Gliomas are the most common malignant brain tumor. Glioblastoma, WHO grade IV astrocytoma, is the most common subtype and unfortunately also the most aggressive subtype with median survival in population based cohorts being only 10 months. Extensive surgical resections followed by postoperative fractioned radiotherapy and concomitant and adjuvant temozolomide prolong survival and is the standard treatment.

The investigators think there is significant potential in individualized surgical decision-making in glioblastoma management. The idea that some patients are amendable to radical surgery, while others should be treated more conservatively, is not controversial in other fields of oncology. The current concept in all patients with glioblastoma is "maximum safe resection of the contrast enhancing tumor", but this may in selected cases be extended to simply "maximum safe resection" tailored to the patient and extent of disease at hand.

Densely proliferating tumor cells have been found from at an average of 10 mm beyond the margins of contrast enhancement in high-grade gliomas. There are now several case series, using various definitions of supramarginal resection, but they have in common that they report a benefit of resection with a margin. This potential benefit also comes together with an associated neurological risk, making this approach unethical and simply not feasible in the patients with glioblastoma as a whole.

Objective of this study is: To investigate if resection with a margin, that is significantly beyond the radiological contrast enhancement, improves survival in selected patients with glioblastoma.

Inclusion Criteria:

1. A suspected diagnosis of supratentorial glioblastoma by MRI.(A)
2. Indication for surgical treatment and where supramarginal resection is considered possible according to the preoperative imaging. This consideration needs to be verified by two specialists in neurosurgery.
3. Negative work-up for other primary tumor(B)
4. Karnofsky performance status of 70 - 100.

A) If randomized to supramarginal surgery, intraoperative frozen section must conclude with "high-grade glioma" to be able to proceed. Surgery in two sessions is also possible in supramarginal group if there is no intraoperative frozen section available or frozen section indicate another diagnosis, but final histopathology reveals a glioblastoma. In case of surgery in two session, there must be no more than 30 days between procedures. See flow-chart in attachment 1.

B) No suspected primary tumor seen on CT chest, abdomen and pelvis. If relevant symptoms/clinical suspicion also supplement with mammography, dermatologist exam, relevant endoscopies etc.

Exclusion Criteria:

1. Not willing to be randomized.
2. Informed consent not possible (e.g. language barriers, aphasia, cognitive severely impaired).
3. Contrast enhancement volume bilateral OR involving corpus callosum.
4. Contrast enhancement along the ependymal lining of ventricles (contact is however not an exclusion criteria).
5. Contrast enhancement involving several lobes.
6. History of major psychiatric disorder such as psychosis, schizophrenia and/or mood disorder (e.g. depression and bipolar disorder) in need of hospitalization
7. Unfit for participation for any other reason judged by the including physician

Participants will be masked until postoperative period. Outcome assessor will be masked until all predefined outcomes have been analysed