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. 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.
The National Cancer Institute estimates there will be about 23,380 new cases of brain cancer diagnosed in 2014.
A tumor is a mass of tissue that's formed by an accumulation of abnormal cells. Normally, the cells in your body age, die, and are replaced by new cells. With cancer and other tumors, something disrupts this cycle.
Tumor cells grow, even though the body does not need them, and unlike normal old cells, they don't die. As this process goes on, the tumor continues to grow as more and more cells are added to the mass.