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A tumor is a mass of new tissue that grows within the body and has no physiological use. As the tumor grows, it may press upon or even dislodge nearby organs, and may "steal" oxygen and other nutrients needed by those organs. "Benign" tumors are those tumors that are not likely to recur after removal; "malignant" tumors are those that are likely to recur and progress, often posing a threat to life. Even benign tumors can be serious, since if they cannot be removed by surgery, they will continue to grow.

The brain is the second most common site of tumors (the uterus is first). Brain tumors are especially serious in a number of ways. First, the brain is enclosed within the confines of the skull. As a tumor grows and presses on surrounding tissues, the brain tissue will become compressed, leading to dysfunction. Second, as a tumor grows, it may destroy normal brain cells and take their place, interfering with the brain processes normally served by those neurons. Finally, brain tumors are often very difficult to remove, since they may be buried under brain tissue; this may make the tumors inaccessible to a surgeon without risk of severe brain damage to the patient.