[MOL] Risks for Brain and NervousSystem Cancer [01866] Medicine On Line

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[MOL] Risks for Brain and NervousSystem Cancer

Brain and Other Nervous System

Terry L. Thomas, Ph.D.,* and
Peter D. Inskip, Sc.D.**

The nervous system consists of two parts: (1) the central nervous system (CNS), which includes the brain and spinal cord, and (2) the peripheral nervous system. Tumors arise in both parts of the nervous system, but nine out of 10 nervous system cancers occur in or around the brain. Most tumors develop from glial cells, which form supporting structures for nerve cells; tumors originating in nerve cells are rare. ÒGliomaÓ is a general term that includes any tumor arising from glial cells. Examples include astrocytic tumors (astrocytoma and glioblastoma multiforme), ependymoma, and oligodendroglioma.

The different types of brain and other nervous system tumors occur with different relative frequencies among children and adults (Schoenberg et al., 1976). The most common tumors of childhood are astrocytic tumors and a group of cancers known collectively as primitive neuroectodermal tumors, of which the most common types are medulloblastoma and neuroblastoma of the CNS (Rorke, 1983). Among adults, the most frequent types are astrocytic tumors, meningioma, acoustic neuroma, and pituitary gland tumors. In general, astrocytic tumors occurring among adults are more malignant and carry a graver prognosis than those seen among children. Meningiomas arise from cells in the membranes that surround the brain and spinal cord. Other types of nervous system tumors include neurofibroma (Schwannoma), neurofibrosarcoma, retinoblastoma, and tumors of the pineal gland. Many nervous system tumors are benign; that is, they do not invade other parts of the body. However, even benign and slow-growing tumors of the nervous system can produce serious neurologic symptoms. The skull has no flexibility to accommodate a growing tumor mass, so as tumors grow, they exert pressure on neighboring nerve tissue.

Brain cancers usually do not spread outside the central nervous system, but cancers from other sites frequently spread, or metastasize, to the central nervous system (Schoenberg, 1982). Breast cancer, for example, often metastasizes to the brain and spinal cord. Melanoma and cancers of the lung, kidney, and gastrointestinal tract also sometimes metastasize to the brain. These tumors are not considered to be primary tumors of the nervous system, because they originate in other parts of the body; therefore, the rest of this chapter concerns only tumors that originate in the nervous system.

There is a small peak in the age-incidence of brain and nervous system cancer under the age of 10--when brain cancer is the second most common cancer--and a much larger peak among adults in the eighth decade of life. The apparent decrease in incidence past the age of 80 is probably indicative of a less aggressive diagnostic approach with patients of very advanced age (Annegers, 1981; Schoenberg, 1978).

Incidence rates for brain and other nervous system cancers vary with sex, race, and country as well as with age. In the United States, the annual incidence rate for brain and other nervous system cancers is about 7.4 cases per 100,000 population among men, and 5.3 among women (standardized to the age distribution of the 1970 U.S. population) (Ries et al., 1994). 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.

In the United States, nervous system cancers occur more often among whites than blacks, and least often among persons of Asian background (Parkin et al., 1992). The incidence of nervous system cancers among whites is about 7.9 cases per 100,000 population among men and 5.7 among women (1970 standard). The incidence rate of nervous system cancers among black men is 4.7 per 100,000 population and among black women is 3.3 (Ries et al., 1994). Whites develop gliomas at a higher rate than do blacks, whereas blacks are more likely to develop meningiomas (Heshmat et al., 1976; Preston-Martin, 1989).

Internationally, the highest rates of brain and nervous system cancer are seen in Scandinavian countries and Israel, while low rates are reported for Asian countries (Parkin et al., 1992). 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. Incidence rates for brain cancer have increased by 25 percent since 1973, at an annual rate of 1.2 percent, while mortality has increased by 16 percent (Ries et al., 1994). Examination of changes in age-specific rates shows the increase in both incidence and mortality to be especially pronounced among older persons (Ries et al., 1994). 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. Certain inherited syndromes, such as neurofibromatosis, predispose persons to develop tumors of the nervous system, but such syndromes are rare. Persons who inherit an altered form of a particular gene are at greatly increased risk of developing retinoblastoma. Parents and siblings of children with brain cancer appear to have a slightly increased risk of developing a brain tumor (Kuijten et al., 1990, 1993).

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