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 111In-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-photon-emission 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 123I-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 111In-pentratreotide and 123I-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 111In-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 111In-pentatreotide has superior kinetics compared to 123I-Tyr-3-octreotide, typically achieving more satisfactory tumour to background ratios, and may thus be more useful in the localization of endocrine tumours.