Patients with a first recurrence of glioblastoma will be included in this study. Patients will be randomized to two arms. The treatment arm will receive stereotactically guided interstitial photodynamic therapy (iPDT), the control arm will receive a biopsy to confirm the recurrence. Both patient collectives can subsequently be treated with any therapy except antiangiogenesis inhibitors. The aim of this study is to test the efficacy of iPDT with regard to progression-free survival.