Within the UMCG the investigators have an active ongoing research line on molecular imaging. The investigators have developed as part of this the 89Zr-bevacizumab PET label for non-invasive measurement of VEGF levels in the tumor and its surrounding microenvironment. This tracer can give insight in the tumors' dependency on angiogenesis as the investigators have already proven for a VEGF-receptor tyrosine kinase inhibitor. Currently this tracer is used in clinical trials. The investigators would like to examine whether all neuroendocrine tumors produce VEGF and whether they differ in their response to inhibition of VEGF by mTOR.