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 Ga-68-DOTATOC, a tracer which reflects the expression of somatostatin receptors (SSTRs), and of [F-18]FDG, a marker of tumour viability, in patients with metastatic neuroendocrine tumours (NETs) in whom Y-90-DOTATOC therapy was planned. Materials and methods: Fifteen patients (63 lesions) with confirmed metastatic NETs were enrolled in this study. Dynamic [F-18]FDG and Ga-68-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 [F-18]FDG uptake was observed in 43/63 lesions. Ga-68-DOTATOC showed pathologically enhanced uptake in all evaluated patients and in 57/63 lesions. Discordant scintigraphic results for [F-18]FDG and Ga-68-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 Ga-68-DOTATOC and 4.6 for [F-18]FDG. The selection of patients for Y-90-DOTATOC therapy was based on the uptake of Ga-68-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 Ga-68-DOTATOC uptake (SUV) as a target variable. Analysis of the [F-18]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 [F-18]FDG and Ga-68-DOTATOC did not show any statistically significant correlation. The only parameter that demonstrated a significant linear correlation between the tracers was V-B. Conclusion: Ga-68-DOTATOC is a promising tool for evaluation of the expression of SSTR2 in NETs. The combination of [F-18]FDG and Ga-68-DOTATOC dynamic PET studies provides different information regarding the biological properties of lesions in patients with metastatic NETs in whom Y-90-DOTATOC therapy is planned. While the global Ga-68-DOTATOC uptake is influenced mostly by K-1, the global [F-18]FDG uptake is mostly influenced by V (B). Only patients with enhanced Ga-68-DOTATOC uptake (SUV > 5.0) were referred to Y-90-DOTATOC therapy.