Metastatic tumors do not start in the brain. They travel to the brain from tumors (usually cancerous) which have started in some other part of the body - like the lung, kidney or bowel. From their primary site individual tumor cells usually get to the brain by the blood stream.  When they reach the capillary blood vessels of the brain they take root and continue to grow as another tumor.  Sometimes metastatic tumors are multiple and can be found in different areas of the brain. The most common sites of the primary tumor which can spread to the brain are: lung, bowel, breast and kidney. Melanomas ( a form of skin cancer) not uncommonly travel to the brain.Some metastatic tumors grow directly into the skull from locations within the paranasal sinuses or can be transmitted to the brain by veins. In this latter instance the tumors usually grow between the skull and the brain.

Metastatic tumors are as malignant as the cancerous primary tumor from which it came. Most grow quite quickly. As the tumor gets larger it recruits its own blood vessels which feed it. In addition, there is some inflammatory response which causes the brain around the tumor to swell (edema).  Usually, but not always, the edema is more pronounced than with a primary tumor which has started in the brain. The edema adds to the mass effect and can cause dysfunction of the surrounding normal brain.
 

Symptoms of a Metastatic Brain Tumor.

Treatment of Intracranial Metastases

The options available to treat a metastatic brain tumor include surgery, radiosurgery, radiation therapy and chemotherapy. The following will briefly discuss each these modalities .
 

Surgery

Surgery is usually recommended when only a single metastatic tumor can be found on computed tomography (CT scanning) or magnetic resonance imaging (MRI). The goal of surgery is to get the tumor out without inflicting a neurological deficit on the patient. A minimally invasive surgical approach - such as stereotactic resection-is usually preferred in order to minimize damage to the brain surrounding the tumor. On some occasions two metastatic tumors can be removed by two small stereotactic craniotomies done at the same sitting.

Patients with a large metastatic tumor of the posterior fossa (in the back of the head) are usually offered surgery even when there may be more than one or two metastatic brain tumors elsewhere. Tumors of the posterior fossa (in the cerebellum) are especially dangerous since the posterior fossa is small and obstruction of the spinal fluid pathways usually occurs early. This results in a build-up of spinal fluid within the brain, a rapid increase in pressure inside the head (intracranial pressure) and coma and death. The surgery to remove one of these is relatively easy and straightforward. Image guidance can be employed to minimize the exposure necessary to get the tumor out and to reduce the risk of damage to healthy tissue.

It is clear that patients who undergo surgery for metastatic tumors do much better than those in whom surgery is not practical or who have refused surgery. The mean survival for patients having radiation therapy alone without surgery is only 3 months after the diagnosis of the brain metastasis. In contrast, the mean survival of patients who have had a solitary metastasis removed is at least one year and longer depending on the type of the primary tumor and how well this is able to be controlled with treatment. Surgery gets the tumor mass out of there and allows the surrounding brain swelling to subside. This gives the patient time for doctors to treat the primary tumor and to complete a course of radiation therapy to the head. This is necessary to kill any possible microscopic tumors not visible on the CT or MRI.

Radiosurgery

There is usually confusion about the "best" method for radiosurgery: Gamma Knife or LINAC. Radiosurgery with the Gamma Knife (LGU) is probably the same as radiosurgery delivered with a linear accelerator (LINAC). IN LINAC radiosurgery, the linear accelerator head moves around its isocenter in a spherical arc. Here a patient's head is also fixed in a stereotactic frame which is used to position the head so that the tumor is in the center of the intersecting beam arcs. In LINAC radiosurgery, the beam is constantly moving but there is only one beam which intersects itself from many different directions. In the Gamma Knife there are many beams from fixed radiation sources which intersect in the center of the unit. In the LINAC, the radiation beam is produced electronically by a linear accelerator. In the LGU, the radiation is produced from decay of a Cobalt-60 radiation source. Both produce the same type of energy.

Radiosurgery can be used to treat more than one metastasis at the same sitting. However, there is a tumor size limitation. Metastatic tumors larger than 1 inch in diameter are probably not appropriate for radiosurgery for two important reasons: First, tumors larger than this must be treated with two or more "shots" and this diffuses the radiation dose so that the surrounding brain gets radiated - not in a fractionated way (see below) but in a single session. This can potentially damage the surrounding brain tissue. Second,  radiosurgery turns a live tumor into a dead tumor. The dead tissue must then be carted away from the brain by an inflammatory reaction. The bigger the mass of dead tissue the greater the inflammation which can make the patient sicker and dependent on high doses of steroids in attempts to keep the swelling under control. Sometimes patients require open surgery to remove this mass of dead tissue because the mass of the dead tumor plus the mass of the surrounding edema (swelling) is causing an elevation of intracranial pressure and/or neurologic deficit.

Radiation Therapy

Chemotherapy