For all tumors of the central nervous system, the figures in Table 30-1, taken from a Central Brain Tumor Registry of the United States (CBTRUS) survey are representative in the current era, keeping in mind again that metastatic tumors are more numerous than primary tumors in the brain. The most common intracranial tumor in most series is meningioma, but it accounts for a very small portion of deaths. This is followed in frequency by pituitary tumors, which are a special case that does not truly belong with brain tumors except that they both are intracranial. However, most serious primary brain tumors are of glia cell origin—that is, gliomas—a category that includes astrocytomas (which occur in several grades of malignancy), oligodendrogliomas, ependymomas (which may have characteristics of both glia and of epithelium), and a number of rarer types. Other brain tumors arise from lymphocytes (CNS lymphoma), or from precursor neuronal elements (neuroblastoma, medulloblastoma), germ cells (germinoma, craniopharyngioma, teratoma). Notable in all published series is the high frequency of certain tumors such as medulloblastoma during childhood.
One of the main changes since the first edition of this book in 1977 is the increase in incidence of primary CNS lymphomas. At that time, the incidence of this tumor, formerly called reticulum cell sarcoma, was negligible. In the last 40 years, the number in our hospitals has more than tripled; in specialized centers such as the Memorial Sloan-Kettering Cancer Center, the increase has been even more dramatic (DeAngelis). It now represents over 2 to 3 percent of all brain tumors and even higher proportion in institutions that care for large numbers of AIDS patients. However, this increase is only partly attributable to the rise in number of immunosuppressed individuals, particularly of those with AIDS, as the tumor appears to be increasing in incidence even in those with normal immune function.
Classification and Grading of Nervous System Tumors
Classifications and grading systems of intracranial tumors abound and are often confusing to the neurologist, in part because in both older (histologic) and newer (genetic) systems there is interconnection between systems for classification and those for prognostication. Until recently, classifications were based on the presumed cell of origin of the neoplasm, while grading systems are meant to be an estimate of the rate of growth and clinical behavior, but the two are often concordant. In the past, the numerical grading system of Daumas-Duport and coworkers (also known as the St. Anne–Mayo system) and the three-tiered system of Ringertz (which correlated most closely with clinical survival) were widely used and cited in the literature. The various classification systems notwithstanding, it is clear that there are practical and prognostic consequences to subclassifying tumors by additional molecular methods that are predictive of outcome and of treatment response (see further on).
The recent World Health Organization (WHO) classification (fifth revision, 2016) represents a departure from previous systems because it depends to a large degree on genetic characteristics of the tumors. Table 30-2 shows the main features of the WHO grading system, using the notation of I-IV to denote the grade and presumed biologic behavior of tumors that incorporates both the histologic tumor type and the main genetic features. A discussion of this system can be found in the article by Louis and colleagues (2016). Each of the main tumor types is discussed below but, if nothing else, the table displays the large variety of tumors that occur in and around the brain. Further details of the genetic features of glial tumors that form the basis of this system are also found in a later section on “Gliomas.”