Third ventricle non-Hodgkin lymphoma: An unusual presentation of primary central nervous system lymphoma

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INTRODUCTION: Primary central nervous system lymphoma (PCNSL) is a rare extra-nodal non-Hodgkin lymphoma that most commonly involves the cerebral parenchyma. Isolated intraventricular involvement is uncommon, and primary localisation to the third ventricle is exceedingly rare, posing significant diagnostic and therapeutic challenges for clinicians. CASE REPORT: A 58-year-old immunocompetent man presented with progressive headache, vomiting, and cognitive decline. Neuroimaging revealed a well-defined, homogeneously enhancing lesion involving the foramen of Monro and the roof of the third ventricle, with associated ventricular obstruction. Differential diagnoses included colloid cyst, central neurocytoma, and glioma. An interhemispheric transcallosal approach was used for the biopsy. Histopathology and immunohistochemistry confirmed diffuse large B-cell lymphoma (CD20 positive, CD45 positive, and Ki-67 index 55-60%). The patient had a stormy postoperative period complicated by dyselectrolytemia and pulmonary embolism and succumbed despite aggressive supportive management. CONCLUSION: Primary CNS lymphoma presenting as an isolated third ventricular lesion is extremely rare and often radiologically mimics other intraventricular tumours. Histopathological confirmation is mandatory before initiating oncological therapy. Surgical intervention should be limited to biopsy if lymphoma is highly suspected, with cerebrospinal fluid diversion when required. Awareness of this rare tumour is essential to avoid unnecessary aggressive resections and to facilitate early initiation of chemotherapy.