The answer to your question is complex.
1-The person's defense mechanism agains't cancer is not working properly for several factor's.
2-People are exposed to far too much radiation.Not just work related but at home with x-ray's,cell phones, and computers.Radiation is cumulative and does not go away.
3-The average American now ingest's more chemicals in one month than thier grandparents did in a year.
4-Americans as a whole do not consume amino acids nearly enough which keep thier peptide and enzyme level's in balance and that is a key cancer fighter.
5-Many American's do not have antioxidants in thier diet which help in damaged cell repair.
6-Genetic's play's a role in some Glioblastoma cases.
I hope this makes things a little clearer for you and god bless your family.