One more reason why cancer is such an evil bastard.
The blood vessels that deliver food and oxygen — and chemotherapy drugs — to a tumor tend to be highly abnormal. Instead of the usual large, straight, simply branched vessels, the ones in and around a tumor are often unevenly distributed, misshapen and tangled. As a result, some parts of the tumor end up far from any blood vessels and thus have little exposure to chemo. Those same regions become starved of oxygen, and this hypoxia suppresses the immune system and also acts as a signal for the tumor cells to metastasize, or disperse to new sites.