Nuclear medicine

Nuclear medicine of neuroendocrine tumors (NETs) includes diagnosis with somatostatin receptor scintigraphy, PET-scanning and MIBG and treatment, so called peptide-radio-receptor treatment (PRRT). NETs are a heterogenous group of neoplasms, comprising at one end of the spectrum well-differentiated and slowly growing tumors, which are the most common, and at the other and less frequent poorly differentiated and malignant neoplasms with aggressive behaviour. The tumors tend to express hormonal activity in accordance with that of the endocrine cell from which they originate and thus the clinical presentation may be diverse. A unique feature of NETs is the expression of peptide hormone receptors, particular somatostatin receptors, the basis for somatostatin receptor scintigraphy. Another specific feature of NETs is the so called APUD- characteristics, which means that they can take up and accumulate amines and precursors for production for various peptides and hormones. This is one of the bases for PET-scanning in these tumors.

Somatostatin receptor scintigraphy (SRS)
SRS is a well-established functional technique for the imaging or NETs. Receptors for somatostatin and analogs have been identified on many cells of neuroendocrine origin. Of the five different subtypes of somatostatin receptors known to date, subtypes 1 and 2 appear to be the most frequent. The latter is the main target for visualization of NETs by SRS. The most commonly used somatostatin analog is octreotide, which is labelled with 111Indium, using the chelator diethylene-triamine-pentaaceticacid (DTPA) to produce 111Indium-DTPA-octreotide which is available as a commercial product - Octreoscan®. Other tracers are 111Indium-DOTA-TOC, 111Indium-DOTA-NOC and 111Indium-DOTA-TATE.



SRS generally employs both planar imaging, with either whole-body scanning or multiple static acquisitions and single-photon emission computed tomography (SPECT). SRS with 111Indium-DPTA-octreotide (Octreoscan®) has shown high accuracy for visualization of NETs. The sensitivity ranges between 80-95% depending on tumor type, with the highest figures for classical midgut carcinoids, and the lowest figure for benign insulin producing tumors (Insulinoma). SRS is considered to be standard of care for patients with NETs and should be performed in all patients with this type of tumor at least once. In the future it may be replaced by 68Ga-DOTA-octreotate PET-scanning, which demonstrates higher sensitivity with the possibility to calculate the number of somatostatin receptors in-vivo in the patient. It is also a “one-stop” procedure since the patient can leave the hospital after 2 hours, instead of coming back for a 24-hour scan as with SRS.

Metaiodobenzylguanidine (MIBG)
MIBG, a catecholamine analog, labelled with 123I or 131I, is a well-established tracer for scintigraphic visualization and treatment of tumors originating from the neural crest. That includes mainly phaeochromocytomas, paragangliomas, but also NETs, such as carcinoids. The sensitivity is lower than for SRS for GEP-NETs with a sensitivity of somewhere between 40-50%. In comparative studies the sensitivity of SRS to detect NETs exceeded that of MIBG scintigraphy.