Schematic representation of a model on drug transport in brain tumour and surrounding normal tissue after treatment with convection-enhanced delivery . Both the tumour and the surrounding normal brain tissue consist of three compartments: the extracellular space, the cell membrane and the intracellular space. F and B represent free and protein-bound drug, respectively. Free drug in the brain extracellular space distributes between the tumour and the surrounding tissue by diffusion and brain ECF bulk flow. Within each region, only free drug is available for transport across the cell membrane to the extracellular and intracellular space.