MON-346 A Hidden Threat: Ectopic Papillary Thyroid Cancer Masquerading as a Mediastinal Mass

Abstract Disclosure: S.A. Sheikh: None. M. Shafiq: None. R.E. Thomas: None. D. Habib: None. S. Sarva: None. V. Gonzalez III: None. Papillary thyroid cancer (PTC) is typically an indolent malignancy with a favorable prognosis, often presenting as a thyroid nodule. However, ectopic or metastatic presentations are diagnostically challenging, especially in the absence of disease in the orthotopic thyroid gland. Carcinoma arising from ectopic thyroid tissue is rare, accounting for less than 1% of all thyroid malignancies. Ectopic thyroid tissue can be found along the embryologic descent of the thyroid gland, with common sites including the lingual and anterior neck regions. However, the mediastinum is an unusual location, and malignant transformation of such tissue is exceedingly rare. Given its rarity, ectopic PTC in the mediastinum presents diagnostic and therapeutic challenges.We present a case of an 81-year-old female who presented with acute chest pain. Initial cardiac evaluation was unremarkable, but a chest X-ray incidentally revealed a right paratracheal density. Further investigation with a CT scan confirmed a mediastinal mass and a mildly atrophic thyroid with a small right lobe calcification. A biopsy of the mediastinal mass confirmed papillary thyroid cancer. This case underscores the importance of considering thyroid malignancy in the differential diagnosis of mediastinal masses and highlights the role of imaging and histopathological evaluation in guiding management. Histopathology revealed papillary thyroid carcinoma. Immunohistochemical staining was positive for thyroglobulin and TTF-1, confirming the thyroid origin. A thyroid ultrasound showed no significant nodules, supporting a diagnosis of ectopic PTC. DiagnosisThe diagnosis of ectopic thyroid carcinoma requires imaging, histopathology, and clinical correlation.•Imaging: Chest X-ray and CT for mass characterization.•Biopsy: Fine-needle aspiration or core biopsy with thyroid-specific immunostaining. •Thyroid Ultrasound & Staging: To assess primary thyroid involvement and metastases. Treatment Modalities •Surgery: Resection of the mediastinal mass, with total thyroidectomy if the orthotopic gland is involved.•Radioactive Iodine (RAI) Therapy: Used postoperatively when resection is incomplete or metastases are present. •Thyroid Hormone Suppression: Levothyroxine to suppress TSH and reduce recurrence risk.•Surveillance: Routine monitoring with thyroglobulin levels, ultrasound, and imaging. Challenges and ConclusionEctopic PTC poses diagnostic and management challenges due to its nonspecific presentation and potential for misdiagnosis. Histopathological confirmation is crucial, as ectopic PTC can mimic metastatic lung or mediastinal tumors. A multidisciplinary approach is essential for optimal patient outcomes. This case highlights the importance of considering thyroid malignancy in mediastinal masses and the role of early, thorough evaluation in guiding treatment. Presentation: Monday, July 14, 2025