Silymarin is a component of milk thistle, which is a plant that uses its seeds and fruit to treat bile duct and liver problems.
One article (1) looked at silymarin and the treatment of various kinds of cancer. It noted that silymarin protects cells against damage and stimulates the repair of the liver. It has long been used in alternative medical therapies, mostly for its effectiveness against liver disease.
Silymarin has also been studied as a way to help chemotherapy kill cancer cells. It has been found to slow the growth of cancer cells and is much less toxic than chemotherapy. Because this was an in vitro study, more in vivo experiments need to be done to see if silymarin can actually kill cancer cells in the human population.
Source: 1.Milk Thistle. http://www.cancer.gov/about-cancer/treatment/cam/patient/milk-thistle-pdq. Accessed 9/6/16.