Inclusion Criteria

1. Age ≥18 and ≤ 80 years
2. Karnofsky Performance Score (KPS) ≥ 60%
3. Supratentorial T1-Gd enhancing lesion(s) amenable to total resection
4. Legal capacity and ability of subject to understand character and individual consequences of the clinical trial
5. Patient's written IC obtained at least 24h prior to surgery
6. For women with childbearing potential: adequate contraception
7. Patients must have adequate organ functions

Bone marrow function:
* Platelets ≥ 75.000/μL
* WBC ≥ 3.000/μL
* Hemoglobin ≥ 10.0 g/dL

Liver Function:
* ASAT and ALAT ≤ 3.0 times ULN
* ALP ≤ 2.5 times ULN
* Total Serum Bilirubin \< 1.5 times ULN

Renal Function:
* Serum Creatinine ≤ 1.5 times ULN

Inclusion Criteria Related to Surgery:
8. IORT must be technically feasible
9. Histology supports diagnosis of GBM

Exclusion Criteria

1. Multicentric disease (e.g. in both hemispheres) or non-resectable satellite lesions
2. Previous cranial radiation therapy
3. Cytostatic therapy / chemotherapy for cancer within the past 5 years
4. History of cancers or other comorbidities that limit life expectancy to less than five years
5. Previous therapy with anti-angiogenic substances (such as bevacizumab)
6. Technical impossibility to use MRI or known allergies against MRI and/or CT contrast agents
7. Participation in other clinical trials testing cancer-derived investigational agents/procedures.
8. Pregnant or breast feeding patients
9. Fertile patients refusing to use safe contraceptive methods during the study

Exclusion Criteria Related to Surgery:
10. Active egress of fluids from a ventricular defect
11. In-field risk organs and/or IORT dose \>8 Gy to any risk organ