Peritoneal Lymphomatosis Mimicking Advanced Ovarian Malignancy: A Diagnostic Challenge in Diffuse Large B-Cell Lymphoma.

Diffuse large B-cell lymphoma (DLBCL) may rarely present as peritoneal lymphomatosis, closely mimicking peritoneal carcinomatosis on imaging and creating significant diagnostic uncertainty. We report the case of an 82-year-old woman presenting with progressive dyspnoea, fatigue, and systemic symptoms following treatment for presumed community-acquired pneumonia. Contrast-enhanced computed tomography demonstrated ascites, diffuse peritoneal thickening, omental involvement, and widespread lymphadenopathy, initially raising suspicion for advanced ovarian malignancy, supported by an elevated CA125 level. Excisional cervical lymph node biopsy confirmed DLBCL with peritoneal involvement. A biopsy was performed shortly after imaging, and there was no significant delay in establishing the diagnosis. Given the patient's frailty, supportive care was pursued. This case highlights the importance of recognising peritoneal lymphomatosis as a key diagnostic mimic of ovarian malignancy and emphasises the role of early tissue diagnosis when imaging findings suggest peritoneal carcinomatosis.