Combining anatomical and functional information from Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET) scans brings great opportunities to improve diagnosis in oncology and treatment planning in radiation oncology. In this work, we propose a PET-guided MR tumor segmentation method minimizing a globally convex energy in a multiphase framework to account for the context variability of lesions. The method was evaluated on four patients with aggressive meningiomas of different shapes, locations and metabolism, and the Dice index obtained by comparing with a manual tumor segmentation performed by an expert was 0:65 +/- 0:13. The results demonstrated a good ability of the method to differentiate tumors from tissues with similar MR intensity values.