Scientists have found many cures for cancer over the decades.
Unfortunately they haven't discovered one that a human can survive as of yet.
Cancer at its base level is a mutation in your body that slowly takes over and mutates more and more of you. It is possible to stop the spread and mutation by killing these cancer carrying genes through a variety of methods, the most common being bombardment by radition. However since they were once normal healthy cells, the radiation needed to kill a mutated cell also kills the healthy cells around the area.
It is a delicate balance to use enough radiation to kill the bad cells while leaving as many of the good, healthy cells unaffected. This why cancer can sometimes "come back" or a patient will no longer be in remission. They simply did not get all of the cancerous cells at time of treatment and the surviving cells have begun to multiply
A lot of the current work in cancer research is focusing on mapping the cancer markers themselves. Rather than killing the infected cells, there is hope of using a retro-virus to in essence remove the cancerous elements from the affected cells and return them to a healthy state.