Is it OK to freeze eggs before cancer treatment that could kill them, then re-implant them after?
There is no artificial fertilization involved. I know a very young girl who may undergo cancer treatment that may render her sterile by killing all her eggs. Is it a sin to remove the eggs and freeze them until the treatment is over, and then re-implant them so that she has a chance to have a family naturally in the future?
If it is indeed a sin, what rule is it breaking?
The document Dignitas Personae addresses this method briefly in respect to IVF (emphasis added):
20. ...In this regard it needs to be stated that cryopreservation of oocytes for the purpose of being used in artificial procreation is to be considered morally unacceptable.
The document does not say that freeing eggs (i.e. oocytes) is intrinsically immoral. Its the purpose that makes the procedure moral or immoral. As long no embryo is created outside the conjugal act and all oocytes are re-implanted to the woman to whom they belong, then no immoral action is taking place.