A stereotactic targeting system is attached to the head under general anesthetic. The operation is planned following intraoperative stereotactic CT and fusion of an MRI examination carried out preoperatively. For the operation it will be necessary to shave a small area of the head, and make an approximately 3-4 cm long cut in the skin and a small opening in the cranial bone (burr hole of approx. 8 mm diameter). Several pinhead sized samples are removed using thin instruments (external diameter approx. 2 mm) from the locations calculated. The tissue that is removed is immediately examined by the pathologist. Depending on the location of the seat of disease, the duration of the stereotactic tissue biopsy (from the start to the end of the anesthetic) is around 2-3 hours. To be on the safe side, the patient will spend the first night after the tissue is removed in an intensive care unit.