It's important to note that aging is not so much a mechanism to protect against cancer as it is a method to avoid cancer (that is, by dying before cancer develops).

Well, following that logic, getting hit by a bus is an even more effective method to avoid cancer, than aging...

Dying as soon as possible is more effective at avoiding cancer, but not as effective at reproducing as much as possible (before getting cancer).

What would the "selfish gene" explanation for this be? The aging mechanism to avoid cancer due to death-before-cancer.

The cell dies or stops working as effectively as before and is potentially replaced by other cells, but the organism lives. So overall, the cell's DNA is more likely to be transmitted.

If the cell lived (selfishly), it could take down the whole organism with it and not reproduce.

This is a good review http://www.nature.com/nature/journal/v448/n7155/full/nature0...