Tumor-to-Blood Ratio for Assessment of Somatostatin Receptor Density in Neuroendocrine Tumors Using 68Ga-DOTATOC and 68Ga-DOTATATE

PET/CT with Ga-68-DOTA-somatostatin analogs has been tested for therapy monitoring in patients with neuroendocrine tumors (NETs). However, SUVs in tumors do not correlate with the net influx rate (K-i), as a representation of the somatostatin receptor expression. In this study, tumor-to-blood ratio (TBR) was evaluated as an alternative tool for semiquantitative assessment of Ga-68-DOTATOC and Ga-68-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 Ga-68-DOTATOC or Ga-68-DOTATATE. K-i was determined using the Patlak method, and TBR was calculated for the 40- to 45-min interval. Results: A linear relation was found between K-i and TBR, with a square of Pearson correlation of 0.98 and 0.93 for Ga-68-DOTATOC and Ga-68-DOTATATE, respectively. Conclusion: A high correlation was found between K-i and TBR. Hence, TBR reflects somatostatin receptor density more accurately than SUV and is suggested as the preferred metric for semiquantitative assessment of Ga-68-DOTATOC and Ga-68-DOTATATE tumor uptake.