Cancer is a random demon and wondering why it's suddenly in your house won't make any difference. It will only consume more energy. Don't beat yourself up or second guess your daughter's actions.
Focus on what you can do--keeping that cool head on display when you can and take care of the practical aspects. And scream. Drive out into the country and sit in the car screaming until your throat is raw and you feel vaguely ridiculous. It's tempting to get a bit of a Doppler effect going by screaming while driving but probably best not to. Hang in there. Mxx