Mathematical modelling of extravasation and comparison with intratumoural accumulation of nanoparticles. To determine if the observed number of holes was sufficient to explain the observed accumulation, we simulated the distribution of nanoparticles in the tumour. Previous models of nanoparticle transport were limited because they could not account for the organization of the blood vessel architecture and density in the tumour. We performed 3D microscopy of U87-MG tumours and visualized the blood vessel architecture using GSL1-Cy3. The tissue processing and staining methods are explained in detail in previous sections. Tissue slices were imaged using a Zeiss Lightsheet Z.1 microscope at an isotropic resolution of 2 μm and a field of view of 1.2 mm in all dimensions. The blood vessel images were segmented using a manually trained random forest algorithm implemented in Ilastik. Binary segmented blood vessel images were used for diffusion simulations.