Ellagic acid (2,3,7,8-tetrahydroxy-chromeno[5,4,3-cde]chromene-5,10-dione) is a natural antioxidant found in numerous fruits, nuts and vegetables.
 So, about cancer
Studies to date have almost all been on cell cultures or animals. It actually shows slight promise, inhibiting growth of some tumours, and one cell culture study suggesting it may act against substances that help tumours form new blood vessels. In the only study so far on humans, it seemed to reduce the side effects of chemotherapy in prostate cancer, but did not actually slow disease progression or improve survival.
If you want to try this, some studies suggest you just eat foods containing it, e.g. a nonrandomized study of men with prostate cancer reported pomegranate juice as slowing the increase in blood level of prostate-specific antigen.