68Ga-DOTATOC Versus 68Ga-DOTATATE PET/CT in Functional Imaging of Neuroendocrine Tumors

Radiolabeled somatostatin analogs represent valuable tools for both in vivo diagnosis and therapy of neuroendocrine tumors (NETs) because of the frequent tumoral overexpression of somatostatin receptors (sst). The 2 compounds most often used in functional imaging with PET are Ga-68-DOTATATE and Ga-68-DOTATOC. Both ligands share a quite similar sst binding profile. However, the in vitro affinity of Ga-68-DOTATATE in binding the sst subtype 2 (sst2) is approximately 10-fold higher than that of Ga-68-DOTATOC. This difference may affect their efficiency in the detection of NET lesions because it is the sst2 that is predominantly overexpressed in NET. We thus compared the diagnostic value of PET/CT with both radiolabeled somatostatin analogs (Ga-68-DOTATATE and Ga-68-DOTATOC) in the same NET patients. Methods: Forty patients with metastatic NETs underwent Ga-68-DOTATOC and Ga-68-DOTATATE PET/CT as part of the work-up before prospective peptide receptor radionuclide therapy. The performance of both imaging methods was analyzed and compared for the detection of individual lesions per patient and for 8 defined body regions. A region was regarded positive if at least 1 lesion was detected in that region. In addition, radiopeptide uptake in terms of the maximal standardized uptake value (SUVmax) was compared for concordant lesions and renal parenchyma. Results: Seventy-eight regions were found positive with Ga-68-DOTATATE versus 79 regions with Ga-68-DOTATOC (not significant). Overall, however, significantly fewer lesions were detected with Ga-68-DOTATATE than with Ga-68-DOTATOC (254 vs. 262, P < 0.05). Mean Ga-68-DOTATATE SUVmax across all lesions was significantly lower than Ga-68-DOTATOC (16.0 +/- 10.8 vs. 20.4 +/- 14.7, P < 0.01). Mean SUVmax for renal parenchyma was not significantly different between Ga-68-DOTATATE and Ga-68-DOTATOC (12.7 +/- 3.0 vs. 13.2 +/- 3.3). Conclusion: Ga-68-DOTATOC and Ga-68-DOTATATE possess a comparable diagnostic accuracy for the detection of NET lesions, with Ga-68-DOTATOC having a potential advantage. The approximately 10-fold higher affinity for the sst2 of Ga-68-DOTATATE does not prove to be clinically relevant. Quite unexpectedly, SUVmax of Ga-68-DOTATOC scans tended to be higher than their Ga-68-DOTATATE counterparts.