POSITRON EMISSION TOMOGRAPHY (PET) IN NEUROENDOCRINE GASTROINTESTINALTUMORS

Positron emission tomography (PET) makes it possible to study effects of medical treatment in vivo. Carcinoid tumors with liver metastases, especially those of midgut origin, produce serotonin via the precursor s tryptophan and 5-hydroxytryptophan (5-HTP) and this overproduction c ontributes to the clinical symptoms of the carcinoid syndrome. Seven p atients with histopathologically verified neuroendocrine tumors and li ver metastases, five of whom with ileal carcinoids, one a lung carcino id and one an endocrine pancreatic tumor, were included in the study. All patients had elevation of urinary 5-HIAA with the exception of one patient with a solitary liver metastasis of midgut origin. After an i ntravenous injection of C-11-5-HTP, PET was performed and the uptake o f radioactivity in tumor tissue, normal liver and plasma were compared . All patients with elevated urinary 5-HIAA and also the patient with a solitary liver metastasis and normal urinary 5-HIAA had high accumul ation and signs of a high rate of binding of 5-HTP in the liver metast ases. The uptake was relatively homogeneous in midgut carcinoid liver metastases but in large necrotic metastases the radioactivity was loca lized to the periphery. In three patients PET examination was repeated after 3 months of interferon treatment and in agreement with circulat ing tumor markers and ultrasonography the uptake of 5-HTP was unchange d. Another patient who received the somatostatin analog somatuline pro gressed on treatment and accordingly the uptake of 5-HTP also increase d. The experience with PET in neuroendocrine gastrointestinal tumors i s very limited. Our results so far indicate that 5-HTP can be used to visualize serotonin-producing neuroendocrine tumors and furthermore it might prove to be of value to monitor the effects of treatment, possi bly also as an early predictive test of the outcome of treatment.