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

Abstract

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 ¹²⁵I‐[tyrosine³]‐octreotide and ¹²⁵I‐[leucine⁸, D‐tryptophan²², tyrosine²⁵]‐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 1 hour after administration of the radioligand in dog 3, to 24 hours after its administration in dog 4; in dogs 1 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. ¹¹¹In‐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.