68Ga-DOTA-Exendin-4 PET/CT and 68Ga-DOTA-NOC PET/CT in localization of clinically suspected insulinomas

Objectives: Head-to-head comparison of 68Ga-DOTA-Exendin-4 PET/CT and 68Ga-DOTA-NOC PET/CT in localization of clinically suspected insulinomas. Methods: Fifteen patients (median age 48years; range 19-70) including 7 men and 8 women with clinical and biochemical suspicion of insulinoma underwent both 68Ga-DOTA-Exendin-4 PET/CT and 68Ga-DOTA-NOC PET/CT. Whole body images were acquired 45-60 minutes after intravenous injection of 2-5mCi of the radiotracers and time of acquisition was 120 seconds per bed. Delayed images of the abdomen was taken at 120 minutes whenever the initial 68Ga-DOTA-Exendin-4 PET/CT scan was negative in 45-60 minute images. Two experienced Nuclear medicine physicians in consensus interpreted the images. Histopathological confirmation was obtained in all patients who had lesions localised in at least one imaging modality. Maximum standardised uptake values (SUV max) of lesion on 68Ga-DOTA-Exendin-4 PET/CT were also correlated with the Ki-67 index of the tumor. The significance of difference between the mean 'lesion SUV max/aorta SUV max' ratio was also analysed in patients who had concordant PET/CT scans. Results: Histopathological confirmation was available in 12/15(80%) patients. Two patients were diagnosed to have insulin autoimmune syndrome and one patient did not had any localizing lesion in any of the PET/CT studies or conventional imaging. The median value of lowest recorded sugar among all the patients was 37mg/dl (95% CI: 36-39.47). Median serum insulin level (of the 12 patients who underwent surgery) was 29.0uU/ml. Two patients with insulin autoimmune syndrome (patients 1 and 2) had serum insulin levels more than 1000uU/ml and median C-peptide level was 5.2(95% CI: 3.85-10.62).On histopathology and imaging 11/12(91.7%) patients had benign insulinoma and one patient had malignant insulinoma. All 12 (100%) patients had 68Ga-DOTA-Exendin-4 PET/CT however only 6/12(50%) were positive 68Ga-DOTA-NOC PET/CT scan. In 8 patients with grade 1 tumor (Ki67 index upto3%) there was a significant correlation (p=0.0054) between the uptake value (lesion to aorta SUV max ratios) to ki-67 index but in 4 patients with grade 2 tumor there was no significant correlation. In 6 patients lesions concordant in both 68Ga-DOTA-NOC and 68Ga-DOTA-Exendin-4 PET/CT scans there was a statistically significant difference in the uptake (SUV max of lesion to aorta ratio) values (p=0.0104). 68Ga-DOTANOC showed a high median uptake value of 20.18 (IQR=7.84-28.70) c