FTA:

One way many cancers grow resistant to treatment is by generating a web of blood vessels that are so jumbled they fail to provide adequate oxygen to the tumor. With oxygen starvation, the tumor gains a sort of cloaking device that protects it from the toxic effects of chemotherapy drugs and radiation, which are designed to seek out well-oxygenated tissue. Researchers have long tested various approaches to improving blood flow to the tumor in the hopes of restoring potency to treatments, however, to date, not much has shown promise.