Somatostatin-Receptor-Scintigraphy
( sstr
- Imaging )
Atleast 5 different somatostatin-receptors ( sstr1 - sstr5 ) characterized by clearly differing binding affinities for natural and synthetic somatostatin (analogues) are known up to date. The receptors are physiologically expressed in various tissues and organs. |
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1. neuroendocrine cells | pancreas, pituitary anterior lobe, thyroid gland, most neuroendocrine tumors (GEP NET) |
2. brain, GI-tract, liver, lung, breast | brain tumors (e.g. meningeoma), intestinal tumors, liver tumors |
3. lymphocytes, leucocytes | lymphomas |
Somatostatin-receptor-scintigraphy ( Octreoscan, SRI ) |
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whole body scans using SPECT-technique by means of ~ 200 MBq of a somatostatin analogue (111-In-DTPA-octreotide, 111-Indium-DOTA, Nal3-Octreotid / DOTANOC ) for visualisation of somatostatin-receptors (sstr's) 2h, 24h and 48h after iv- injection of ~10 µg of the peptide. 1. planar scintigraphy Imaging may be performed on an outpatient basis (costs !) |
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Normal: imaging of liver, spleen, thyroid, pituitary gland kidney and bladder by glomerular filtration, intestine and gallbladder by hepatobiliary elimination |
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Pathological: most endocrine pancreatic tumors except the majority of insulinomas, carcinoid tumors and pituitary tumors, medullary thyroid carcinoma (MTC) oncological disease: SCLC, breast cancer, malignant lymphomas, brain tumors, primary liver cell carcinoma (HCC) |
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sstr-imaging represents a valuable localisation procedure for small neuroendocrine tumors - e.g.gastrinoma; carcinoids. important indication: all malignant neuroendocrine tumors with metastases, preferentially "non-functioning" GEP-NET during the course opf therapeutic strategy planning. Due to the specific receptor - expression of most insulinomas SRI does not play a significant role in the localisation of insulinoma, except specific indications in patients with the rare malignant insulinoma |
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Images |
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PET is comparable to scintigraphy using radioactive tracers in order to visualize metabolic events in certain tissues. The use of 18-FDG-PET with fluorized deoxyglucose enables visualization of unspecifically increased glucose metabolism seen also in many tumors. A more specific visualization of neuroendocrine tumors should be easily available in the near future by combination of a somatostatin analogue and PET = DOTATOC-PET. In some centers simultaneous investigation by means of a CT (computed tomography) and PET enables image fusion (PET-CT) allowing to locate positive signals of the PET in specific structures of the CT. |
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Emitters of radioactive gamma- or beta radiation - Yttrium-90, Indium-111, Gallium-68, Lutetium-177 - may be coupled to somatostatin analogues like octreotide (octreotate) via chemical conjugators (DOTA). In near future therapy by means of 177-Lutetium-DOTATOC or 177-Lutetium-DOTANOC seems to become the standard of radiopeptide-therapy due to advantages of this gamma radiation emitter in contrast to the standard 90-Yttrium-Dotatoc-therapy. Depending upon the presence of somatostatin-receptor positive tumor tissue endogenous local radiation may be applied - radiopeptide therapy. Especially metastases outside the liver - normally not accessible other than by systemic aggressive chemotherapy, may be reached with this type of therapy. The procedure in general is comparable to the known and established radio-iodine therapy of thyroidal disease. Instead of the specifically enriched iodine in the thyroid the peptide - somatostatin or its synthetic analogues - is used for which specific receptors have been detected by means of an octreotide scan or complex combined nuclear PET studies. DOTA: = Tetra-aza-cyclo-dodecane-tetraacetic acid This promising form of therapy is under clinical evaluation and being applied in a few centers in Europe and the US, but already on the verge of becoming a standard therapy for diffusely metastasized NET due ot the clear underlying rationale.. |