Here in Canada, before assisted suicide was legalized, my grandfather (in his late 80s) refused any treatment for his kidney failure. He was ready to die and could barely walk or eat on his own anymore. There was a wink wink situation where as the kidney failure worsened, his morphine was increased to the point where it was fatal. The death certificate still said renal failure, though.
For me, if I ever got terminal cancer, I'd weigh the quality of life of treatment versus non-treatment. I've seen people go both ways and I've seen the results being right and wrong both ways. I don't want to spend my final months semi-alive on a bed or constantly messed up, though.