Somatostatin Receptor Imaging In Vivo by Planar Scintigraphy Facilitates the Diagnosis of Canine Insulinomas

Somatostatin receptors expressed by insulinomas in 5 dogs were imaged in vivo by means of indium In 111 pentetreotide (OctreoScan) scintigraphy. The diagnosis in each dog was supported by the presence of hypoglycemia ( 20 μIU/mL), and histopathologic review of neoplastic tissue. All insulinomas expressed high-affinity somatostatin receptors of subtype sst2, as shown by receptor autoradiography in vitro using 1 2 5 I-[tyrosine 3 ]-octreotide and 1 2 5 I-[leucine 8 , D-tryptophan 2 2 , tyrosine 2 5 ]-somatostatin-28 with an sst2 subtype-selective analogue. Scintigrams were obtained at 1, 4, 12, and 24 hours after the IV administration of 74-222 MBq of OctreoScan to each patient. Abnormal foci of activity were 1st observed from I hour after administration of the radioligand in dog 3, to 24 hours after its administration in dog 4; in dogs I and 2, abnormal foci of activity were visible from 12 hours. Dog 5 showed a questionable abnormal focus of activity at 12 hours, but not at 24 hours. Scintigraphy enabled accurate prediction of the anatomical location of the primary tumor in 1 of 4 dogs, but was unable to differentiate a right- from a left- pancreatic lobe tumor, or vice versa, in 3 dogs; the 5th dog had equivocal results. 1 1 1 In-pentetreotide scintigraphy is a useful diagnostic adjunct to the clinical evaluation of the insulinoma patient, but is unable to localize the tumor in some cases.