Mediastinal Ectopic Pancreas Mimicking Lymphoma with Discordant Histology and Flow Cytometry: A Diagnostic Challenge

<b>Background:</b> Mediastinal ectopic pancreas (EP) is an exceptionally rare entity that can mimic malignancy. Diagnosis is typically established post-operatively; pre-operative confirmation is challenging. <b>Case Presentation:</b> We describe a 28-year-old man presenting with life-threatening airway obstruction due to a progressive mediastinal mass, requiring emergency tracheal stenting. Diagnostic workup revealed a critical discordance: while CT-guided core biopsy confirmed benign ectopic pancreatic tissue, concurrent flow cytometry identified a monoclonal B-cell population with a high Ki-67 index (~86%), raising concern for a high-grade lymphoid process. However, no morphological evidence of lymphoma was found, and PET-CT showed only moderate metabolic activity (SUVmax 4.6), making an untreated aggressive lymphoma less consistent. The patient declined surgical resection. Management proceeded with a conservative strategy of structured clinical surveillance based on the benign histology. At 6-month follow-up, the patient remained clinically stable without chemotherapy, supporting the diagnosis of benign ectopic pancreas and suggesting the flow cytometric findings represented reactive "pseudo-monoclonality" secondary to inflammation. <b>Conclusions:</b> This case highlights mediastinal EP as a rare airway emergency and illustrates a major diagnostic pitfall: flow cytometric clonality and high proliferative fractions can occur in inflammatory settings and must not override benign architectural histology. When discordance persists and definitive tissue cannot be obtained, management should emphasize multidisciplinary review, deliberate specimen triage, and structured surveillance with predefined triggers for repeat higher-yield biopsy or surgical sampling and airway-stent reassessment.