Objective: The main objective of this study is to determine if functional MRI parameters change within 6 months, and earlier than volumetric changes after start of proton beam therapy. This would allow timely differentiation between affected and unaffected (viable) tumor components, which can be used for therapy adjustment. Secondary objectives: Determine which set of parameters (PET-CT and secondary MRI) can predict clinical outcome (tumor specific mortality, development of metastases, morbidity secondary to tumor activity and morbidity secondary to treatment); determine what type of imaging can accurately identify viable tumor nodules relative to critical anatomical structures; improving understanding of relevance of changing imaging parameters by correlating these with resected tumor. Study design: Prospective cohort study Study population: LUMC patients diagnosed with primary or recurrent chordoma or chondrosarcoma in the axial skeleton. A number of 20 new patients per year is expected. Main study parameters: Volumetric and functional MR imaging parameters including permeability parameters. Secondary parameters are generated by PET-CT (SUV, MTV and TLG), MR (perfusion, permeability and diffusion), therapy (proton beam dose mapping, surgery) and clinical outcome. End points are disease specific survival, progression free survival (including development of metastases), side effects of treatment, and functional outcome (see CRF). In patients who are treated with surgical resection following neo-adjuvant therapy, the surgical specimen will be correlated with imaging findings. Nature and extent of the burden and risks associated with participation, benefit and group relatedness: Treatment and clinical management will not be affected in this study, thus the additional burden, risks, and benefits associated with participation in this study are minimal. Two extra MRI and one PET-CT examination will be planned during proton therapy.