What would the Church allow in the following situation? A pregnant woman has cancer. Chemotherapy may cure her but it would harm or kill her child. But if she refuses chemotherapy she will die before the child is viable outside the womb. I understand the principle that one cannot do something intrinsically evil (abortion) but one can do something that does both good and harm as long as the harm is not intended (principle of double effect).
The principle of double effect can apply here. Stage four cancer is often terminal. In the situation you describe, she could still use the treatment for the reasons you give. St. Gianna Beretta Molla chose not to, but she could have. Read about St. Gianna at www.wf-f.org/StGianna.html.