Abstract
<jats:p>Mantle cell lymphoma (MCL) is a rare, aggressive subtype of B-cell non-Hodgkin lymphoma that can involve the gastrointestinal tract as multiple lymphomatous polyposis (MLP). MLP is often associated with advanced Ann Arbor stage III/IV disease and can mimic benign or neoplastic polyposis syndromes, making early recognition and biopsy essential for diagnosis and management. We present a 61-year-old man with a positive fecal immunochemical test 4 months prior to presentation, for which gastroenterology evaluation had not occurred. He presented with one week of progressively worsening left upper quadrant abdominal pain, one month of fatigue, 10-pound weight loss, and still changes. In the ED, he was found to have elevated lactate and severe symptomatic anemia. Computed tomography (CT) angiography of the abdomen and pelvis showed marked splenomegaly, hepatomegaly, and bulky mesenteric and retroperitoneal lymphadenopathy. Esophagogastroduodenoscopy (EGD) revealed severe erosive and nodular gastritis with diffuse friability. Colonoscopy revealed a large ulcerated fungating mass in the distal transverse colon with multiple additional large ulcerated polyps throughout the colon and rectum. Subsequent CT imaging revealed additional colonic masses and intussusception. Biopsies taken during EGD and colonoscopy revealed immunohistochemistry positivity for BCL1, CD20, CD5, and SOX11, confirming MCL with diffuse gastrointestinal involvement presenting as MLP. He was started on Bendamustine and rituximab with tumor lysis prophylaxis given his high tumor burden. This case illustrates MCL presenting as MLP with diffuse gastrointestinal involvement, bulky lymphadenopathy, splenomegaly, and high tumor burden, consistent with Ann Arbor stage IV disease. This case highlights how MCL presenting as MLP may initially manifest with non-specific gastrointestinal symptoms rather than classic lymphoma features, contributing to diagnostic uncertainty. Since MLP can mimic colonic neoplasms or other polyposis syndromes, recognition of this endoscopic pattern with prompt biopsy is essential for diagnosis and initiation of appropriate systemic therapy.</jats:p>