The major factors that influence survival seem to be the type of cancer, its location, whether it can be surgically removed or reduced, your age, and your other medical problems.
The long-term survival rate (greater than 5 years) for people with primary brain cancer is less than 10% despite aggressive surgery, radiation, and chemotherapy.
But these treatments do prolong survival in the short term and, perhaps more importantly, improve your quality of life while you are still alive.
Most people with metastatic brain cancer die from their primary cancer rather than from the brain lesions.
In this case as well, radiation and chemotherapy increase your life expectancy modestly.
People who have seizures generally do poorly over the following 6 months.
Despite seemingly dismal chances of long-term survival, these chances are clearly greater with treatment than without. Discuss treatment options and best-estimated prognosis with your cancer team.