Primary. Primary brain tumors originate in the brain or in close proximity to it, such as in the skull, brain membranes, cranial nerves or pituitary or pineal gland. Most brain tumors in children are primary. About 25 percent of brain tumors in adults 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 astrocytomas, oligodendrogliomas, meningiomas, acoustic neuromas (schwannomas) and medulloblastomas.

Secondary. Secondary (metastatic) brain tumors are tumors that result from cancer that started elsewhere in the body and then spread (metastasized) to the brain. Cancers of the lung, colon and breast, along with melanoma (a form of skin cancer ), are most likely to spread to the brain. About 75 percent of brain tumors in adults are metastatic. Sometimes, brain metastases are the first sign of cancer elsewhere in the body.