SUN-553 Atypical Spindle Cells on a Thyroid Aspirate: A Rare Case of a Thyroid Schwannoma Presenting as a Progressively Expanding Thyroid Nodule

Background: Mesenchymal tumors represent around 1% of all thyroid neoplasms. Schwannoma is one such tumor that arises from the peripheral nerve sheath within the thyroid gland. Case: A 40-year-old lady was referred to our center for evaluation of a thyroid mass that was initially noticed 2 years prior and had been rapidly enlarging over the past several months. The patient experienced occasional dysphagia, but no shortness of breath nor change in voice. There was no family history of thyroid cancer or exposure to head and neck radiation. On physical exam, a firm, non-tender, 3 cm nodule was palpated in the right thyroid lobe. There was no cervical lymphadenopathy. Pemberton sign was negative. There was no evidence of café-au-lait spots or cutaneous neurofibromas on skin exam. The thyroid function tests were normal (NL), with serum TSH of 2.06 µIU/mL (NL: 0.27 - 4.2) and free thyroxine of 1.2 ng/dL (NL: 0.9 - 1.7), anti-TPO antibody was elevated (483 IU/mL; NL: 0 - 34.9), serum calcitonin was undetectable (<5 pg/ml, NL <= 7.6 pg/ml), and serum PTH was NL (29.8 pg/ml; NL: 15 - 65). Thyroid ultrasound (US) revealed a solid, heterogeneous, hypoechoic, vascularized nodule, with regular borders and no microcalcifications, measuring 3.2cm x 1.7cm x 3.9cm - consistent with intermediate suspicion for malignancy per the American Thyroid Association risk stratification. There was no cervical lymphadenopathy. A CT scan of the neck revealed the mass causing mild tracheal deviation. A fine needle aspiration biopsy of the nodule revealed atypical spindle cells with hyperchromatic elongated nuclei and rare colloid, representing a spindle cell neoplasm. The patient underwent total thyroidectomy, right central neck lymph node dissection and a right cervical thymectomy. Histopathology was consistent with thyroid schwannoma characterized by a low Ki67 proliferation index, along with background Hashimoto’s thyroiditis. Immunohistochemical staining showed that the tumor cells were focally positive for TTF-1, negative for thyroglobulin and calcitonin and diffusely positive for S-100 and vimentin, further supporting the diagnosis of a schwannoma. Next generation sequencing of the DNA and RNA derived from tumor tissue using Oncomine Comprehensive Assay v3 detected NF2 p.Leu505fs frameshift mutation. The post-operative course was uneventful, and the patient was discharged on replacement levothyroxine. Upon 8 months follow-up, the patient continued to do well without any symptoms. Conclusion: Thyroid schwanommas are exceptionally rare, with around 20 cases reported to this day. Schwannomas may progressively enlarge causing compressive symptoms and mimicking more aggressive spindle cell tumors such as spindle cell papillary, medullary or anaplastic thyroid cancer and thyroid sarcomas. Prompt imaging and cytologic evaluation with appropriate ancillary studies is crucial in the management of these tumors.