Well-Differentiated Malignant Neuroendocrine Cell Tumor Originating From the Jejunum With Excessive Insulin Secretion

Background: Insulinomas are rare, well-differentiated insulin-secreting neuroendocrine tumors of the pancreas. Insulin-secreting, extra-pancreatic tumors have rarely been reported in the literature. We present a case of a well-differentiated neuroendocrine tumor with clinical, laboratory, and immunohistochemical tumor findings supportive of ectopic insulin production in a non-pancreatic neuroendocrine tumor of the small bowel. Clinical Case: A 70 year-old female with past medical history of end stage renal disease, Type II diabetes mellitus, and hypertension presented to our facility with multiple episodes of dizziness and syncope due to hypoglycemia. She had initially managed her diabetes with metformin but stopped taking the medication due to increasingly frequent episodes of hypoglycemia. Significant labs during the hospitalization include blood glucose <20 mg/dL, insulin 75.9 IU/mL (2.6-21.9 IU/mL) and C-peptide 57.7 ng/mL (1.1-4.4 ng/mL). Her symptoms persisted despite dextrose supplementation. Abdominal and Pelvic CT showed proximal small bowel obstruction and sclerosing mesenteritis with a calcified lymph node but no pancreatic mass. A diagnostic laparoscopy was performed, showing 18 inches of small bowel with malignant appearing masses that were adherent to the mesentery. A 21.0 cm segment of jejunum was resected. There were at least 30, variably sized masses throughout the bowel, involving the mucosa, submucosa, muscularis propria and subserosal fat; along with lymphatic, venous, and perineural space tumor permeation, and metastases involving multiple regional lymph nodes. The tumors ranged in size from 0.5 up to 1.5 cm. They showed well-differentiated nuclear morphology, a mitotic count of 0 per 80 high power fields, and a Ki-67 proliferation index of 1%, indicative of a G1 NET. Immunohistochemical stains confirmed neuroendocrine differentiation with positive staining for chromogranin, synaptophysin, and INSM1 (insulinoma-associated protein 1) transcription factor, while insulin staining was negative. The patient was diagnosed with a pT3N1, G1, ectopic insulin-secreting neuroendocrine tumor of small bowel origin. She was started on diazoxide with significant improvement in her hypoglycemia. Conclusion: Insulin production by non-pancreatic tumors is extremely rare. Clinical, laboratory, and immunohistochemical tumor findings in this case support ectopic insulin production by a well-differentiated neuroendocrine tumor of small bowel origin.