'Reverse discordance' between 68Ga-DOTA-NOC PET/CT and 177Lu-DOTA-TATE posttherapy scan: the plausible explanations and its implications for high-dose therapy with radiolabeled somatostatin receptor analogs.

In this technical note, an unusual discordance between diagnostic and posttherapeutic scan resulting from the use of different somatostatin receptor ligands in two settings is described. Such observation, we believe, is multifactorial, but most importantly arises due to different receptor affinity profile of the ligands and different somatostatin receptor subtype expression in different tumors. It is important for the treating physician to be aware of this phenomenon that would aid in improving our understanding of complex ligand-receptor interactions in various somatostatin receptor-positive tumors with its possible implications for therapeutic decision making with radiolabeled somatostatin receptor analogues.