Yeah, they were actually both left temporal lobe glioblastomas. Every time that comes up with a doctor they get a funny look and ask where they lived, obviously suspecting some environmental cause.

The short answer is people almost never know the cause of a glioblastoma. My parents only lived in the same place for like 6 years, and they moved around a fair bit. Nobody else in my family had one so far, so the next likely data points are me and my brother. Not a lot you can do anyhow, so ¯\_(ツ)_/¯