Functional imaging of endocrine tumors: role of positron emission tomography

This article provides an update on functional imaging approaches for diagnostic localization of endocrine tumors, with emphasis on positron emission tomography (PET). [18F]Fluorodeoxyglucose PET scanning is now a widely accepted imaging approach in clinical oncology. Benefits include improved patient outcome facilitated by staging and monitoring of disease and better treatment planning. [18F]Fluorodeoxyglucose PET is also useful in some endocrine tumors, particularly in recurrent or metastatic thyroid cancer where the degree of accumulation of the radionuclide has prognostic value. However, this imaging approach does not take full advantage of the unique characteristics of endocrine tumors. Endocrine tumor cells take up hormone precursors, express receptors and transporters, and synthesize, store, and release hormones. These characteristics offer highly specific targets for PET. Radiopharmaceuticals developed for such approaches include 6-[18F]fluorodopamine, and [11C]hydroxyephedrine for localization of pheochromocytomas, [11C]5-hydroxytryptophan and [11C]L-dihydroxyphenylalanine for carcinoid tumors, and [11C]metomidate for adrenocortical tumors. These functional imaging approaches are not meant to supplant conventional imaging modalities but should be used conjointly to better identify specific characteristics of endocrine tumors. This represents a relatively new and evolving approach to imaging that promises to answer specific questions about the behavior and growth of endocrine tumors, their malignant potential, and responsiveness to different treatment modalities.