It depends on the cancer, and the treatment. Radiation therapy is what people dread typically, but it's normally used in conjunction with surgery, i.e. a cancerous tumor is removed, and radiotherapy is used to hopefully cause total celldeath to any remaining cancerous cells, and stop them for spreading.
Is dying of cancer less painful if you refuse treatment? How much less painful? I am wondering because I know of two elderly people that refused treatment and wonder how "peacefully" they passed.
Older people do react more negatively to radiotherapy generally, but surgery, presuming they're fit enough for it, shouldn't have any long lasting physical discomfort, and shouldn't have affected how peaceful or not their death was.
It's really impossible to say, but fundamentally, the side effects of radiotherapy are not pleasant, especially in the elderly. I think there's dignity in declining treatment in later years.