That's one opinion from one doctor. Was this a pediatric ophthalmologist? If not, please ask for a referral to one. Even if it was, please ask for a second opinion.
The wonderful thing about being in a major hospital is that you can (and should) put together a strong, inter-disciplinary team. I would think that would already be set up for a child with a significant issue like spina bifida. If not, talk to one of the social workers about your confusion/concern and ask for help in getting a team approach. Your child should have been assigned a case manager - again, if not, get one.
Good luck! No way you should be put into a panic about retinoblastoma after just one exam.