Q: Could you describe what a Brainstem is? I had to put down my Boxer because of a Brainstem, but I don't really understand what causes them. A: Is it possible that your vet diagnosed your boxer with a brainstem tumor? The brainstem is a part of the brain that consists of the midbrain, pons, and medulla. This part of the brain then connects to the spinal cord. Boxers are one of the breeds that are predisposed to brain cancers. I am very sorry for the loss of your pet and I hope this answered your question.