A risk factor is anything that may increase your chance of having a disease. Risk factors for a certain type of cancer might include tobacco use, diet, family history, or many other things. The exact cause of someone's cancer may not be known. But risk factors can make it more likely for a person to have cancer.

Things you should know about risk factors for cancer:

• Risk factors can increase a person's risk, but they do not always cause the disease.
• Some people with one or more risk factors never get cancer. Other people have cancer and no known risk factors.
• Some risk factors are very well known. But there is ongoing research about risk factors for many types of cancer.

Some risk factors, such as family history, may not be in your control. But others may be things you can change. Knowing about risk factors can help you make choices that might lower your risk. For instance, if an unhealthy diet is a risk factor, you may choose to eat healthy foods. If excess weight is a risk factor, your doctor may help you lose weight.

Most brain tumors form in people without any known risk factors. But some factors may increase your risk for brain tumors, such as:

• Radiation exposure. People who have had radiation to the head are at higher risk for brain tumors. Most often this exposure comes from radiation therapy used to treat another type of cancer, like leukemia during childhood.
• Certain inherited syndromes. People who are born with certain syndromes, such as neurofibromatosis, von Hippel-Lindau disease, Li-Fraumeni syndrome, and tuberous sclerosis, have an increased risk of brain tumors.
• Family history of brain tumors. Most people with brain tumors don't have a family history of the disease. But in rare cases, tumors can run in families.
• Weak immune system. People who have a weak immune system have a higher risk of developing central nervous system lymphoma. This includes people who have AIDS or who have had an organ transplant.

Researchers have also looked at many other factors that might increase a person's risk of brain tumors. This includes:

• Contact with chemicals, such as vinyl chloride, pesticides, and petroleum products
• Cell phone use
• Living near power lines
• Infection with certain viruses

So far there is no strong evidence linking any of these to brain tumors. Research on these possible links is ongoing.

Talk with your doctor about your risk factors for brain tumors and what you can do about them. Most people with brain tumors do not have known risk factors, so it's not clear if there is any way to prevent these tumors. Even among the known risk factors, most are not under your control.

If you are concerned about your risk, talk with your doctor about what you can do.