An endoscopically proven ventriculitis-type, cyst-like intraventricular primary lymphoma of the central nervous system.

Primary ventriculitis form of primary CNS lymphoma (PCNSL) is very rare and difficult to diagnose from radiological findings. A 55-year-woman presented with a cystic mass in the lateral ventricle without evidence of demonstrable malignant cells in the cerebrospinal fluid (CSF). The endoscopic biopsy showed a ventriculitis-like thin web on the surface of the ventricular wall around the foramen of Monro, resulting in obstruction. Pathological diagnosis was non-Hodgkin's lymphoma, B cell type. This case suggests that PCNSL should be considered as a differential diagnosis for a cystic ventricular lesion even with no malignant cells in the CSF.