Schwanomma For more information, please visit our Comprehensive Brain & Spinal Tumor site A schwannoma (also known as an "acoustic neuroma," "neurilemmoma," "neurinoma," "neurolemmoma," and "Schwann cell tumor") is a benign nerve sheath tumor 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 push the nerve aside and/or up against a bony structure (thereby possibly causing damage). Schwannomas are relatively slow growing. For reasons not yet understood, schwannomas are mostly benign and less than 1% become malignant, degenerating into a form of cancer known as neurofibrosarcoma. Schwannomas can arise from a genetic disorder called neurofibromatosis. They are universally S-100 positive. Schwannomas can be removed surgically, but can then recur.