To improve detection of premalignant lesions in the gastrointestinal tract (the rectum and the esophagus) there is a need for better endoscopic visualization and the ability for targeted biopsies. The University Medical Center Groningen (UMCG) developed a fluorescent tracer by labelling the VEGF-A-targeting humanized monoclonal antibody bevacizumab, currently used in anti-cancer therapy, with the fluorescent dye bevacizumab-800CW (IRDye800CW). In several phase I studies and phase II studies, either completed or currently running, in the UMCG, the use of VEGF-A-guided near-infrared (NIR) fluorescence molecular endoscopy (FME) in combination with high-definition white light endoscopy (HD-WLE) shows an improved detection rate of early premalignant lesions. In this study the safety and feasibility of a next generation imaging system will be tested. This system uses immune optical coherence tomography (immuno-OCT) and near infrared fluorescence (NIRF) with the targeted tracer (Bevacizumab-800CW) for improvement of the detection of dysplastic lesions in Barret's esophagus (BE) and colorectal polyp detection. The system provides more depth information and can eventually be used without the guidance of the regular endoscopy system.

Inclusion Criteria:

• Indication for a therapeutic endoscopy procedure (EMR or ESD);
• Age ≥ 18;
• Written informed consent.

Exclusion Criteria:

• Patients younger than 18 years old;
• Submucosal and invasive esophageal adenocarcinoma (EAC) or colorectal carcinoma (CRC);
• Radiation therapy for esophageal or colorectal cancer;
• History of infusion reactions to Bevacizumab or other monoclonal antibodies;
• Chemotherapy, immunotherapy or surgery 28 days before administration of the tracer;
• Non-adjustable hypertension;
• Medical or psychiatric conditions that compromise the patient's ability to give informed consent;
• Pregnancy or breastfeeding; a negative pregnancy test must be available for women of childbearing potential.