Most malignant brain tumours are caused by a cancer that started somewhere else in the body and spread to the brain, through the bloodstream. These are known as secondary tumours.
Cancers that can spread to the brain include lung cancer, breast cancer, bowel cancer and melanoma skin cancer.
Some primary malignant brain cancers (cancerous tumours that start in the brain) are caused by a previously benign brain tumour becoming cancerous, but usually the exact reason why a primary tumour develops is unknown.
Who's most at risk?
Some of the things that can increase your risk of brain tumours are outlined below.
Brain tumours affect people of all ages, including children, but the risk tends to increase as you get older. Most tumours affect people over 50 years of age.
People with a family history of brain tumours may be at a slightly increased risk of developing a brain tumour themselves.
If your brain is exposed to radiation during radiotherapy, your risk of developing certain types of brain tumour later on is increased.
Some genetic conditions can increase your risk of a brain tumour, such as:
Unlike most brain tumours, tumours associated with these conditions tend to develop in childhood or early adulthood.
Can mobile phones cause brain tumours?
There have been reports in the media about a possible connection between brain tumours and the radiofrequency (RF) energy emitted by mobile phones. RF energy produces heat, which can increase body temperature and damage tissue exposed to it.
However, it's thought that the amount of RF energy people are exposed to from mobile phones is too low to produce significant tissue heating or an increase in body temperature.
Research is underway to establish whether RF energy has any long-term health effects, but the balance of evidence currently available suggests that it's unlikely mobile phones cause health problems.
For more information, read about mobile phone safety.