What is vascularization of tumors?

Source of invasion: All tumors tend to spread, mostly by metastasizing through surrounding blood supply. The tumors do not need the oxygen supplied by the vessels to survive since they grow by anaerobic metabolism. When Folkman developed angiostatin to diminish blood supply to tumors there was no change in growth and lesions progressed. This confirmed the role of vascularization to the tumor as sources for invasion

Blood vessel growth: Tumors arise when there is abnormal proliferation of cells within tissue. As tumors grow they stimulate angiogenesis or new blood vessel growth into them to sustain their growth since they need this for further growth. With neo-vascularization, existing blood vessels are recruited and remodeled & new blood vessel grow into the tumor to provide blood supply for the tumor tissue survival.