Both malignant and benign brain tumors can damage adjacent brain tissue.   Either directly, by damaging nearby healthy cells or indirectly, as a consequence of inflammation / swelling / crowding.  Confined to the space inside the bony skull, there is no room for expansion for a growing tumor, which puts pressure on and displaces brain tissue. This may cause various symptoms and problems, some of which may be life-threatening. E.g. can block cerebrospinal fluid flowing through the brain, increasing intracranial pressure and enlarging ventricles (hydroencephalus)

• A benign brain tumor:  grows slowly, has distinct boundaries, and rarely spreads to nearby brain areas.
• A malignant brain tumor (i.e. Brain cancer): grows quickly, has irregular boundaries, and usually spreads to nearby brain areas, but rarely to organs outside the brain or spine.

Tumor cells are analyzed under a microscope and graded according to:

• Cell muliplication rate;
• Extent of blood supply to cells;
• Amount of necrosis (dead cancer cells) in the middle of the tumor;
• If the cells are confined and how different they are to normal cells.

Primary or metastatic brain cancer?

• Primary brain tumors: originate in brain tissue;  can be either malignant (brain cancer) or benign. Primary brain cancer does not usually metastasize to areas outside the brain or spine.
• Metastatic brain tumors:  cancer cells in brain tumor have traveled from another location via the blood or lymph; nearly always malignant.  Lung and breast cancers are the most common cancers that spread to the brain.