Initial direct comparison of 99mTc-TOC and 99mTc-TATE in identifying sites of disease in patients with proven GEP NETs

The imaging of neuroendocrine tumors has become one of the most significant areas in nuclear oncology. In an attempt to provide high-quality imaging and possible sensitivity at a reduced cost, time, and radiation dose, several Tc-99m agents have been proposed. The aim of this initial study was to compare the tumor uptake and biodistribution of 2 new 6-hydrazinopyridine-3-carboxylic acid (HYNIC)-derivatized Tyr(3)-octreoticle analogs, Tc-99m-[HYNIC,Tyr(3)]octreotide (Tc-99m-TOC) and Tc-99m-[HYNIC, Tyr(3),Thr(8)]octreotide (Tc-99m-TATE), in patients with somatostatin receptor-expressing tumors. Methods: Each of 12 patients with proven gastrointestinal pancreatic neuroendocrine tumors received a mean activity of 520 MBq of Tc-99m-TOC and Tc-99m-TATE. Scintigraphy with both tracers was performed 3-4 h after their injection using standard whole-body and SPECT imaging. The images were reviewed subjectively by 2 readers, who reported tumor uptake lesion by lesion. Results: Both radio-tracers demonstrated concordance between the results in 7 patients (58%). In total, 110 sites of disease were identified with Tc-99m-TOC, compared with 115 with Tc-99m-TATE. There was 1 case in which Tc-99m-TOC identified sites of disease not seen on 99mTc-TATE imaging but 4 cases in which some sites of disease were seen with Tc-99m-TATE and not Tc-99m-TOC. Conclusion: In this initial study, both tracers seem to show similar sites of tumor, with Tc-99m-TATE having a slight edge in the total number of lesions seen, especially in lymph node metastases.