Comparison of the pharmacokinetics of 68Ga-DOTATOC and [18F]FDG in patients with metastatic neuroendocrine tumours scheduled for 90Y-DOTATOC therapy

Purpose The purpose of this study was to evaluate and compare, by means of dynamic PET, the pharmacokinetics of ⁶⁸Ga-DOTATOC, a tracer which reflects the expression of somatostatin receptors (SSTRs), and of [¹⁸F]FDG, a marker of tumour viability, in patients with metastatic neuroendocrine tumours (NETs) in whom ⁹⁰Y-DOTATOC therapy was planned.Materials and methods Fifteen patients (63 lesions) with confirmed metastatic NETs were enrolled in this study. Dynamic [¹⁸F]FDG and ⁶⁸Ga-DOTATOC PET scans were performed on two different days in the same week. The data analysis was based on qualitative and quantitative analysis using a two-tissue compartment model with a blood compartment and a non-compartment model based on the fractal dimension (FD). Multivariate analysis was used for evaluation of the kinetic data.Results Enhanced [¹⁸F]FDG uptake was observed in 43/63 lesions. ⁶⁸Ga-DOTATOC showed pathologically enhanced uptake in all evaluated patients and in 57/63 lesions. Discordant scintigraphic results for [¹⁸F]FDG and ⁶⁸Ga-DOTATOC were observed in 6/15 patients. Global SUV was defined as the SUV measured in the last frame (55–60 min p.i.) of the dynamic series, for each tracer. The median global SUV uptake was 7.9 for ⁶⁸Ga-DOTATOC and 4.6 for [¹⁸F]FDG. The selection of patients for ⁹⁰Y-DOTATOC therapy was based on the uptake of ⁶⁸Ga-DOTATOC. Multiple linear regression analysis was applied to determine the effect of each kinetic parameter (K ₁–k ₄, V _B) on the global SUV of both tracers. The highest positive t-ratio was found for K ₁ (receptor binding), followed by k ₃ (cellular internalisation) and V _B (fractional blood volume), when using the global ⁶⁸Ga-DOTATOC uptake (SUV) as a target variable. Analysis of the [¹⁸F]FDG data revealed the highest positive t-ratio for V _B, followed by k ₃ (phosphorylation) and K ₁ (influx). The comparison of global SUV, K ₁–k ₄ and the FD for [¹⁸F]FDG and ⁶⁸Ga-DOTATOC did not show any statistically significant correlation. The only parameter that demonstrated a significant linear correlation between the tracers was V _B.Conclusion ⁶⁸Ga-DOTATOC is a promising tool for evaluation of the expression of SSTR2 in NETs. The combination of [¹⁸F]FDG and ⁶⁸Ga-DOTATOC dynamic PET studies provides different information regarding the biological properties of lesions in patients with metastatic NETs in whom ⁹⁰Y-DOTATOC therapy is planned. While the global ⁶⁸Ga-DOTATOC uptake is influenced mostly by K ₁, the global [¹⁸F]FDG uptake is mostly influenced by V _B. Only patients with enhanced ⁶⁸Ga-DOTATOC uptake (SUV >5.0) were referred to ⁹⁰Y-DOTATOC therapy.