Morphologically 'invisible' proinsulin - secreting adenoma detected by Ga-68 Exendin-4 (GLP-1 Receptor) positron emission tomography/CT.

This case report of a young man suffering from recurring hypoglycaemia illustrates a rare condition of a neuroendocrine tumour, predominantly secreting proinsulin and invisible to conventional imaging approaches. Only a GLP-1 receptor PET/CT using Exendin-4 visualized the pancreatic lesion and enabled curative therapy, confirming the diagnostic value of this tracer for detection of neuroendocrine tumours. As only few publications on this topic are available, an overview of the available data is also given. The known cut-off value of 60% for proinsulin level indicating malignancy is critically discussed.