Let the girl live
This is why I believe not every human being has the right to be a parent.
No doctor is going to give radiation therapy to a young girl unless it is deemed necessary for treating the cancer. It's not as though the doctor said, "Oh, you have Hodgkin's disease. I'm going to give you chemotherapy and then three nodal radiations, whether you need it or not, because that is standard practice."
Apparently, these parents are going to be really rich someday since they have the magical ability to look at their daughter and know, by staring really closely at her, if her cancer is completely gone. They have been blessed with the divine power to just know that the cancer has gone into permanent remission. They don't need no stinkin' cancer specialists.