SAT542 TDC or Not TDC: Isolated Ectopic Papillary Carcinoma in the Midline Neck with a Benign Native Thyroid

Disclosure: M. Fasen: None. L. Kosmiski: None. Background: Thyroglossal duct carcinoma is a rare entity, occurring in about 1% of all thyroglossal duct cysts (TDC). Several cases have reported PTC within a thyroglossal duct cyst. Initially thought to be a TDC, we present an unusual case of a midline ectopic papillary thyroid carcinoma with absence of features characteristic of a TDC in the context of benign native thyroid tissue. Clinical Case: A 19 year old female presents to the clinic with a progressively growing midline neck mass over the past five years associated with mild dysphagia. Physical exam was notable for 2 x 2 cm mobile nontender midline mass. Neck ultrasound demonstrated a complex cystic lesion measuring 3.8 x 3.4 x 2.5 cm with partial calcification and mildly vascular. The thyroid itself had no nodules. She underwent a Sistrunk procedure with follow-up pathology revealing 1.6 cm classical PTC that extended into surrounding soft tissue and focal invasion into skeletal muscle without a definitive remnant thyroglossal duct lining despite cystic architecture. The differential included PTC completely replacing the thyroglossal duct cyst, PTC arising from ectopic thyroid tissue or metastatic PTC. Due to the unclear origin of the PTC, decision was made to perform total thyroidectomy. Pathology of the thyroid and four lymph nodes were negative for malignancy confirming a PTC origin from an extra-thyroidal remnant. The patient’s course was complicated by post-operative hypocalcemia resolved with calcitriol and calcium carbonate. She is pending discussion with endocrinology regarding adjuvant radioactive iodine. Conclusion: Thyroglossal duct carcinoma, most commonly papillary carcinoma, is a rare condition that should be considered in patients presenting with cystic midline neck masses. Sistrunk surgery should be performed in all cases for tumor removal. Thyroidectomy in individuals with normal thyroid can be considered due to the probability of concomitant intraglandular thyroid cancer and enables management with adjuvant radio-iodine if needed. In this particular case, PTC was found without evidence of a TDC and suggests PTC arose from ectopic thyroid tissue. Ectopic thyroid tissue can undergo malignant transformation along with the native thyroid gland. There are only a few other reports of isolated ectopic thyroid tissue malignancy with a benign thyroid gland. Presentation Date: Saturday, June 17, 2023