CAV1 and meningioma: (4) “the hydrodynamic theory”, according to the idea that tumor stasis occurs not only because of the compression of an adjacent cortical vein, but mostly from poor development of the tumor’s drainage system; when tumor blood supply becomes insufficient, meningiomas secrete angiogenic factors (such as VEGF-A, Endothelin-1, Caveolin-1) resulting in immature permeable neovessels, leakage of plasma proteins, and PTBE development in the surrounding brain parenchyma [45,49,63].