Melanoma and cancers of the lung, kidney, and gastrointestinal tract also sometimes metastasize to the brain. Incidence rates for brain and other nervous system cancers vary with sex, race, and country as well as with age. The corresponding annual death rates are 5.1 for men and 3.5 for women. Gliomas are more common among males, but meningiomas are more common among females, which suggests the possibility of hormonal factors in the development of one or both of these types of tumor. Interpretation of variations in brain cancer rates among cancer registries from different countries is uncertain, because of inconsistencies in the completeness of ascertainment, in methods of reporting, and in limitations in the diagnostic methods available to distinguish primary from metastatic tumors.
Similar issues complicate the interpretation of trends in brain cancer rates over time.
This is almost certainly due, in large part, to improved detection of brain tumors among the elderly. Technological advances have made it easier to detect brain tumors without having to do a biopsy. Tumors among the elderly that might previously have been missed or misdiagnosed are now being ascertained. However, the possibility cannot be ruled out that a small part of the apparent increase in the incidence rate is real and indicative of increased exposure to environmental neurocarcinogens. The causes of tumors of the brain and nervous system are largely unknown, but genetic factors and a variety of environmental factors have been implicated to varying degrees.