Peritoneal lymphomatosis mimicking complicated hydatid cyst on imaging: A rare presentation of diffuse large B-cell lymphoma.

Hepatic hydatid cysts are often asymptomatic; however, uncommon complications such as intraperitoneal rupture may present with massive ascites and mimic other intra-abdominal conditions. Diffuse large B-cell lymphoma rarely involves the peritoneum and omentum, a presentation referred to as peritoneal lymphomatosis, which can closely resemble complicated hydatid disease or peritoneal carcinomatosis on imaging. We describe a 73-year-old Iranian man who presented with progressive abdominal distension and lower extremity edema. Vital signs were stable, and no systemic features of infection were noted. Laboratory evaluation revealed mild anemia with preserved liver function tests. Ultrasonography and contrast-enhanced computed tomography identified a large multiloculated hepatic cyst in segments VI and VII, accompanied by extensive ascites and omental thickening. Analysis of the ascitic fluid demonstrated an exudative, protein-rich effusion with markedly elevated lactate dehydrogenase, raising suspicion for a malignant process. Omental biopsy followed by immunohistochemical studies showed diffuse positivity for CD45, CD20, and PAX5, with negative staining for mesothelial, epithelial, and T-cell markers. These findings confirmed a high-grade B-cell lymphoma consistent with diffuse large B-cell lymphoma. This case highlights the diagnostic challenge of differentiating peritoneal lymphomatosis from complicated hydatid disease or peritoneal carcinomatosis, particularly in patients with a prior history of hepatic pathology. A multidisciplinary approach integrating clinical, radiologic, and histopathologic data is crucial for establishing an accurate diagnosis. Early recognition enables prompt initiation of appropriate immunochemotherapy and may significantly improve clinical outcomes in diffuse large B-cell lymphoma.