The role of octreoscan in thyroid eye disease

Until recently there was no imaging technique available which could demonstrate pathological changes in orbital tissues and could be regarded as a reliable measure of inflammation in thyroid eye disease (TED). Pentetreotide (a synthetic derivative of somatostatin) labelled with 111In has been used to localize tumours which possess surface or membrane receptors for somatostatin in vivo using a gamma camera (1). This technique visualizes somatostatin receptors in endocrine-related tumours in vivo and predicts the inhibitory effect of the somatostatin analogue octreotide on hormone secretion by the tumours (1). By applying 111In-DTPA-d-Phe octreotide scintigraphy (octreoscan), accumulation of the radionuclide was also detected in both the thyroid and orbit of patients with Graves' disease (2-4). If peak activity in the orbit 5h after injection of radiolabelled octreotide is set at 100%, a decrease to 40+/-4% is found at 24h, significantly different from the decrease in blood pool radioactivity, which is 15+/-4% at 24h. Accumulation of the radionuclide is most probably due to the presence in the orbital tissue of activated lymphocytes bearing somatostatin receptors (5). Alternative explanations are binding to receptors on other cell types (e.g. myoblasts, fibroblasts or endothelial cells) or local blood pooling due to venous stasis by the autoimmune orbital inflammation.