Peripheral Nerve Sheath Tumors of the Thyroid Gland: A Series of Four Cases and a Review of the Literature.

Primary peripheral nerve sheath tumors (PNSTs) of the thyroid gland are exceptionally rare. Two schwannomas and two malignant PNSTs (MPNSTs), arising primarily within the thyroid gland, were identified in the files of the Endocrine Tumor Registry at the Armed Forces Institute of Pathology. The patients included two females, age 69 and 80 yr, and two males, age 18 and 33 yr. The patients presented with a mass in the thyroid gland confined to a single lobe of the thyroid without involvement of the cervical neck region. None of the patients had a history of neurofibromatosis. The benign tumors were encapsulated, one of them cystic, with the characteristic cellular and nuclear features of schwannomas. The MPNSTs were invasive tumors, effacing the thyroid parenchyma, with a fascicular pattern of growth composed of neural appearing cells with increased cellularity, increased mitotic activity, and with focal necrosis. Immunoreactivity for 5100 protein and vimentin was seen in all tumors. The patients with schwannomas, treated only by surgical resection, were alive without evidence of disease, over a period of 5-33 yr. Both patients with MPNSTs died of the disease 8 mo and 42 mo, respectively, with widely disseminated disease. Primary thyroid PNSTs are exceptionally rare tumors. MPNSTs, in this limited experience, have a fatal outcome irrespective of aggressive adjuvant therapy.