Optionally and when available, we will ask patients if they would like to undergo optoacoustic endoscopy. This is a form of endoscopic ultrasound which is able to detect bevacizumab-IRDye800CW up to 2 cm in depth. The procedure is comparable with NIR fluorescence endoscopy. If patients agree, after removal of the NIR fluorescence endoscope the optoacoustic endoscope will be introduced in the rectum of the patient for detection of bevacizumab-IRDye800CW in deeper areas of the tumor.