Disclaimer: am not a doctor + know nothing about this branch of medicine, but this fairly new drug cured one of my family member's (stage 2?) lung cancer that metastisized to brain: https://en.wikipedia.org/wiki/Osimertinib aka Tagrisso

My understanding is that it's a 3rd generation evolution of immune-system-boosting drugs that "teach your immune system to attack cancer".

I don't know how fundamentally different brain cancer that starts in the brain is vs lung cancer that moves to the brain, but maybe similar approaches are possible?