Early ${}^{18}$F-FDOPA PET/CT imaging after carbidopa premedication as a valuable diagnostic option in patients with insulinoma

Data on the diagnostic value of ${}^{18}$F-FDOPA PET/CT in patients with insulinoma are limited and are focused on small patient populations explored using different PET/CT protocols and the inconsistent use of carbidopa premedication. The aim of this study was to improve the current knowledge about the diagnostic value of ${}^{18}$F-FDOPA PET/CT combined with oral carbidopa premedication and early pancreatic imaging for tumour localization in patients with insulinoma-related hyperinsulinaemic hypoglycaemia (HH). The relationships among ${}^{18}$F-FDOPA quantitative uptake parameters, insulin secretion and tumour pathological features were also investigated. Of 34 patients with suspicion of insulinoma-related HH examined by dual time-point carbidopa-assisted ${}^{18}$F-FDOPA PET/CT, 24 with histologically proven insulinoma were retrospectively included. One patient underwent two PET/CT examinations for relapsing insulinoma after surgical excision. Thus, 25 preoperative ${}^{18}$F-FDOPA PET/CT studies were finally retained and analysed. All studies were performed under carbidopa premedication (200 mg orally, 1-2 h prior to tracer injection). The PET/CT acquisition protocol included an early acquisition (5 min after ${}^{18}$F-FDOPA injection) over the upper abdomen and a delayed whole-body acquisition starting 20-30 min later. The cytological and/or histopathological diagnosis of insulinoma was the diagnostic standard of truth. ${}^{18}$F-FDOPA PET/CT localized insulinoma in 21 of the 25 studies, leading to a primary lesion detection rate of 84%. Four lesions (19%) were detected only on early acquisitions. The false-negative tumour detection rates were, respectively, 22% and 12.5% in patients receiving and not receiving treatment for hypoglycaemic symptoms at the time of PET/CT. In benign insulinomas, the early maximum standardized uptake value (SUVmax) was significantly higher than the delayed SUVmax. Compared to the 21 benign lesions, four malignant insulinomas showed significantly higher ${}^{18}$F-FDOPA uptake. Lesion size, fasting-end insulin and C-peptide levels correlated with tumour ${}^{18}$F-FDOPA uptake, dopaminergic tumour volume and metabolic burden. The present study showed that ${}^{18}$F-FDOPA PET/CT combined with carbidopa premedication and early pancreatic acquisitions is a valuable diagnostic option in patients with insulinoma when GLP1R-based imaging is not available. The results also provide new insights into the relationships between tumour secretion and imaging phenotype in insulinomas. (orig.)