S4044 Under the Surface: Asymptomatic Mantle Cell Lymphoma Presenting as Diffuse Colonic Polyposis – A Case Report

Introduction: Diffuse gastrointestinal polyposis is typically associated with hereditary syndromes such as familial adenomatous polyposis, Peutz-Jeghers syndrome, juvenile polyposis, and inflammatory conditions like pseudopolyps in inflammatory bowel disease. The malignant potential of these polyps varies by etiology. Rarely, diffuse polyposis can initially manifest an underlying extraintestinal malignancy. This report describes such a rare case where an asymptomatic patient was found to have mantle cell lymphoma (MCL) presenting as diffuse colonic and ileal polyposis. Case Description/Methods: A 70-year-old man with a medical history of hyperlipidemia and benign prostatic hyperplasia underwent a screening colonoscopy. He was completely asymptomatic, without abdominal symptoms, weight loss, or B symptoms. Endoscopic evaluation revealed diffuse, small, polypoid lesions from the rectum to the terminal ileum. The ileal mucosa appeared nodular, and the colonic mucosa showed generalized erythema and nodularity. Biopsies demonstrated an atypical, small-sized lymphoid infiltrate predominantly in the subepithelial layer. Immunohistochemical staining was positive for CD20 and BCL1 (cyclin D1), with a low Ki-67 proliferation index (∼<10%). CD3 highlighted scattered reactive T-cells. The findings were diagnostic for mantle cell lymphoma. Positron emission tomography-computed tomography T confirmed widespread nodal and extranodal involvement. The patient was referred to oncology and was started on systemic chemoimmunotherapy. Discussion: This case underscores the need for heightened awareness of lymphoma as a cause of diffuse gastrointestinal polyposis. Though primarily considered a systemic disease, Mantle cell lymphoma can present initially with isolated gastrointestinal findings. A hallmark endoscopic pattern of MCL is multiple lymphomatous polyposis (MLP)—numerous small polypoid lesions throughout the gastrointestinal tract. While this entity is well-described in literature, it remains underrecognized, especially in asymptomatic patients. This case illustrates the importance of considering lymphoproliferative disorders, particularly MCL, in the differential diagnosis when diffuse polyposis extending into the terminal ileum is encountered on routine colonoscopy, even without systemic symptoms.