Abstract: The invention is based on the realization that brain cancer cells spread preferentially along paths of elevated water diffusion, such as along nerve fiber bundles, that can be measured by magnetic resonance (MR) diffusion-weighted imaging (DWI) and the migration of cancer cells away from the primary tumor can be predicted using computational models that incorporate DWI information. The invention therefore applies DWI and models cell migration to develop appropriate non-symmetric margins for radiation treatment of malignant brain tumors.