Tumor-to-blood ratio for assessment of somatostatin receptor density in neuroendocrine tumors using ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE

Abstract

PET/CT with ⁶⁸Ga-DOTA-somatostatin analogs has been tested for therapy monitoring in patients with neuroendocrine tumors (NETs). However, standardized uptake values (SUV) in tumors do not correlate with the net influx rate (Ki), as a representation of the somatostatin receptor (SSTR) expression. In this study, tumor-to-blood-ratio (TBR) was evaluated as an alternative tool for semi-quantitative assessment of ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE tumor uptake and as a therapy monitoring tool for patients with NETs. Methods: Twenty-two NET patients underwent a 45-min dynamic PET/CT scan after injection of ⁶⁸Ga-DOTATOC and/or ⁶⁸Ga-DOTATATE. Ki was determined using the Patlak method and TBR was calculated for the 40-45 min time interval. Results: A linear relation was found between Ki and TBR, with a square of Pearson correlation (R2) of 0.98 and 0.93 for ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE, respectively. Conclusion: High correlation was found between Ki and TBR. Hence, TBR reflects SSTR density more accurately than SUV and is suggested as the preferred metrics for semi-quantitative assessment of ⁶⁸Ga-DOTATOC and ⁶⁸Ga-DOTATATE tumor uptake.