Does PRRT with standard activities of super(177)Lu-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 super(177)Lu-[DOTA super(0), Tyr super(3)]octreotate ( super(177)Lu-octreotate) is generally performed using a fixed activity of 7.4 GBq (200 mCi) per course bound to 180 to 300 mu g 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 super(68)Ga-DOTATOC positron emission tomography (PET) imaging might guide potential dose escalation. Methods: Five patients with metastatic NET of the pancreas underwent super(68)Ga-DOTATOC PET/CT before and directly after standard-dose PRRT with super(177)Lu-octreotate. In each patient, four target tumor lesions, normal liver parenchyma, and the spleen were evaluated and the ratios of SUV sub(max) of the target lesions to liver (SUV sub(T/L)) and spleen (SUV sub(T/S)) were calculated; paired Student's t test was performed with p