Radiopeptide therapy with Y-90 or Lu-177 DOTATOC: Is treatment response predictable by pretherapeutic uptake of Ga-68 DOTATOC?

212 Objectives In this retrospective study we analyzed whether pretherapeutic Ga-68 DOTATOC-PET/CT is able to predict response to radiopeptide therapy with fixed doses of Y-90 (3700 MBq) or Lu-177 DOTATOC (7400 MBq). Methods Thirty-six patients with advanced stage neuroendocrine tumors were treated with either Y-90 (30/36) or Lu-177 DOTATOC (6/36). Prior to therapy each patient received PET/CT with 150 MBq Ga-68 DOTATOC. Treatment results were evaluated after 3 months by CT, tumor markers and clinical course and correlated with Ga-68 DOTATOC uptake (SUV) of tumor manifestations and a target dose equivalent derived from PET (therapeutic radioactivity / body weight x pretherapeutic SUV). Results According to conventional criteria (tumor shrinkage, decrease of tumor markers) 13 patients were classified as responders, 15 as non-responders, and in 8 patients follow-up findings were equivocal. Using a SUV of 20 as cut-off for favorable outcome PET was able to predict treatment response of all responders and 13/15 non responders. Two non-responders presented SUV´s >20 (22.5 and 26.7). In 5/8 patients with equivocal findings SUV was 20 (24.8; 34.9 and 35.6). Using the target dose equivalent as predictor and 2 MBq/g as cut-off 12/13 responders and 14/15 non-responders were correctly classified. Seven of 8 patients with equivocal findings had a target dose equivalent Conclusions Pretherapeutic DOTATOC tumor uptake as well as target dose equivalent were highly predictive for the results of subsequent radiopeptide therapy. This might provide the rationale for prospective studies to evaluate individual dosing and selecting patients with high likelihood of favorable treatment outcome.