The purpose of the study is to evaluate the predictive value of 18F-fluoromisonidazol-PET (FMISO-PET), 2-18F-fluoro-2-deoxy-d-glukose-PET-CT (FDG-PET-CT), Diffusion-Weighted Magnetic Resonance Imaging (DWI-MRI) and Dynamic Contrast-Enhanced Magnetic Resonance Imaging (DCE-MRI) for patients with anal cancer receiving radiation therapy +/- chemotherapy (Cisplatin and 5-FU).