Tumor vascular networks look different from normal vascular networks, but the mechanisms underlying these differences are not known. By studying the scale-invariant behavior of normal and tumor vascular networks we show that vascular networks exhibit three classes of fractal behavior.Tumor networks display percolationlike scaling. Normal arteriovenous networks display diffusionlimited scaling, and normal capillary networks are compact structures. The mechanisms responsible for these differences are suggested using a growth model.

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