I have had a recurrent brain tumor for 16 years and have had both standard radiation in 2007 and proton radiation in 2015 at Mayo Clinic. I've always been told exactly what you have, that it can be very difficult to determine if it is necrosis or tumor on the MRI. So sometimes they watch with serial MRI's to watch for tumor growth. The tricky thing with necrosis is that it can present with very similar symptoms as the tumor because they both take up unwanted space in the brain. I know sometimes they even opt to do surgery to debulk necrosis if it is causing a lot of unwanted symptoms. Hoping the best for your daughter!