There is a need for improved visualization of presence and extent of pituitary neuroendocrine tumor (PitNET) tissue during transsphenoidal surgery (TSS), especially in tumors invading the cavernous sinus (CS). Optical molecular imaging of PitNET associated biomarkers is a promising technique to accommodate this need. Vascular Endothelial Growth Factor (VEGF-A) is overexpressed in PitNET tissue compared to normal pituitary tissue and has proven to be a valid target for molecular imaging. Bevacizumab is an antibody that binds VEGF-A. By conjugating a fluorescent dye to this antibody, the fluorescent tracer molecule bevacizumab-800CW is created, which binds to VEGF-A. The investigators hypothesize that bevacizumab-800CW accumulates in PitNET tissue, enabling visualization using a molecular fluorescence endoscopy system. In this pilot intervention study the investigators will determine the feasibility of using microdoses (4.5, 10 and 25 mg) of bevacizumab-800CW to detect PitNET tissue intraoperatively.

Inclusion Criteria:

• Patients with an established diagnosis of PitNET with a Knosp grade of 3 or 4 who are scheduled to undergo TSS.
• WHO performance status 0-2
• Signed written informed consent

Exclusion Criteria:

• Medical or psychiatric conditions that compromise the patient's ability to give informed consent
• Pregnant or lactating women. Documentation of a negative pregnancy test must be available for woman of childbearing potential. Woman of childbearing potential are pre- menopausal women with intact reproductive organs and women less than two years after menopause
• History of infusion reactions to bevacizumab or other monoclonal antibody therapies.
• Inadequately controlled hypertension with or without current antihypertensive medication
• Within 6 months prior to inclusion: myocardial infarction, TIA, CVA, pulmonary embolism, uncontrolled chronic hepatic failure, unstable angina pectoris