Nuclear medicine imaging of neuroendocrine tumours
Computed tomography (CT) is the basic method for morphological imaging of neuroendocrine tumours (NETs) for visualization of local tumour extent and staging of regional and distant metastases. Magnetic resonance imaging (MRI) and intravenously contrast-enhanced ultrasound (CEUS) are complementary methods but are less frequently available and are therefore predominately used when CT does not suffice. This, however, varies depending on the local situation and expertise. Endoscopic US (EUS) is superior for detection and to evaluate the local extent of gastric, duodenal and pancreatic NETs but has similarly limited availability.
Scintigraphy by 111In-labelled octreotide (OctreoScan) remains the mainstay for evaluation of the NETs somatostatin receptor status and generally facilitates tumour staging and is important to assess the patients eligibility for treatment with somatostatin analogues. Recently, various 68Ga-labelled somatostatin analogues have been tested for NET imaging by positron emission tomography (PET) in combination with CT (PET/CT). Generally, 68Ga-labelled octreotide (68Ga-DOTA-TOC, 68Ga-DOTA-NOC) and octreotate (68Ga-DOTA-TATE) are used as PET tracers. Somatostatin receptor imaging by PET has in several comparative studies performed better than scintigraphy. Centres where PET/CT with 68Ga-labelled somatostatin analogues is available are still few but the technique is fairly rapidly increasing and scintigraphy with OctreoScan has in these centres generally been abandoned. Because of existing protocols, scintigraphy with OctreoScan is still performed for patient selection when peptide receptor radio therapy (PRRT) with 177Lu-DOTA-TATE is considered.
Because of the 68Ga-labelled somatostatin analogue preparations favourable pharmacokinetics, PET imaging can be performed already 3060 min after tracer injection as compared to scintigraphy, which is generally performed at 4 and 24 h. Also the spatial resolution of PET and the image contrast is better.