This is something I’ve given some thought to, having lost loved ones to cancer, and having dear ones going through treatments now. I think it depends on many things folks have mentioned - the age of the person, family members dependent on the person whether physically or emotionally, and how they would fare, the severity of the cancer itself - including how far it has spread, the success rate of the treatment and the number of side effects from the treatment, etc.
I too, have seen people suffer through treatments that seemed worse than the disease. With me it wouldn’t be about avoiding suffering, because either way you would have that. It would be more about being in good enough shape to finish things I needed to or felt the Lord wanted me to take care of, before dying. If chemo would make me weak and sick enough I couldn’t do those things, then I would probably opt out, even if It shortened my life. Tho, as my aunt told me who is currently dying from breast cancer that has spread to a number of other organs, and is on experimental drugs, we never know for sure what we’ll decide until we are in the situation! She never expected to still be on drugs at this stage - but sometimes you do it for others. She feels ready to die, but others are still struggling with the whole thing very bitterly - she’s praying for the Lord to prepare their hearts for acceptance without anger. That, too, is an offering and sacrifice.