Patients with glioblastoma are eligible for the study at the earliest six weeks after concomitant radiochemotherapy. Patients will receive Lu-177-labeled 6A10-Fabs in three fractions with an interval of four weeks between injections, which will be administered into the tumor cavity via an implanted reservoir. A patient-specific dosing strategy is used based on the individual volume of the resection cavity. A modified 3+3 design is used. The dose of Lu-177-labeled 6A10-Fab-Fragments administered into the resection cavity is escalated in three cohorts until the maximum tolerated dose is determined.