A Rare Case of Asymptomatic Insulinoma with Mesocolonic Lymph Node Metastases and Long-Term Stability

Malignant insulinomas are rare, and lymph node metastases are particularly uncommon in small, low-grade tumors. We report an asymptomatic insulinoma of the pancreatic body with multiple lymph node metastases confined to the transverse mesocolon, which remained radiologically stable for at least 3 years prior to the diagnosis. This case highlights the potential for metastatic disease, even in indolent insulinomas.

A 75-year-old man was referred after repeated findings of low fasting glucose levels during annual health screenings over 3 years. Despite persistent hypoglycemia, the patient remained asymptomatic and untreated. On admission, the fasting blood glucose level was 45 mg/dL, the immunoreactive insulin level was 8.7 μU/mL, and the serum C-peptide level was 2.0 ng/mL, with insulin secretion indices within normal limits. Dynamic contrast-enhanced CT revealed a 1-cm hypervascular lesion in the pancreatic body and 3 similar lesions in the transverse mesocolon. A retrospective review of earlier scans confirmed their long-term stability. Selective arterial calcium injection testing revealed insulin secretion from both the dorsal pancreatic and accessory middle colic arteries, corresponding to the pancreatic and mesocolonic lesions, respectively. Central pancreatectomy with en bloc mesocolon resection was performed. Intraoperative portal venous insulin levels declined from 102 μU/mL before resection to 10 μU/mL before closure, confirming the complete tumor removal. Histopathological analysis revealed a well-differentiated neuroendocrine tumor composed of islet cell-like neoplastic cells with a Ki-67 labeling index below 1%. Four metastatic lymph nodes were identified in the patient. The patient has remained recurrence-free for 7 years, with normal fasting glucose and insulin levels.

This case demonstrates that even small, low-grade insulinomas can metastasize to the lymph nodes through atypical drainage pathways. Favorable tumor biology may mitigate the adverse prognostic implications of nodal disease in well-differentiated pancreatic neuroendocrine tumors. Comprehensive lymph node assessment combined with functional localization techniques, such as selective arterial calcium injection testing and intraoperative insulin monitoring, may be essential for achieving curative resection and long-term disease control.