The Catholic church teaches that direct, deliberate abortion (intended to kill the child) is to be judged the same as any other deliberate killing of a human being, that is, it is wrong whether as a means to an end, or as an end in itself.
The baby's life is considered equal in value to the mother's life (or the father's, or the Queen of England's for that matter): neither more nor less.
Since the doctor has an equal obligation to save the mother and the baby, if the baby is save-able at all, then he has to try to save both.
That's not always possible, but it has to be the intention.
The most common solution is to try to deliver the baby prematurely, as early as possible, consistent with its survival, and then treat the mother's cancer as aggressively as you have to with radiation, chemo, surgery or whatever.
In rare circumstances (tubal pregnancy, but other cases as well) --- especially if the situation is such that the baby won't survive no matter what you do--- you can surgically remove the mother's pregnant but diseased organ (fallopian tube, uterus or whatever) and bring about what is called a "double effect": the baby perishes, but it was perishing in its mother anyway, so it is not considered "killing" the baby if you act to save the mother in this way.
I hope I have not made this seem too complicated. The principle is simple. (1) You try to save both. (2) If is is strictly not possible to save both,then you do what you need to do to save the one you can.
Thank your for your response.