UK Mail has a nice article about women who discovered they had breast cancer when they were pregnant.
You treat the cancer, even though there is a small risk to the baby inside mom. Usually you want until the second or third trimester, when the baby is completely formed, to give chemo, nbut you do give chemo during pregnancy.
All the cases reported were successful; however, my cousin developed a lump during pregnancy but didn't get it checked until after delivery... she died tow years later, alas. So not all cases have a happy ending.
For tumors needing radiation, you put a screen to protect the baby.
The worst case scenerio that I have seen was a patient who years before had been discovered with stage 3 cervical cancer during pregnancy. Now, in Catholic ethics, you are not allowed to abort the baby, but you are allowed to give mom radiation (or chemo) even if the kid might be affected, since you are treating mom, not aiming to kill the kid.
My patient had been given the choice, and since she had three other kids opted for radiation and lost the baby. I saw her years later, for complications from the radiation that made her prone to vaginal infections and large bowel/rectal problems. She still mourned that baby....