I've heard that if you sign a contract under the threat of death that it is not legally binding. Now I know the doctors aren't making the threat, but I think the situation is remarkably similar, right?
No, they're polar opposites. If I hold a gun to your head and demand you do something, I am (i) forcing you to do something that is (ii) against your interests. If you're sick and I offer you an experimental treatment, I am (i) not-forcing you to do something that (ii) is in your interests and benefit. It could maybe save your life. A cancer patient is not legally incapacitated like a child; she has the ability and right to make decisions influencing her own life and death.