A primary brain tumor is one that originates in the brain, and not all primary brain tumors are cancerous; benign tumors are not aggressive and normally do not spread to surrounding tissues, although they can be serious and even life threatening.
No one knows what causes brain tumors; there are only a few known risk factors that have been established by research.
Children who receive radiation to the head have a higher risk of developing a brain tumor as adults, as do people who have certain rare genetic conditions such as neurofibromatosis or Li-Fraumeni syndrome.
But those cases represent a fraction of the approximately 35,000 new primary brain tumors diagnosed each year.
Age is also a risk factor -- people over the age of 65 are diagnosed with brain cancer at a rate four times higher than younger people.