I'm guessing surviving a brain tumor gives them more of a "you only live once" attitude, so if they didn't want kids, this near-death experience might make them have a change of heart, or if they wanted kids, this could make them want kids sooner before time runs out. In other words, "OMG I almost died! Time to do ALL OF THE THINGS!"
Plus, while I'm sure their doctors warned them about the risk of recurrence due to pregnancy, I'd wager a lot of these women feel invincible and empowered after surviving a brain tumor, so they figure if they can beat a tumor, they can do anything! Or they figure that lightning can't strike in the same place twice and they'll roll the dice anyway. I was curious if perhaps treatment of the tumor caused neurological dysfunction that made them think breeding with such issues was okay, but since this kind of cancer/tumor occurs in the membrane covering the brain and not in the brain itself, I figure that's probably not the case.
Regardless, these fucktards shouldn't be reproducing when doing so could not only rekindle the same goddamn cancer, but there's probably a chance their kids would be predisposed to it too. Generally speaking, when any kind of tumor comes back for an encore, it's usually curtains for the patient. It sucks if you really wanted kids, but all these morons think of is, "ME ME ME, GIMME GIMME GIMME!" with absolutely no thought toward how it would affect those kids to either wind up developing the same problem or to lose their mothers to recurring cancer.
Then when they or their kids get the same fucking kind of cancer, they'll be sticking their hands out on GoFundMe for this tragic, unforseen illness. I would never wish such a horrible disease on anyone, but I can't feel sorry for them when they get told, "Do not do this" and then they go and do it.