Carbidopa pretreatment improves image interpretation and visualisation of carcinoid tumours with 11C-5-hydroxytryptophan positron emission tomography

Purpose: Positron emission tomography ( PET) with C-11-5-hydroxytryptophan (5-HTP) as tracer is a promising imaging instrument in the management of patients with neuroendocrine tumours (NETs). However, high radioactivity concentrations in the urinary collecting system sometimes produce image reconstruction artefacts that can make detection of small NETs difficult. As a means to decrease urinary excretion of radioactivity and thereby improve image quality, we examined the effect of pretreatment with carbidopa ( CD), a peripheral inhibitor of aromatic amino acid decarboxylase (AADC), which converts 5-HTP to serotonin (5-hydroxytryptamine, 5-HT). Methods: Six patients with midgut carcinoid metastases were examined with C-11-5-HTP PET before and 1 h after oral administration of 100 or 200 mg of CD. Results: There was a fourfold significant reduction of tracer uptake in the urinary collecting system after CD administration ( p= 0.0277, n= 6), with a mean standard uptake value (SUV) of 155 +/- 195 before CD and 39 +/- 14 after CD. In tumour lesions there was a significant increase in SUV after CD administration ( p< 0. 0001, n= 18), with a mean SUV of 11 +/- 3 before CD and 14 +/- 3 after CD. There was no difference between the doses ( 100 and 200 mg) of CD in this respect. In all patients, image interpretation and tumour detection were markedly improved after CD administration. Conclusion: We conclude that CD premedication improves C-11-5-HTP PET image quality and facilitates detection of NET lesions. Because of the similarity of metabolic pathways, this method could probably be applied to improve PET imaging using other tracers like F-18-DOPA and C-11-DOPA.