Assessing the imaging boundary of glioblastoma multiforme (GBM) has potential to characterize phenotypic invasiveness relevant to outcomes. Preoperative apparent diffusion coefficient (ADC) changes across this boundary predicts outcome in humans. The tissue specificity of this finding is unknown, hindering the interpretation, further development, and application of the technique. We selected and assessed a relevant preclinical murine infiltrating GBM orthotopic human xenograft model with a novel histological tissue tumor load assessment to investigate relationships between ADC on imaging and cellular infiltration. A robust and strong inverse correlation between the histological tumor infiltration measure and ADC transition is demonstrated, supporting the hypothesis that ADC changes across GBM boundaries represent tumor infiltration and therefore relate to the previously-proposed invasive phenotype imaging biomarker.