Imaging of neuroendocrine tumours with gamma-emitting radiopharmaceuticals

Nuclear medicine can image some tumors by means of receptor specific radiopharmaceuticals, and offers the possibility to characterize cancer through the detection of its receptor expression. This is the case of neuroendocrine tumours (NETs), that are visualized by different radiolabelled somatostatin analogues that bind 5 distinct somatostatin receptor types (named sstr1-5) that show different tissue distribution. The subtypes sstr2 and sstr5 are the most commonly expressed in NETs. Until now the most widely used radiolabelled somatostatin analogue for planar and single photon emission computed tomography (SPECT) has been [In-111]pentetreotide, because of its commercial availability. Other analogues labelled with gamma emitting radionuclides are [Tc-99m]EDDA/HYNIC-TOC, [Tc-99m]P829, [In-111]DOTA-lanreotide, [In-111]DOTA-NOC-ATE, [In-111]DOTA-BOC-ATE. However, these compounds have not been successful for the routine use. Moreover, NETs express various receptors that can be depicted by different radiopharmaceuticals, such as [I-123]VIP and [In-111]GLP-1. Besides this, some precursors of the catecholamines metabolism, as meta-iodo-benzyl-guanidine (MIBG), labelled with I-123 or I-131, accumulates in neuroendocrine tissues, in particular those of sympathoadrenal lineage. MIBG scintigraphy is currently indicated for neuroblastoma, paraganglioma and phaeocromocitoma. An impressive technological progress has been achieved recently with PET and, in particular, with the development of hybrid instrumentations (PET/CT) combining nuclear imaging with radiological imaging providing both functional and morphologic information. Among positron emitting tracers, the [F-18]FDG is the most diffuse in oncology, but other more effective tracers are available for NETs, such as the analogues labelled with Ga-68. The diagnostic sensitivity and accuracy of these technology is superior to that of gamma emitting radio-pharmaceuticals, but the fact that they are not still registered limits their use in the clinical practice. This overview summarizes the state of art of NETs imaging, focusing the attention mainly on gamma-emitting tracers.