LOCALIZATION OF NEUROENDOCRINE TUMORS AND INSULINOMAS USING RADIOLABELED SOMATOSTATIN ANALOGS, I-123 TYR(3)-OCTREOTIDE AND IN-111 PENTATREOTIDE

OBJECTIVE A number of neoplasms are known to express somatostatin receptors; the use of somatostatin receptor imaging in their localization has recently been described, but the resolution and discrimination of the isotopes used remains sub-optimal. We have looked at the use of a new In-111-labelled analogue of somatostatin, pentatreotide, in the visualization and functional characterization of a number of neoplastic conditions. PATIENTS Thirteen patients with proven neoplasms were scanned using this agent. Planar and single-photonemission computerized tomographic (SPECT) images of the relevant part of the body were obtained using a gamma-camera at 10 minutes and 4 and 21 hours after injection of the radiopharmaceutical. In six patients (three carcinoid, three insulinomas) scanning was also performed using I-123-Tyr-3-octreotide. RESULTS Primary tumours or metastases were visualized in six of the seven patients with neuroendocrine tumours, and three of six patients with insulinoma. One patient with an insulinoma who had a positive scan showed absent uptake when rescanned after tumour removal. A rise in blood glucose (more than twice basal) in response to octreotide was seen only in those insulinoma patients with positive scans. In cases where both In-111-pentatreotide and I-123-Tyr-3-octreotide scans were performed, both radiopharmaceuticals identified the same 4/6 tumours; however, tumour definition (reflecting high tumour to background ratio) was better with pentatreotide on the 21-hour images with minimum biliary and gut activity, allowing better resolution of the tumour image. CONCLUSION It appears that In-111-pentatreotide scintigraphy is a rapid and safe procedure for the visualization of neuroendocrine tumours possessing somatostatin binding sites. A positive scan may be predictive of neuroendocrine responsiveness to octreotide therapy. In addition, it also appears that In-111-pentatreotide has superior kinetics compared to I-123-Tyr-3-octreotide, typically achieving more satisfactory tumour to background ratios, and may thus be more useful in the localization of endocrine tumours.