Visualization of Rectal Cancer During Endoscopy, Using a Fluorescent Tracer

In this non-randomized, non-blinded, prospective, single center feasibility study, patients with locally advanced rectal cancer who are included in the RAPIDO study (NL36315.042.11) will undergo two times epi-illumination endoscopy (in other words flexible NIR fluorescence endoscopy). The study consists of a total of five study procedure related visits: * Visit 1: During a screening visit, eligibility will be evaluated and patient characteristics will be collected. * Visit 2: During the second visit 4.5 mg of bevacizumab-IRDye800CW will be administered intravenously. The patient will then be observed for 1 hour post administration. * Visit 3: First endoscopy will be performed at baseline (two days after tracer administration); before the start of chemoradiotherapy. * Visit 4: After chemoradiotherapy patients will receive a second dose of 4.5 mg of bevacizumab-IRDye800CW (second tracer administration) * Visit 5: A second flexible NIR fluorescence endoscopy procedure will be performed (two-three days after the second tracer injection), preferably right before surgery. 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.