18F-FDOPA PET/CT imaging of insulinoma revisited

18F-FDOPA PET imaging is increasingly used in the work-up of patients with neuroendocrine tumours. It has been shown to be of limited value in localizing pancreatic insulin-secreting tumours in adults with hyperinsulinaemic hypoglycaemia (HH) mainly due to 18F-FDOPA uptake by the whole pancreatic gland. The objective of this study was to review our experience with 18F-FDOPA PET/CT imaging with carbidopa (CD) premedication in patients with HH in comparison with PET/CT studies performed without CD premedication in an independent population. A retrospective study including 16 HH patients who were investigated between January 2011 and December 2013 using 18F-FDOPA PET/CT (17 examinations) in two academic endocrine tumour centres was conducted. All PET/CT examinations were performed under CD premedication (200 mg orally, 1 - 2 h prior to tracer injection). The PET/CT acquisition protocol included an early acquisition (5 min after 18F-FDOPA injection) centred over the upper abdomen and a delayed whole-body acquisition starting 20 - 30 min later. An independent series of eight consecutive patients with HH and investigated before 2011 were considered for comparison. All patients had a reference whole-body PET/CT scan performed about 1 h after 18F-FDOPA injection. In all cases, PET/CT was performed without CD premedication. In the study group, 18F-FDOPA PET/CT with CD premedication was positive in 8 out of 11 patients with histologically proven insulinoma (73 %). All 18F-FDOPA PET/CT-avid insulinomas were detected on early images and 5 of 11 (45 %) on delayed ones. The tumour/normal pancreas uptake ratio was not significantly different between early and delayed acquisitions. Considering all patients with HH, including those without imaging evidence of disease, the detection rate of the primary lesions using CD-assisted 18F-FDOPA PET/CT was 53 %, showing 9 insulinomas in 17 studies performed. In the control group (without CD premedication, eight patients), the final diagnosis was benign insulinoma in four, nesidioblastosis in one, and no definitive diagnosis in the remainder. 18F-FDOPA PET/CT failed to detect any tumour in these patients. According to our experience, CD administration before 18F-FDOPA injection leads to low residual pancreatic 18F-FDOPA activity preserving tumoral uptake with consequent insulinoma detection in more than half of adult patients with HH and more than 70 % of patients with a final diagnosis of insulinoma. If 18F-FDOPA PET/CT is indicated, we strongly recommend combining CD premedication with early acquisition centred over the pancreas. (orig.)