NEUROPATHOLOGY FOR MEDICAL STUDENTS
Presented by William I. Rosenblum, MD
CHAPTER 9--TUMORS OF THE NERVOUS SYSTEM
Section 3: Metastatic Tumors
This chapter contains four interrelated sections. The other three sections are:
METASTATIC TUMORS They account for roughly 20% of all intracranial tumors, but this figure varies widely from series to series and is probably low. Indeed, in certain cancer clinics, they are the most common brain tumor. 3/4 of them are seen in the 40- to 60-year-old age group and the incidence is much greater in men than in women. It is not unusual that the first symptoms or signs of malignancy are referable to brain metastases. The majority of metastatic CNS tumors originate in the lungs and the next most important site is the breast, followed by the kidney (hypernephroma), skin (melanoma), and GI tract (stomach), etc. Others are the prostate and testis, etc. However, in women lung cancer is now vying with breast cancer for first place. Most tumors metastasize to the brain through the cerebral arterial system, but they are also thought to spread via the spinal venous plexus. Lymphatic spread to the CNS does not occur since the CNS has no lymphatic system. Metastases are more frequently multiple than single. Breast metastases are also common in the dura as are melanomas in the leptomeninges. Metastatic tumors do not usually spread diffusely in the brain, but are rather well circumscribed. Their centers are often necrotic and yellow (image below).
Brain edema may be all out of proportion to the size of the tumor metastasis. In this image, the edematous white matter is much more prominent on the side of the tumor. Sometimes, significant hemorrhage may occur within a metastasis. This is frequently the case with malignant melanoma and choriocarcinoma. HERNIATION EFFECTS Regardless of cause (tumor, hemorrhage, abscess, edema, etc. [see Cerebrovascular Disease section]), the presence of a mass in the intracranial cavity may lead to herniation, brain stem damage and death. This often results from one of the tumors described above. Due to breakdown of the blood brain barrier, the attendant edema and swelling may aid and abet this event. There are few alternatives for the brain to take when placed under pressure because it resides in a closed, unyielding box. Unless a craniotomy is done or a ventricular shunt is placed to remove the pressure, the brain will try to escape through the only exits possible. There is a hole or incisura in the tough tent-like dural covering of the cerebellum (tentorium cerebelli) through which the brain stem passes. The temporal lobes (unci) will herniate through this cavity under pressure and distort the brain stem and push it caudally. In the process, vessels which supply the brain stem are stretched, begin to spasm and are torn. Secondary (Duret) hemorrhages occur in the midbrain and pons, resulting in coma and death. Pressure is also exerted on the exiting oculomotor nerve which results in dilatation of the homolateral pupil. At the same time, the adjacent posterior cerebral arteries may be compressed leading to infarction of the occipital poles and visual cortex.