Physicians generally divide brain tumors into the following categories to explain where or how they originate:

• Primary. Primary brain tumors originate in the brain or close to it, such as in the skull, brain membranes (meninges), cranial nerves, or pituitary or pineal gland. Most brain tumors in children are primary. About one-fourth of all brain tumors are primary. It's not known what causes these brain tumors. Studies are being done to determine whether heredity, environmental factors, viruses or other factors play a role in their development.

• Some common types of primary tumors — which are named after the type of brain cells from which they originate — include acoustic neuromas (schwannomas), astrocytomas, medulloblastomas, meningiomas and oligodendrogliomas.

• Secondary. Secondary (metastatic) brain tumors are tumors that result from cancer that starts elsewhere in the body and then spreads (metastasizes) to the brain. Cancers of the lung and breast are most likely to spread to the brain. About three-fourths of brain tumors are metastatic. Sometimes, brain metastases are the first sign of cancer elsewhere in the body.

Because doctors don't know exactly what causes primary brain tumors, it's difficult to pinpoint risk factors. Brain tumors sometimes strike several members of a family, suggesting heredity may be a risk factor. Heredity appears to account for a small minority of brain tumors. Overall, males and whites are more likely to develop a brain tumor.

Some types of brain tumors appear to occur more frequently in people who are exposed to radiation or certain chemicals, such as those who work in oil refining, rubber manufacturing, and chemical and nuclear industries. But a definite link between exposure to chemicals and brain tumors hasn't been proved. Similarly, electromagnetic fields and the use of cell phones have been studied as causes of primary brain tumors, but no definitive medical evidence indicates that either causes brain tumors.

Although brain tumors can occur at any age, they're most common in people older than 65. Children younger than 8 appear to be at a higher risk for brain tumors than are older children.

Doctors and researchers don't believe that head injuries increase the risk of developing a brain tumor.

Most headaches don't signal a serious medical condition. But if you have headaches that progressively worsen, that are always in the same location or that are worse in the morning than in the afternoon — accompanied by nausea or vomiting or by blurred or double vision — bring this to the attention of your doctor.

Brain tumors are relatively rare. But the prognosis is often better the earlier the tumors are diagnosed and treated.