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AIM: Both MRI and PET have been increasingly used for monitoring treatment response and adjustment of therapeutic strategies in glioma. The purpose of the currentstudy was to elucidate a possible prognostic influence of the initial tumor volume derived both from 18FET-PET as well as MRI on outcome in de-novo glioblastoma (GBM) patients. PATIENTS AND METHODS: 71 patients with newly diagnosed GBM who received 18FET-PET and MRI prior to microsurgery/biopsy and subsequent radio/chemotherapy according to the EORTC/NCIC protocolwere analysed. After co-registration, volumetrical analysis was performed on MRI and 18FET-PET data. Biological tumor volume (BTV) was defined on 18FET-PET using brain-to tumor ratio of 1.6. For MRI,T2/FLAIR as well as Gd-enhanced T1-weighted sequences with and without necrotic parts were correspondingly obtained in each patient. Correlation of initial tumor volume on each image modality with overall survival and progression free survival was performed. Log rank test was used for statistical analysis. RESULTS: Volumetrical analyses revealed the following distribution pattern: FLAIR volume > BTV1.6 >Gd-enhanced T1 volume with necrosis >Gd-enhanced T1 volume without necrosis. Patients with smaller BTV1.6 experienced longer OS and PFS (p= 0.01/0.03), while Gd-enhanced T1-weighted MRI without necrosis was the only MRI parameter associated with an improved OS and PFS (p = 0.03/p = 0.05). CONCLUSION: Biological tumor volume as determined by 18FET-PET and Gd-enhanced T1 volume without necrosis are associated with outcome in GBM. BTV seems to surmount volumetry obtained by conventional MRI techniques in depicting vital tumor tissue. This research was in part supported by Deutsche Krebshilfe/GGN/ 70-3163-Wi 3