Once the brain cells stop dividing in childhood, they are never meant to divide again. You can see that the division of brain cells is under strict regulation and control. When this control is lost in a single cell, then it starts dividing in an uncontrolled manner.
Brain cancer starts in just one cell.
A benign tumor stays where it starts, although it can grow very large and press on crucial areas. Like any cancer, the exact reason why one person gets brain cancer and another doesn't is unknown. Exposure to radiation has also been connected to developing brain cancers as well as other malignancies. The risk is greater if the person is exposed to at a young age to a higher dose. This depends upon where in the brain the cancer arises, and how big it becomes before coming to medical attention.