Dual 18F-fluorodeoxyglucose/68Gallium DOTATATE (FDG/DOTA) PET grading and histological grade in neuroendocrine tumours (NET)

157 Objectives [F-18]-FDG and [Ga-68]-DOTATATE PET scans have been increasingly utilised in NETs to aid diagnosis, prognostication and guide management. FDG PET avidity is associated with high-grade disease and poor prognosis [1], and DOTATATE avidity is associated with low-grade disease and improved prognosis [2]. However, there are no established methods to interpret dual FDG/DOTATATE PET and no studies investigating its correlation with histological grade. We aimed to develop a scoring schema for dual FDG/DOTATATE PET and to investigate its correlation with grade in patients with metastatic NETs. Methods We identified patients with histologically diagnosed NET, who had both whole body FDG and DOTATATE PET/CT scans within 31 days of each other. A novel scoring scheme (NETPET score) was developed. Two experienced nuclear medicine physicians scored the scans independently according to the lesions that were most FDG positive/DOTATATE negative. Paired scans were scored from P1 (DOTATATE+, FDG- disease only), P2-P4 (both DOTATATE+ and FDG+ disease where P2 is FDG uptake DOTATATE) to P5 (significant FDG+, DOTATATE- disease). The correlation between NETPET score and histological grade from initial diagnostic biopsy or surgical resection was assessed. Results 58 patients (median age 60 years, 35% female; 17 Grade 1, 26 Grade 2, 15 Grade 3) were eligible for inclusion. Distribution of NETPET score was as follows: Score P1 - 11, P2- 17, P3 - 1, P4 - 8, P5 - 20. NETPET score was strongly correlated with histological grade (Chi-squared test, p=0.0001). The predominant score in Grade 1 disease was P1 (8/11, 73%), in Grade 2 was P2-P4 (18/26, 69%) and in Grade 3 was P5 (12/20, 60%). Significant numbers of patients with Grade 2 disease (6/26, 23%) had a NETPET score of P5 indicating significant FDG+DOTATATE- disease. Conclusions NETPET score correlates strongly with histological grade, with increasing FDG positivity/DOTATATE negativity indicative of higher grade disease. In addition, significant numbers of patients with Grade 2 disease had FDG+DOTATATE- lesions identified on PET, pointing to possible areas of de-differentiation. Dual FDG/DOTATATE PET is a promising tool for “whole body molecular biopsy” of NET and should be tested prospectively.