Imaging of Neuroendocrine Tumors.

Neuroendocrine tumors (NETs) comprise a heterogeneous group of malignancies with a very variable clinical expression and progression. They present unique properties that are important to consider for radiological and nuclear imaging, such as APUD-characteristics (amine precursor uptake and dearboxylation), as well as the expression of somatostatin receptors. The most common localizations are the lungs, gastrointestinal tract and pancreas. The only curative treatment is surgery, but more than 50% present metastatic disease at the time of diagnosis. The systemic treatment includes chemotherapy and targeted agents, as well as peptide receptor radiotherapy. The diagnosis and follow-up of these tumors necessitate a large number of different imaging methods, such as CT, MRI, US, SRS and PET. Ultrasonography offers the possibility to take guided biopsies from different lesions. Somatostatin receptor scintigraphy was developed in the 1990s and nowadays presents the standard of care for NETs in most countries. The procedure offers a total body examination and a better staging of the disease. However, it has been replaced in most centers by PET/CT with 68Ga-DOTA-somatostatin analogues with a superior spatial resolution and faster imaging (one-stop procedure). Another tracer used for PET/CT is 18FDG, particularly for high-grade tumors. Other more specific tracers are 18F-L-DOPA, 11C-L-DOPA and 11C-5-hydroxytryptophan, which have demonstrated excellent imaging results. The new targeted agents present a challenge in the evaluation procedure of treatment and, therefore, new imaging techniques and an improvement of currently available techniques are mandatory.