My son has GBM and seizures for almost 5 years. He was never well controlled. After surgery for biopsy his seizures disappeared. We figured out that it must be the Decadron has been effective to control it. Although steroids have side effects, we are planning to keep him on . At least he gets some quality of life. A friend of mine, Cindy, has a similar situation. Child with GBM, severe seizures with airway restriction. She took the same approach and seizures are so far more controlled.
Good luck,
Shevy