Insulinoma with suspected mutant somatostatin receptor expression according to histological examination.

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This case highlights the possibility of an insulinoma expressing an aberrant form of SSTRs resulting in a discrepancy between the preoperative octreotide assessment and postoperative SSTR expression. Insulinoma is a pancreatic disease that causes hyperinsulinemic hypoglycemia. The first-line treatment is surgery; however, somatostatin derivatives are administered in cases where surgery is not a viable option and to prevent preoperative hypoglycemia. Here, we report a case in which preoperative examination indicated a potential tumor with low somatostatin receptor 2 (SSTR2) and SSTR5 expression, whereas postoperative pathological examination indicated strong SSTR expression. We report the case of a 69-year-old Japanese female who experienced hypoglycemia-like symptoms for a decade such as sweating, fatigue, hunger, and confusion with an increase in episode frequency per year. Dynamic computed tomography revealed a 13-mm diameter nodule and aberrant blood flow in the pancreatic tail. Subsequently, the patient was diagnosed with pancreatic insulinoma. A preoperative octreotide test did not relieve hypoglycemia, and no uptake of 111indium-pentetreotide was observed, suggesting an insulinoma with low SSTR expression. However, postoperative histological studies suggested that the intracellular domain of SSTRs were highly expressed, while the extracellular domain may be mutated. We present a rare case of insulinoma expressing an aberrant form of SSTRs resulting in a discrepancy between the preoperative octreotide assessment and postoperative SSTR expression.