This might be a stupid question - but what is cancer?
To the best of my understanding, cancer occurs when cells divide beyond the normal constraints the body has on them. This can occur in two ways, either cells can refuse to die when they're supposed to, e.g., problems with apoptosis, or the cells can divide more than they're supposed to. However, the end result is roughly the same, uncontrolled cell growth. I may be a bit fuzzy on this, but I recall that a tumor changes from benign to malignant once the tumor has undergone metastasis. This occurs when the tumor, a glob of uncontrolled rapidly dividing cells, becomes infiltrated with new blood vessels, i.e., the process of angiogenesis, and some of the cells in the tumor break off and infect other parts of the body.