These are far more common than are primary brain tumors. The presence of multiple intracranial lesions is also more suggestive of metastases compared to primary tumor. Among secondary intracranial tumors, only metastatic carcinoma occurs with high frequency. Lymphoma and leukemia are less frequent than carcinoma and they have far less proclivity to spread to the brain or its coverings. Occasionally one encounters a rhabdomyosarcoma, Ewing tumor, carcinoid, and others that have metastasized, but these have such low incidence that cerebral metastases seldom become a matter of diagnostic concern. The interesting pathobiology of metastatic carcinoma—the complex biologic mechanisms that govern the detachment of tumor cells from the primary growth, their transport to distant tissues, and their implantation on the capillary endothelium of the particular organ in which they will eventually grow is of research interest. Suffice it to say that tumor cell adhesion molecules, the vasculature, and a number of other cellular events participate in the implantation of what is essentially a neoplastic embolus.
Autopsy studies disclose intracranial metastases in approximately 25 percent of patients who die of cancer (Posner and Chernik 1978). Approximately 80 percent are in the cerebral hemispheres and 20 percent in posterior fossa structures, corresponding roughly to the relative size and weights of these portions of the brain and their blood flow. Intracranial metastases assume three main patterns—those to the skull and dura, those to the brain itself, and those spreading diffusely through the craniospinal meninges (leptomeningeal metastases, which includes carcinomatous meningitis [meningeal carcinomatosis] and lymphomatous meningitis). As common as intracranial metastases are metastases to the vertebral column, which eventually cause compression of the spinal cord and nerve roots are more frequent. This separate problem is discussed in Chap. 42. Metastatic deposits within the spinal cord itself are infrequent but are seen from time to time; they are more common, however, than another cancer-associated spinal cord lesion, paraneoplastic necrotic myelopathy (see in the following text).
Metastases to the skull and dura occur with any tumor that metastasizes to bone, but they are particularly common with carcinoma of the breast and prostate and in the special case of multiple myeloma. Secondary deposits usually occur without metastases to the brain itself and reach the skull either via the systemic circulation (as in carcinoma of the breast) or via the Batson vertebral venous plexus—a valveless system of veins that runs the length of the vertebral column from the pelvic veins to the large venous sinuses of the skull, bypassing the systemic circulation (the route presumably taken by carcinoma of the prostate). Metastatic tumors of the convexity of the skull are usually asymptomatic but those at the base may implicate the cranial nerve roots or the pituitary gland. Bony metastases are readily recognized on various imaging techniques. Occasionally, a carcinoma metastasizes to the subdural surface dura and compresses the brain, comparable to a subdural hematoma. Many metastases in the skull and dura, perhaps most, are asymptomatic.