Diffuse Gastrointestinal Polyposis Revealing Mantle Cell Lymphoma: A Case Highlighting a Diagnostic Pitfall

Mantle cell lymphoma (MCL) is an uncommon subtype of B-cell non-Hodgkin lymphoma characterized by heterogeneous clinical behavior and frequent extranodal involvement. Gastrointestinal (GI) manifestations are well recognized but often underdiagnosed, particularly when they present as multiple lymphomatous polyposis (MLP), which can closely resemble benign or hereditary polyposis syndromes. We report the case of a 47-year-old man presenting with chronic abdominal pain, diarrhea, pruritus, and unintentional weight loss. Endoscopic evaluation revealed diffuse sessile polypoid lesions throughout the GI tract, initially raising concern for familial adenomatous polyposis. Histopathological examination with immunophenotyping demonstrated MCL. Staging studies confirmed isolated GI involvement without distant organ infiltration. The patient was treated with rituximab-based combination immunochemotherapy, resulting in significant clinical improvement and partial endoscopic regression of lesions after two cycles. This case highlights GI-predominant MCL as a critical diagnostic consideration in adults presenting with diffuse polyposis. Histologic confirmation with immunohistochemistry is essential to avoid misdiagnosis and delays in systemic therapy. Early recognition of this rare presentation enables prompt treatment and improved outcomes.