[ajb]

The reason preventative treatments are held to a higher standard is because they are given to a very large number of people who might never get the disease anyway. Hence, a 1 in 1000 risk is significant. If you are treating people who have had cancer, their risk of getting it again in much higher than 1 in 1000.

[Turing_Machine]

Right. There's also the fact that the FDA will accept much worse side effects for a cancer treatment than they would for (e.g.) an athlete's foot treatment.

For a bad cancer like some melanomas, just about anything that doesn't kill the patient outright is gonna be on the table.