Granted, not cancer, but I always told myself I would NEVER have surgery, that I'd rather just die.. and then my appendix burst. I was an adult and didn't have to sign the surgical consent forms. I was having panic attacks and hysterically crying up until I was unconscious, but when it actually came down to it, I had the surgery (and then two more. Don't ever let your appendix burst. In my own defense, however, I did go to the hospital BEFORE it burst -- they were just slow to diagnose it and figured out what was going on too late).

My first instinct here is to say "NOOOOO NEVER" but I probably would. I just hope I never have to face that decision.