Rectal cancer is a frequent but curable malignancy in the Western world. The golden standard in treating these patients consists of neoadjuvant chemoradiotherapy (CRT) followed by extensive surgery regardless of tumor response. The main question is whether extensive surgery can be avoided holding in mind that already a significant amount of patients reach a pathological complete response after radiochemotherapy. The goal of this study is dual. First of all, the investigators want to investigate the value of DW-MRI and 18FDG-PET in the assessment of response after neoadjuvant CRT in 100 patients with rectal cancer, to select those patients eligible for less invasive surgery. In the same patient group, the investigators will examine the biomarker potential of molecular characteristics of the tumor in blood and tissue. Using both molecular and radiological findings, the investigators want to predict pathological response after chemoradiotherapy and to select patients who may benefit from treatment adjustments during chemoradiotherapy.