SOMATOSTATIN RECEPTOR IMAGING OF ENDOCRINE GASTROINTESTINAL TUMORS

Somatostatin receptors are present on various tumors of neuroendocrine origin. We recently developed a technique for the in vivo visualization of somatostatin receptor positive tumors, which offers a powerful alternative to tumor imaging with labeled monoclonal antibodies. Instead of injecting radiolabeled antibodies against the somatostatin receptor, we labeled a somatostatin analogue ([Tyr3]-octreotide) which is known to bind specifically to the somatostatin receptor, and injected this labeled hormone analogue in order to visualize somatostatin receptor positive tumors. We previously reported the successful visualization of the primary tumors or metastases of various endocrine gastrointestinal tumors after injection of the iodinated somatostatin analogue [I-123-Tyr3]-octreotide. The primary tumors or metastases of 12 out of 13 carcinoids, 3 out of 3 gastrinomas, 2 out of 4 insulinomas, and 1 out of 1 somatostatinoma were visualized. Using In-111-coupled octreotide, we were able to visualize 19 out of 19 carcinoids, 7 out of 7 gastrinomas, 4 out of 7 insulinomas. 1 out of 1 glucagonoma, and 3 out of 3 non-functioning endocrine pancreatic tumors, but none of 18 exocrine pancreatic tumors. In a large proportion of patients with endocrine gastrointestinal tumors, previously unrecognized metastases were demonstrated. Also, the absence or presence of in vivo visualization of these tumors after the injection of radiolabeled octreotide seems to predict the ability of octreotide therapy to control symptoms caused by hormonal secretion from these tumors. In conclusion: In-111-octreotide scintigraphy is a simple and sensitive technique for localizing of the primary tumor and its metastases in the majority of patients with carcinoids or endocrine pancreatic tumors. Apart from its merit in tumor localization, in vivo somatostatin receptor imaging could, in view of its ability to demonstrate somatostatin receptor positive tumors, be used to select patients likely to respond favorably to octreotide treatment.