For clinicians, several generalizations create a helpful framework for understanding these diseases. First, many types of tumor, both primary and secondary, occur in the cranial cavity and spinal canal but certain ones are much more frequent than others and are prone to occur in particular age groups. For example, secondary metastatic deposits are more common than primary brain tumors in adults and the opposite is true in children. Furthermore, certain cancers (breast, lung, melanoma, renal cell cancer) display a tendency to metastasize to nervous tissue and many others rarely do so. Second, some primary intracranial and spinal tumors, such as craniopharyngioma, meningioma, and schwannoma, have a disposition to grow in particular parts of the cranial cavity, thereby producing characteristic neurologic syndromes. Third, the presence of a state of immunosuppression such as AIDS or cancer chemotherapy, special inherited disorders such as neurofibromatosis, and exposure to radiation each predispose to the development of tumors of the nervous system. Fourth, the growth rates and invasiveness of tumors vary; some, like glioblastoma, are highly malignant, invasive, and rapidly progressive and others, like meningioma, are most often benign, slowly progressive, and compressive. These different qualities have substantial clinical implications, frequently providing the explanation of slowly or rapidly evolving clinical states as well as potential surgical cure and prognosis.
Furthermore, a special class of disorders results from the production of autoantibodies that are elaborated by systemic, nonneural tumors and these antibodies target cerebral and spinal neurons. These remote effects, referred to as paraneoplastic, often constitute the initial or only clinical manifestation of the underlying neoplasm. To this can be added the numerous direct and indirect effects on the nervous system of treatments for systemic cancer.
Comprehensive references on brain tumors and on the effects of cancer on the nervous system are the text edited by Kaye and Laws, and one by DeAngelis and Posner.
Incidence of CNS Tumors and Their Types
Currently, in each year there are an estimated 600,000 deaths from cancer in the United States. Of these, the number of patients who died of primary tumors of the brain seems comparatively small (approximately 20,000, half of them malignant gliomas), but in roughly another 130,000 patients the brain is affected at the time of death by metastases. Thus, in approximately 25 percent of all the patients with cancer, the brain or its coverings are involved by neoplasm at some time in the course of the illness. Among causes of death from intracranial disease in adults, tumor is exceeded in frequency only by stroke, whereas in children primary brain tumors constitute the most common solid tumor and represent 22 percent of all childhood neoplasms, second in frequency only to leukemia. Viewed from another perspective, in the United States, the yearly incidence of all tumors that involve the brain is 46 per 100,000, and of primary brain tumors, 15 per 100,000. In part, the current high incidence of neoplasms of the brain in comparison to past epochs is probably the result of the availability and high utilization of cranial imaging.
It is difficult to obtain accurate statistics as to the types of intracranial tumors, for most of them have been obtained from university hospitals with specialized cancer and neurosurgical centers. One perspective of several decades ago comes from the figures of Posner and Chernik, from which one can infer that secondary tumors of the brain greatly outnumber primary ones. In the autopsy statistics of municipal hospitals, where one would expect a more natural selection of cases, the figures for metastatic tumors vary widely, from 20 to 42 percent (Rubinstein, 1972). Even these estimates probably underestimate the incidence, particularly of metastatic disease.