Salaam,
I think that it depends on what stage the cancer is at and if there is a good chance that you can go into remission, then I think it is important that you have treatment as preserving life is important. Also with chemotherapy advancements nowadays, the side effects may not be as bad as they used to be. You may feel worse initially but will be better in the long term.
If though the outlook is very bleak and you only have months to live, then I do not see it being an issue for wanting to be free of medical treatments in the final stages of your life.