Aim of this prospective observation study is to strengthen the hypothesis that the TP53 genotype is a promising marker to predict response to radiation therapy in rectal cancer patients. Consequently, the expected results will justify prospective, randomized intervention trials to obtain level of evidence I for the p53 marker hypothesis. Trial endpoint is downstaging and pCR rate. Tumor stage and pathological remission will be evaluated by MRT and pathohistology and correlated to the TP53 genotype of the diagnostic biopsy. Additionally, we will investigate the applicability of novel imaging modalities in magnet resonance tomography to monitor response to radiotherapy.