P-279 A rare subset of well-differentiated gastro-entero-pancreatic neuroendocrine tumors with positive FDG PET/CT: Is it a sign of an aggressive disease?

Abstract

Neuroendocrine tumors (NETs) are relatively rare and heterogeneous, mostly asymptomatic and discovered incidentally. PET-CT is one of the gold diagnostic standards and typically detects well-differentiated NETs using 68Gallium-Dotatate (Ga-PET/CT) and more aggressive undifferentiated NETs using 18F-FDG (FDG-PET/CT). However, some well-differentiated NETs are displaying atypical high-grade features and are being detected on FDG-PET/CT. In this study, we aim to study the diagnostic role of FDG-PET/CT in patients with well-differentiated gastro-entero-pancreatic (GEP) NETs, with positive uptake on FDG-PET/CT. A retrospective chart review of all GEP NETs cases diagnosed, from 2014 to 2021, at the American University of Beirut Medical Center was done. Patients who have low (G1) or intermediate (G2) well-differentiated NETs with positive findings on FDG-PET/CT were included based on WHO classification with Ki 67 ≤2 for low grade and >2-≤20 for intermediate grade well-differentiated NETs. Out of 36 patients with G1 or G2 GEP NET diagnosed during that period, 9 (25%) patients met the inclusion criteria for this study. Median age at diagnosis was 60 years, and 6 (66%) were males. 8 (88.9%) patients were both FDG-PET/CT and Ga-PET/CT positive, while 1 (11.1%) was FDG-PET/CT positive with Ga-PET/CT negative. 1 (11.1%) of the patients had grade 1 tumors and 8 (88.9%) had grade 2 disease. All patients had metastatic disease except for 1 patient who had locally advanced disease. The tumor originated from the small bowels in 6 (66%) of patients and from the pancreas in the remaining 3 (33%). As for the initial treatment after diagnosis, 2 patients received octreotide, 2 patients received octreotide, capecitabine and temozolomide, 2 patients received octreotide and underwent surgical resection, 1 patient received octreotide and everolimus, 1 patient received capecitabine and temozolomide and 1 patient with locally advanced disease underwent Whipple surgical procedure. 5 patients had disease progression while on initial treatment plan. In patients positive for both Ga-PET/CT and FDG-PET/CT, median progression free survival (PFS) was 40.1 months (95% CI, 15.4 – 64.9) and mean PFS was 36.2 months (95% CI, 23.1 – 49.2). The PFS in these patients is significantly lower that the PFS reported in the literature for G1/G2 NETs with positive Ga-PET/CT and negative FDG-PET/CT (36.2 vs 71 months, p = 0.0162). Patients with G1/G2 GEP NETs with positive FDG PET/CT have shorter PFS than in the literature. Should we include FDG PET/CT in the GEP NET prognostic score?.