A-507 09:21 C. Complications of treatment in the CNS P. Demaerel; Leuven/BE (philippe.demaerel@uz.kuleuven.ac.be) Radiotherapy and chemotherapy are the common central nervous system cancer treatments. Other possible treatments, including recombinant humanised monoclonal antibodies and autologous dendritic cell-based immunotherapy have been used. Complications can occur soon after initiating a treatment, but delayed complications several months or years after treatment are increasingly being recognised. One of the difficulties concerns the differentiation between progressive disease/tumour recurrence and treatment-related changes, e.g. radionecrosis or inflammatory/immune-mediated changes. The term "pseudoprogression" is often used for the latter. The Response Assessment in Neuro-Oncology working group has recently updated the response criteria. Advanced MR imaging techniques are being used in an attempt to find imaging biomarkers and to gain more insight into this difficult differential diagnosis. Diffusion and perfusion imaging and MR spectroscopy have shown promising results. Elevation of choline and depression of N-acetyl aspartate is usually associated with the presence of tumour. Post-therapy necrosis is often characterised by the presence of lipid/lactate and the absence of other metabolites. Measurements of the apparent diffusion coefficient on diffusion imaging and cerebral blood volume on perfusion imaging have been investigated as potential biomarker too. Cancer treatments may also affect the central nervous system and cause neurotoxicity by direct injury to the glial cells, vascular injury or through immune-mediated mechanism. The imaging findings can include entities such as posterior reversible encephalopathy syndrome, progressive multifocal leukoencephalopathy and Wernicke encephalopathy. Learning Objectives: