Background: Insulinomas, which are tumors derived from the beta cells of the islets of Langerhans in the pancreas, cause hypoglycemia secondary to hyperinsulinemia. The detection rates of insulinoma using any imaging examinations are not high.
Case Report: A 45-year-old woman who had experienced abnormal behavior, such as suddenly standing up, fidgeting, and repeating the same words admitted to our hospital because of evaluation for hypoglycemia. She had gained 15 kg over the past two years (45–60 kg), and she was unable to tolerate not having 3 full meals a day. Sixty two mg/dL of plasma glucose and 16.4 µIU/mL of immunoreactive insulin were observed. Levels of plasma glucose and immunoreactive insulin were 42 mg/dL and 10.4 µIU/mL, respectively, in response to 15-hr fast. No mass lesions were detected by any imaging examinations. Because an arterial stimulation with venous sampling (ASVS) showed increased insulin secretion in response to Ca stimulation on the splenic artery, we decided that surgical resection of pancreas body and tail would be necessary. Intraoperative ultrasonography and palpation of the pancreas revealed a clearly demarcated mass measuring approximately 9 mm on the pancreatic body. Four months after the surgery, body weight had decreased to 50 kg and good glycemic control was achieved. Pathological diagnosis was an insulinoma.
Conclusions: We presented a case of insulinoma in which it was possible to localize the lesion by ASVS and by intraoperative ultrasonography, even though no mass lesion was observed on the diagnostic imaging examinations.

Keywords: insulinoma, Arterial stimulation and venous sampling, Selective Intra-arterial Calcium Injection Test