High-Grade B-Cell Lymphoma Presenting as Peritoneal Lymphomatosis with Characteristic Multiple Colonic Subepithelial Lesions: A Case Report

Introduction: Peritoneal lymphomatosis (PL) is a rare manifestation of lymphoma that frequently mimics peritoneal carcinomatosis, making early diagnosis challenging. We report a case of high-grade B-cell lymphoma (HGBL) presenting with PL, in which characteristic multiple colonic subepithelial lesions (SELs) provided the key diagnostic clue. Case Presentation: A 76-year-old woman presented with epigastric pain, abdominal fullness, and altered bowel habits. CT showed massive ascites and omental masses suggestive of peritoneal carcinomatosis. Colonoscopy revealed multiple non-contiguous subepithelial lesions (SELs) from the terminal ileum to the sigmoid colon. Underwater endoscopic mucosal resection established a diagnosis of diffuse large B-cell lymphoma. PET/CT demonstrated peritoneal lymphomatosis with Lugano stage IV disease. Subsequent cytogenetic analysis of bone marrow specimens confirmed high-grade B-cell lymphoma with MYC and BCL2 rearrangements. EPOCH-R therapy induced marked regression. Discussion/Conclusion: Multiple colonic SELs may provide an important clue to diagnosing HGBL-associated PL. Recognition of this characteristic endoscopic finding and prompt tissue acquisition by endoscopic resection may facilitate early diagnosis and timely initiation of appropriate chemotherapy.