PET-CT Beyond FDG
PET-CT Beyond FDG
" Chapter 7 Somatostatin Receptor PET-CT (p. 96-97)
Somatostatin receptor PET-CT employs positron emitting tracers (68Ga-DOTA-peptides) that specifi cally bind to somatostatin receptors (ssr) expressed on tumor cell surface and has been used mainly for the assessment of neuroendocrine tumors (NETs). NETs derive from neuroendocrine cells widely dispersed in the human body and present an increased expression of ssr, particularly of ssr2. 68Ga-DOTA-peptides are very accurate for the assessment of well-diff erentiated NETs, since being slowgrowing tumors, they are not clearly visualized on 18FFDG PET-CT scans.
Several diff erent somatostatin analogues derived from octreotide, lanreotide, or vapreotide (DOTA-TOC, DOTA-TATE, DOTA-NOC) have been employed in the clinic. The most relevant diff erence among the various compounds is the variable binding affi nity to ssr- subtypes. Although DOTA-TOC, DOTA-TATE, and DOTA-NOC can bind to ssr2, the predominant receptor-type in NET, and to ssr5, only 68Ga-DOTA-NOC presents also a good affi nity for ssr3.
At present, there are no published studies directly comparing 68Ga-DOTA-peptides for the assessment of NET or papers investigating if ssr-subtypes expression diff erences on tumor cells are related to any specifi c clinical setting. However, considering the more favorable dosimetry and the wider ssr-subtypes spectrum affi nity, 68Ga-DOTA-NOC seems to be the most promising tracer for NET imaging. 100 68Ga-peptides present several technical and biological advantages for NET PET imaging, compared to other commonly used tracers (such as 18F-FDG and 18F-DOPA) and to SRS (somatostatin receptor scintigraphy).
First of all, the synthesis and labeling process is quite easy and economic. Gallium can be easily eluted from a commercially available Ge-68/Ga-68 generator, therefore not requiring an on-site cyclotron. 68Gallium ( t 1/2 = 68 min) presents an 89% positron emission and negligible gamma emission (1,077 keV) of 3.2%.
The long half life of the mother radionuclide 68Ge (270.8 days) makes it possible to use the generator for approximately 9–12 months depending upon the requirement, rendering the whole procedure relatively economic. In particular, for 68Ga-DOTA-NOC, a radio-labeling yields of 95% can usually be achieved within 15 min and 300–700 MBq of 68Ga DOTA-NOC can be obtained within 20 min. 68Ga-DOTA-peptides PET-CT technical procedure consists of the intravenous administration of approximately 100 MBq (75–250 MBq) of the radiolabeled peptide (such as 68Ga-DOTA-NOC, DOTA-TOC, etc). Image acquisition usually starts aft er an uptake time of 60 min (30–180 min).
DOTA-peptides are primarily excreted through the kidneys. 68Ga-DOTA-NOC physiologic uptake areas include the pituitary gland, the spleen, the liver, the adrenal glands, the head of the pancreas, the thyroid (very mild uptake), and the urinary tract (kidneys and urinary bladder). Some centers require somatostatin analogue treatments to be interrupted before PET. A better visualization of gastro-entero-pancreatic tumors can be achieved by oral pre-medication with gastrografi n."