How is sustained angiogenesis in cancer?

In order for a tumor to enlarge more than 1-2 mm it needs angiogenesis, occurs by vessel-sprout from preexisting capillaries (neo-vascularization), endothelia newly made promote growth of tumors by secreting GF, vessels are leaky and dilated and have anarchic network. Hypoxia usually triggers this angiogenesis by increasing angiogenic factors, by tumor cells or inflammatory cells or resident stromal cells Factors regulating angiogenesis (p53 [angio inhibitor thrombospondin 1], RAS, MYC, MAPK upregulate VEGF, VEGF [inhibitors treat some cancers])