MON-327 Where Is the Primary Papillary Thyroid Cancer?

Abstract Disclosure: N. Dhir: None. A. Safdar: None. M. Cillo: None. Introduction: Papillary thyroid cancer (PTC) is a common thyroid malignancy, which usually presents with the detection of a thyroid nodule on physical exam or imaging. It can metastasize to lymph nodes or other sites. PTC found in lymph nodes without a primary tumor in the thyroid gland is a rare occurrence. This can be distinguished from metastatic PTC by the absence of the primary tumor in the thyroid gland and the presence of thyroid tissue within the lymph node. Case Presentation: A 38-year-old woman with no significant past medical history presented to the clinic for a non-tender right submandibular mass. She noticed it 3 months prior to her visit and it has increased in size. Head and neck ultrasound revealed a complex cystic structure measuring up to 4.5 cm in the right side of the neck without any evidence of cervical lymphadenopathy. CT scan of neck showed no thyroid nodules but confirmed a complex right neck level IIa mass/lymph node with an enhancing solid component and thin wall cystic component. A dedicated thyroid ultrasound did not reveal any evidence of discrete thyroid nodules. Fine needle aspiration of the mass showed concerns for PTC, which was confirmed on the excisional biopsy revealing metastatic cystic PTC, 2.5 cm in largest dimension, without extranodal extension (1/7 lymph nodes were positive), and positive BRAF V600E mutation. The differential diagnosis included primary thyroid cancer that spread to this lateral lymph node versus ectopic thyroid cancer. Although ectopic thyroid cancer is a rare presentation, given the possibility she had a primary thyroid cancer that then spread to the lateral node, her risk of recurrence was deemed not to be insignificant. After shared decision making, she was recommended to proceed with total thyroidectomy with central and right sided lymph node dissection with adjuvant radioactive iodine therapy. She has yet to follow up on this recommendation and will likely prefer observation. Discussion: Ectopic PTC in a lymph node involves cancerous papillary thyroid cells arising from ectopic thyroid tissue within a lymph node. During embryogenesis, thyroid tissue could be displaced due to disrupted migration. There have been reports of ectopic thyroid tissue being found in the kidney, pancreas, lungs, and heart. The carcinoma could develop in the ectopic thyroid tissue, whether in a lymph node, the mediastinum, or even lungs, while the thyroid itself is normal. This is a rare and difficult to diagnose phenomenon. It is important to diagnose the source with appropriate imaging and careful histopathological and immunohistochemical analyses. Differentiating ectopic PTC from metastatic PTC is vital in providing an appropriate treatment plan and prognosis. This case highlights the importance of confirming the primary origin of PTC to further direct management. Presentation: Monday, July 14, 2025