SUN-552 Ectopic Thymoma Masquerading as a Thyroid Mass

Background: Ectopic cervical thymomas are often confused with thyroid or parathyroid swellings due to their anatomical positioning (1). One of the rarest congenital abnormalities is intrathyroidal ectopic thymic tissue, cytologically confirmed in few reported cases (2). Fine Needle Aspiration Cytology in predominantly lymphocytic thymomas have also often been misinterpreted as Hashimoto's thyroiditis or malignant lymphoma (3). Clinical Case: We report a 12-year-old girl with ectopic cervical thymoma mimicking a thyroid mass. She was hospitalized with shortness of breath, swelling, facial flushing, weight loss, fatigue, and difficulty swallowing. Ultrasound and the MRI of the neck reported it to be an ectopic thyroid gland. CT scan of the neck and the chest reported the mass likely to be a thymoma that impinged on the carotid artery along with a mediastinal thymoma. Endocrine work up including complete blood count, PTH, gastrin, fasting glucose, insulin, thyroid function tests were all within normal limits. The patient underwent surgical resection of the mass and it was noted to be banana shaped, traversing from the neck into the mediastinum. Surgical pathology revealed well-defined thymic cortices and medullae, many Hassall corpuscles with admixed fat cells and thymic cyst lined by a layer of low columnar cells. The CD3 and Pax 5 immunostaining were positive for cells of thymic origin. Conclusion: Ectopic thymomas should be considered in the differential diagnosis of thyroid masses in children and adolescents. In such cases, apart from the clinical examination and the imaging studies, it is also important to perform the cytology and immunohistochemistry. Thyroid cell specific proteins such as thyroglobulin, thyroid transcription factor-1, thioperoxide and dipeptidyl aminopeptidase-4, neuroendocrine markers chromogranin, calcitonin and parathyroid hormone could be used to rule out cells of thyroid or parathyroid origin(4). Reference: 1. Thakur, A, et al. Ectopic cervical thymoma mimicking as papillary thyroid carcinoma: A diagnostic dilemma. Indian J Pathol Microbiol. 2010; 53; 305-307 2. Durmaz, et al. Intrathyroidal ectopic thymic tissue may mimic thyroid cancer: A case report. Journal of Pediatric Endocrinology and Metabolism. 2012; 25(9-10); 997-1000. 3. Lee YY, et al. Aspiration cytology of an ectopic cervical thymoma misinterpreted as a lymphoproliferative lesion of the thyroid: A case report. Oncol Lett. 2015; 10; 1255- 8. 4. De Micco C, et al, The value of thyroid peroxidase immunohistochemistry for preoperative fine-needle aspiration diagnosis of follicular variant PTC. Surgery. 1999; 126; 1200-4