Calcitonin-producing Duodenal Neuroendocrine Tumor: A Case Report and Literature Review

Introduction: Neuroendocrine tumors (NETs) exhibit a wide range of clinical presentations, including the production of various hormones. Calcitonin, a sensitive marker for medullary thyroid cancer (MTC), is nonspecific and may be elevated in extra-thyroidal NETs. Case Presentation: We present the case of a 64-year-old female patient who underwent total thyroidectomy due to a nodule in the isthmus, with a fine-needle aspiration biopsy indicating follicular neoplasia. Pathological examination revealed macro- and micro-nodular thyroid hyperplasia, along with a parathyroid adenoma. During postoperative follow-up, a progressive elevation of calcitonin was observed, reaching 64.2 pg/ml, while carcinoembryonic antigen levels remained normal. Since no MTC foci were found upon reviewing the thyroidectomy specimen, an investigation into the origin of the elevated calcitonin was initiated. Serum chromogranin A and specific neuronal enolase levels were within normal ranges. Tc-99m HYNIC-TOC scintigraphy yielded negative results. Additionally, an upper gastrointestinal endoscopy revealed a submucosal lesion in the second portion of the duodenum, with a biopsy confirming a grade 1 NET. The patient underwent Whipple surgery and hepatic metastasectomy. Postoperatively, a decrease in baseline serum calcitonin levels was observed. Seven years after surgery, she continues specialized monitoring with no biochemical or imaging evidence of disease. Conclusion: Serum calcitonin contributes to the diagnosis and monitoring of anterior intestine NETs.