MON-039 A Race Against Time: The Rapid Course of Thymic Neuroendocrine Carcinoma

Abstract Disclosure: R. Devbhandari: None. M. Shah: None. S. Tripathee: None. H. Akbari: None. A. Kang: None. R.M. Jhingan: None. Introduction: Neuroendocrine tumors of the thymus (NETTs) are very rare, with fewer than 1,000 reported cases. Over 80% are malignant, and nearly 50% show metastasis or local invasion, primarily in men aged 40-50. NETTs are classified as well-differentiated (typical/atypical) and poorly differentiated (small-cell/large-cell) and are often associated with endocrinopathies like Cushing’s syndrome, acromegaly, and Multiple Endocrine Neoplasia 1. The prognosis is poor due to high recurrence and metastasis. Case Report: We present a case of a 29-year-old male with no significant medical history presented with 2 months of intermittent right upper quadrant abdominal pain and unintentional 40 lb weight loss over 3 months. Physical examination was unremarkable. Lab results showed a hemoglobin level of 9 g/dL (14-18 g/dl), indicating mixed iron deficiency and anemia of chronic disease. HIV and tuberculosis tests were negative. A computed tomography (CT) scan of the chest/abdomen/pelvis revealed thickened distal ileal loops, a right diaphragmatic lymph node, and two small cardiophrenic lymph nodes. Bidirectional endoscopies with biopsies and small bowel follow-through were unremarkable. Due to persistent symptoms and worsening anemia, a CT-guided biopsy of the diaphragmatic lymph node was done which revealed metastatic NET. Immunohistochemical stains for chromogranin A and synaptophysin were negative. A positron emission tomography (PET) scan identified a 1 cm thymic nodule without tracer uptake elsewhere. The case was discussed at a multidisciplinary tumor board, and the thymus was suggested as the likely primary site. We planned a diagnostic laparoscopy to rule out occult abdominal metastasis with ongoing abdominal pain but tragically, the patient deteriorated rapidly and passed away before definitive treatment could be initiated. Conclusion: NETTs are highly aggressive malignancies. Our patient's right upper quadrant pain was likely referred from an enlarged diaphragmatic lymph node rather than a gastrointestinal (GI) cause—an atypical presentation not previously reported. UnremarkabIe GI studies, including biopsies, supported this. A PET scan identified the thymus as the primary source. The patient's rapid decline supports the diagnosis of NETTs, a clinical course uncommon in NETs of other origins. This case emphasizes the importance of a prompt, thorough, and meticulous evaluation in young patients with unexplained symptoms, as early detection of rapidly progressing conditions may improve prognosis. Presentation: Monday, July 14, 2025