Twofish, I am not for internet medical evaluation, but there is lots of information out there that can help. I just googled "unltrasound showed cyst on baby's brain" and up popped a lot of positive outcomes for this. This is just one site...take a read...and let your daughter read up on this and relax too...this happens a lot around 20 weeks. She will be okay and so will the baby.
I am just shy of 20 weeks pregnant and my ultrasound showed a cyst on the babies brain | BabyCenter