Quantitative and Qualitative Intrapatient Comparison of 68Ga-DOTATOC and 68Ga-DOTATATE: Net Uptake Rate for Accurate Quantification

Quantitative imaging and dosimetry are crucial for individualized treatment during peptide receptor radionuclide therapy (PRRT). Lu-177-DOTATATE and Ga-68-DOTATOC/Ga-68-DOTATATE are used, respectively, for PRRT and PET examinations targeting somatostatin receptors (SSTRs) in patients affected by neuroendocrine tumors. The aim of the study was to quantitatively and qualitatively compare the performance of Ga-68-DOTATOC and Ga-68-DOTATATE in the context of subsequent PRRT with Lu-177-DOTATATE under standardized conditions in the same patient as well as to investigate the sufficiency of standardized uptake value (SUV) for estimation of SSTR expression. Methods: Ten patients with metastatic neuroendocrine tumors underwent one 45-min dynamic and 3 whole-body PET/CT examinations at 1, 2, and 3 h after injection with both tracers. The number of detected lesions, SUVs in lesions and normal tissue, total functional tumor volume, and SSTR volume (functional tumor volume multiplied by mean SUV) were investigated for each time point. Net uptake rate (K-i) was calculated according to the Patlak method for 3 tumors per patient. Results: There were no significant differences in lesion count, lesion SUV, K-i, functional tumor volume, or SSTR volume between Ga-68-DOTATOC and Ga-68-DOTATATE at any time point. The detection rate was similar, although with differences for single lesions in occasional patients. For healthy organs, marginally higher uptake of Ga-68-DOTATATE was observed in kidneys, bone marrow, and liver at 1 h. Ga-68-DOTATOC uptake was higher in mediastinal blood pool at the 1-h time point (P = 0.018). The tumor-to-liver ratio was marginally higher for Ga-68-DOTATOC at the 3-h time point (P = 0.037). Blood clearance was fast and similar for both tracers. SUV did not correlate with K-i linearly and achieved saturation for a K, of greater than 0.2 mL/cm(3)/min, corresponding to an SUV of more than 25. Conclusion: Ga-68-DOTATOC and Ga-68-DOTATATE are suited equally well for staging and patient selection for PRRT with Lu-177-DOTATATE. However, the slight difference in the healthy organ distribution and excretion may render Ga-68-DOTATATE preferable. SUV did not correlate linearly with K-i and thus may not reflect the SSTR density accurately at its higher values, whereas K-i might be the outcome measure of choice for quantification of SSTR density and assessment of treatment outcome.