English: In tumor tissues (lower panels), endothelial cells are poorly aligned with wide fenestrations, and there is a lack of lymphatic clearance and a smooth muscle layer compared to normal tissues (top panels). Therefore, macromolecules (10 to 200 nm or 40 to 800 kDa) tend to accumulate in tumor tissues much more than in normal tissues. In contrast, small molecules diffuse freely in and out of blood vessels in both normal and tumor tissues due to their small sizes, leading to their low concentrations over time.^[1]