Imaging of Neuroendocrine Neoplasms: Monitoring Treatment Response-

Neuroendocrine neoplasms (NENs) encompass a broad spectrum of tumors throughout the body and range in biologic behavior from indolent to aggressive. Consequently, a wide spectrum of treatment options are available for NENs, including observation, somatostatin analogues, targeted therapy, chemotherapy, surgical resection, liver-directed therapy (embolization and ablation), and peptide receptor radionuclide therapy. Given the wide variety of tumor behaviors and treatments, precise criteria for treatment response in NENs are lacking. Though conventional anatomic imaging with CT and MRI remains important for NEN response assessment, the use of somatostatin receptor (SSR) PET is increasing and often provides synergistic and complementary information. Additionally, in certain clinical scenarios, a particular imaging strategy may prove superior or inferior to others for the detection of metastatic disease and evaluation of therapy response. A strong need exists to further define appropriate and standardized assessment criteria for tumor response and progression in NEN. This article presents the strengths and weaknesses of individual imaging modalities for evaluating NEN therapy response, including conventional anatomic imaging, SSR PET, FDG PET, dual-tracer PET, and PET/MRI. Ongoing challenges and unmet needs in the use of imaging for NEN response evaluation are explored.