A brain tumor is a mass of abnormal tissue in the brain that grows out of control. Primary brain tumors start in the brain and are made of brain cells. Secondary brain tumors start somewhere else and spread to the brain. For example, breast cancer can spread to and cause tumors in the brain, but those tumors are made of breast, not brain, tissue. This isn’t brain cancer; it’s metastatic breast cancer. What causes brain tumors isn’t well-understood, although they are more common in dogs than most other domestic animals. Genetics probably play a role, as may dietary, environmental, viral, bacterial, parasitic, chemical, immunologic and/or traumatic factors. Regardless of cause, brain tumors are space-occupying masses that displace, compress and compromise healthy brain tissue. Affected dogs may develop seizures, blindness, disorientation, aggression, incoordination, weight loss, weakness, nose bleeds and/or breathing problems. Unfortunately, brain tumors are often fatal.