AGING AND PRIMARY CENTRAL NERVOUS SYSTEM NEOPLASMS

We are entering an era of increased longevity. The elderly now make up 12% of the population, and this number is expected to rise to 20% by the year 2030. 6 In 30 years, one of every five Americans will be 65 years old or greater. Cancer is a group of diseases that predominantly affects older individuals. Metastatic tumors account for 61% of intracranial tumors in the over-65 population. Statistically speaking, metastasis should be the first consideration when addressing an intracranial lesion in the elderly patient; however, most metastases occur in patients with known systemic cancer. Therefore, the new diagnosis of an intracranial tumor is more likely a primary brain tumor than a metastasis. Of the remaining 39% of primary intracranial tumors, gliomas account for the majority, followed by meningiomas, acoustic neuromas, and primary central nervous system lymphomas (PCNSL). 38 PCNSL in the immunocompetent patient has risen in incidence from approximately 1% of all intracranial tumors to three times this number in the past 20 years. 15 There is a marked increased incidence of PCNSL among immunodeficient patients, especially those with AIDS, but there is an independent rise in the non-AIDS population as well. In non-AIDS patients, the peak incidence of lymphoma occurs in the 55 to 60-year-old population, and older patients are disproportionately affected. Recent studies show clearly increased incidence and mortality rates for brain tumors in the elderly over the past two decades (Tables 1 and 2). Davis et al 10 showed an increase of 36% in malignant brain tumors in women and 68% in men from 1970 to 1985–1989. In a separate study, Davis and Schwartz 8 showed increasing incidence trends in brain cancer from 1968 to 1983, with the highest percentile changes (8.4%) noted in women aged 75 to 84. Boyle et al 4 reported twofold, fourfold, and fivefold increases in brain tumor incidence among those aged 75 to 79, 80 to 84, and 85 and older, respectively, from 1973 to 1985. Studies from other industrialized countries show similar increases in brain tumor incidence in all age groups but especially in older persons. There are several factors that may explain or contribute to the increased incidence of brain tumors in the elderly population. Increased surveillance, improved imaging, and increased availability of imaging may cause an apparent increased incidence of brain tumors due to better detection; however, there is evidence that the increased incidence of brain tumors in older patients is not due solely to enhanced diagnosis by computed tomography (CT) or magnetic resonance (MR) imaging. Studies of Central European countries, where the introduction of CT scanning was delayed, show increases in brain tumor incidence in the elderly prior to the general availability of modern neuroimaging. 4 A study by Desmeules et al 11 evaluating the influence of diagnostic methods on the incidence of primary brain tumors in the past 20 years found CT and MR imaging to be responsible for detection of an additional 20% of brain tumors, but there was a twofold increased incidence of brain cancer, indicating that improved diagnosis was not responsible for most of the increase. A study of primary brain tumor incidence in elderly Floridians 40 compared the increased incidence of primary brain tumors from 1981–1984 to 1986–1989 with the incidence of pancreatic cancer from the same period. Pancreatic cancer is a neoplasm that is often clinically silent initially and requires CT scanning for identification. The incidence of pancreatic cancer rose 3.4% during these years, which was not significant compared with an increase of 23.4% ( P An alternative explanation to the rising brain tumor incidence seen in older patients has been proposed by Riggs, 33 who has attributed the increasing brain tumor mortality rates to a differential survival phenomenon of the elderly gene pool. Because patients with malignant brain tumors die from their CNS neoplasm, mortality rates for this population are closely linked to brain tumor incidence rates. Riggs has shown that as age increases, age-specific primary malignant brain tumor mortality rates are increasingly dependent on age-group population size in a linear fashion. He has proposed an "anti-Darwinian survival of the less fit" theory as an explanation for this phenomenon. As factors of natural selection become less important to long-term survival of older patients who might have succumbed to other illnesses in a less developed medical setting, these patients may have increased biologic vulnerability to brain tumors, causing an apparent increased incidence. The age-specific increase of brain tumors in the elderly is greater in men than in women. This further supports the differential survival theory, because men usually have a higher mortality than women from other more common diseases; their relatively better survival would have a stronger effect on the overall brain tumor incidence. Cultural and ethnic differences in brain tumor incidence have been identified. Several studies of both African and American blacks show decreased incidence of brain tumors overall compared to whites. Gliomas were decreased in incidence, but meningiomas and pituitary adenomas were slightly increased. In one Nigerian study of 48 primary brain tumors from 1974 to 1979, only one third occurred in patients more than 30 years old. There may be race-specific genetic factors that may influence the incidence seen in older patients. Environmental exposures and increased levels of carcinogens have been proposed as possible factors responsible for increased brain tumor incidence. This may preferentially affect the elderly if accumulated toxicity or a prolonged latency is operational; however, the data that suggest environmental carcinogens cause brain tumors are weak. Petroleum workers have been identified in several studies as having increased mortality rates from brain tumors. 22, 39 Rubber and plastic workers have been noted to have elevated risk of brain cancers in several studies. 27, 39 Individual studies have shown increased rates of primary brain tumors in blue collar workers, especially in textile workers and motor vehicle operators. 22 Meningiomas were seen with greater frequency in weavers and tailors. Increased glioma rates have been noted in aircraft industry employees. 32 Cranial irradiation is a well-documented risk factor for all brain tumors, but it is not known to increase risk preferentially in older individuals. There has been concern that electromagnetic fields, cellular telephones, and food products such as aspartame (e.g., Nutrasweet) may lead to brain tumors, but enhanced risk has never been confirmed. Moreover, the elderly have less exposure to these factors and are not thought to have increased vulnerability to them.