Does PRRT with standard activities of 177Lu-octreotate really achieve relevant somatostatin receptor saturation in target tumor lesions?: insights from intra-therapeutic receptor imaging in patients with metastatic gastroenteropancreatic neuroendocrine tumors

Background Peptide receptor radionuclide therapy (PRRT) with 177 Lu-[DOTA 0 ,Tyr 3 ]octreotate ( 177 Lu-octreotate) is generally performed using a fixed activity of 7.4 GBq (200 mCi) per course bound to 180 to 300 μg of the peptide. While this single activity may lead to suboptimal radiation doses in neuroendocrine tumors (NET) with advanced or bulky disease, dose escalation has been withheld due to concerns on potential tumor somatostatin receptor saturation with reduced efficacy of the added activity. In vivo saturation effects during standard-dose PRRT based on quantification of pre- and intra-therapeutic 68 Ga-DOTATOC positron emission tomography (PET) imaging might guide potential dose escalation. Methods Five patients with metastatic NET of the pancreas underwent 68 Ga-DOTATOC PET/CT before and directly after standard-dose PRRT with 177 Lu-octreotate. In each patient, four target tumor lesions, normal liver parenchyma, and the spleen were evaluated and the ratios of SUV max of the target lesions to liver (SUV T/L ) and spleen (SUV T/S ) were calculated; paired Student's t test was performed with p