Centred upon the septum pellucidum just above the foramen of Monro, which it obstructs, is and a peripherally enhancing mass with central non-enhancement. The enhancing component demonstrates intermediate diffusion restriction (1000-1200 x 10^-6 mm^2/s) with facilitated diffusion centrally. MR spectroscopy demonstrates some elevation of choline and a lipid/lactate peak. Cerebral blood volume appears somewhat elevated in the enhancing component of the mass. Enhancement appears to creeping up the septum pellucidum superiorly. Oedema is seen primarily inferior to the mass outlining the anterior commissure and the columns of the fornices which are involved by the mass. It is an isolated focus of the abnormality, with no other ventricular or leptomeningeal areas of abnormal enhancement. The remainder of the brain in the posterior fossa content are unremarkable. Normal flow voids are seen. The lateral ventricles are enlarged with transependymal oedema consistent with obstruction at the level of the foramen of Monro.
Conclusion: Mass in the septum pellucidum with resultant obstructive hydrocephalus. The lesion is unusual and a number of entities should be considered. The classic lesion to arise from the septum pellucidum is a central neurocytoma, and although this should be considered, imaging features are quite atypical is thought unlikely to represent this entity. A more likely diagnosis is that of a pilocytic astrocytoma, or a high-grade glial tumour (anaplastic oligodendroglioma, anaplastic astrocytoma, glioblastoma). Lymphoma is another possibility and although the appearance is a little atypical, in the setting of immunosuppression (renal transplant) this remains a very viable diagnosis along with post-transplant lymphoproliferative disorder (PTLD).