerland
5. Department of Neurosurgery, University Hospital Heidelberg , Heidelberg , Germany
6. Department of Neurosurgery, Technical University Munich , Munich , Germany
7. Department of Neurosurgery, Massachusetts General Hospital , Boston, Massachusetts , USA
8. Department of Neurosurgery, University of California, San Francisco , San Francisco, California , USA