Might I suggest reading "Being Mortal" by Dr. Mukharjee (sp?), and he discusses this very question.
KM, I was going to make that same suggestion but Halbert beat me to it. The author is Atul Gawande.
You might also think about
talking with an oncology social worker. They're trained and experienced in helping patients sort through tough feelings and decisions. I went to one at a difficult point in my journey and she was very helpful.