MON-031 A Rare Mediastinal Neuroendocrine Neoplasm presenting with Hypercalcemia and Severe Duodenal Ulcer Bleeding

Abstract Disclosure: N. Nikravangolsefid: None. A.K. Karn: None. M.G. Elhassan: None. Neuroendocrine neoplasms (NEN) are a heterogeneous group of tumors derived from neuroendocrine cells, frequently originating in the gastrointestinal or pulmonary systems. These tumors present a spectrum of clinical manifestations, ranging from incidental findings to endocrinopathies and hormone-related complications. Neuroendocrine neoplasms of the mediastinum are extremely rare. We report the case of a 31-year-old male with a history of duodenal ulcer with incidental hypercalcemia two years prior, who presented with abdominal pain, hematemesis, and a month history of systemic symptoms, including night sweats, and unintentional weight loss of 10 pounds. Physical examination revealed tachycardia, and melena on rectal examination. Laboratory investigations showed severe anemia (Hb 6 g/dL), hypercalcemia (12.1 mg/dL), and hypophosphatemia (2.0 mg/dl). Imaging revealed a large anterior mediastinal mass (17.3 × 9.3 × 14.8 cm) with hypodense components. During hospitalization, esophagogastroduodenoscopy performed for acute gastrointestinal bleeding identified a large duodenal ulcer with visible vessels, which was successfully managed with epinephrine injection and hemostatic spray. Further evaluation of hypercalcemia demonstrated elevated parathyroid hormone (PTH 148.1 pg/mL) and a parathyroid adenoma on sestamibi scan. A CT-guided biopsy of the mediastinal mass confirmed a well-differentiated neuroendocrine tumor, positive for Cam5.2, CD56, synaptophysin, and PAX8. Tragically, the patient developed massive hematemesis a week after admission and went into cardiac arrest before the biopsy result was available. It is likely that the large mediastinal mass could have hindered chest compression efforts during resuscitation. This case highlights the complexity of diagnosing NEN neoplasms and emphasizes the importance of thorough diagnostic workup during the first episode of gastrointestinal bleeding with concurrent hypercalcemia. Presentation: Monday, July 14, 2025