A Schwannoma also known as a neurilemmoma, neurinoma,a neurolemmoma, Schwann cell tumor or nerve sheath tumor are composed of Schwann cells, which normally produce the insulating myelin sheath covering peripheral nerves. Schwannomas are very homogeneous tumors, consisting only of Schwann cells. The tumor cells always stay on the outside of the nerve, but the tumor itself may either displace the nerve aside or put pressure on it possibly causing symptoms or possibly causing damage. Schwannomas are slow growing mostly benign tumors. Less than 1% become malignant, degenerating into a form of cancer known as neurofibrosarcoma.