Gallium-68 somatostatin receptor PET/CT: Is it time to replace ¹¹¹Indium DTPA octreotide for patients with neuroendocrine tumors?

Excerpt

In the era of theranostics which tries to integrate some form of diagnostic testing to determine the presence of molecular targets for which a specific compound is intended, molecular imaging serves these diagnostic functions and provides powerful means for non-invasively detected disease. The reason for the tremendous excitement of theranostics is its revolutionary approach that promises improved therapy selection on the bases of specific molecular features of disease, greater predictive power for adverse effects, and new ways to objectively monitor therapy response [1]. A unique feature of neuroendocrine tumors (NETs) is the expression of different receptors on the tumor cells for peptide hormones such as somatostatin receptors as well cholecystokinin (CCK) and gastrin releasing peptide (GRP) receptors [2]. These receptors can be targeted with radiolabeled peptides for imaging and treatment. Somatostatin receptors are G-protein coupled membrane glycoproteins and at the moment five subtypes of human somatostatin receptors have been cloned (sstr 1–5). The expression of somatostatin receptor type 2 is present in 70–90 % of NETs [3]. Therefore, radioactive-labeled somatostatin analogs allow the visualization and staging of these tumors. The gold standard for detection and staging of most NETs is ¹¹¹Indium-DTPA-octreotide (Octreoscan^®) (SRS) recently performed by co-registration with computerized tomography [4]. Nowadays, SPECT images are obtained using a triple-headed camera. ¹¹¹In-DTPA-octreotide can be used to visualize receptor-bearing tumors efficiently after 24 and 48 h, by which times interfering background radioactivity has been reduced by renal clearance. The sensitivity of SRS varies for different subtypes of NETs to be somewhere between 50 and 95 % [4]. There are of course limitations for lower sensitivity in some tumor types not only due to low expression of somatostatin type 2 receptors but also the size of the tumor. In clinical practice, tumors less than 1 cm in size are not detected by SRS. …