Scaling laws describe how a metabolic or geometrical feature of a living organism changes with its size. As a classic example, Kleiber’s law explains how the metabolism of an organism grows as a function of its size. It is remarkable that metabolism changes at a slower pace than size does, the result of the existence of an underlying hierarchical vessel network. No one knows what the scaling law in tumors is and whether it could provide information on their aggressiveness. Positron emission tomography (PET) provides a way to assess metabolism macroscopically and could be used to investigate metabolic scaling laws in tumors.