Cells become cancerous through a multi-step process. The cells pick up several mutations, each clearing a natural limit on cell division and usually increasing the rate at which the cells divide. By the time a person gets old their body has many pre-cancerous clumps of cells, and cancer occurs when one of the cells in one of the clumps picks up a final mutation and becomes fully cancerous.
Cancer has proven very difficult to treat, but perhaps it is easier to treat at the pre-cancerous stage. The idea would be to treat healthy people at middle age or later and kill most of their pre-cancerous cells. This would make the pool of cells that can develop into cancer much smaller and reduce the incidence of cancer.
Chemical chemotherapy drugs would be a poor choice for this–I expect they are not effective on slowly dividing pre-cancerous cells and these drugs are also damaging.
Instead, it may be possible to trigger apoptosis (cell suicide) in pre-cancerous cells. These cells are losing their differentiation and activating abnormal signaling pathways. They are likely stressed and may already be primed to undergo apoptosis. One of the organism’s anti-cancer mechanisms is to trigger apoptosis in pre-cancerous cells. The idea here is to supercharge this mechanism.
So the idea would be to treat the person with a cocktail of drugs that induces apoptosis by activating the apoptotic signaling pathways. The treatment should be strong enough to trigger a wave of apoptosis in the most susceptible cells clearing most pre-cancerous cells from the body. There would be some normal cells killed as well but they will be replaced by normal tissue processes.