P-095 High Risk Papillary Thyroid Carcinoma in a Branchial Cleft Cyst Presenting With Cervical Lymph Node Metastasis

Abstract Introduction Branchial cleft cysts (BCC) are congenital cystic lesions of the lateral neck resulting from incomplete obliteration of the branchial clefts during embryogenesis. They are typically located anterior to the sternocleidomastoid muscle and are benign in nature. Papillary thyroid carcinoma (PTC) is the most common histological subtype of thyroid malignancies. Due to its propensity for cervical lymph node metastasis, it may mimic cystic lesions of the lateral neck, posing a diagnostic challenge in adults. Clinical Case A 35-year-old female presented to the otolaryngology clinic with a painless neck swelling. Physical examination revealed a 3cm lesion in the right level IV region. There was no history of autoimmune thyroid disease, radiotherapy, or family history of thyroid cancer. Ultrasonography showed a normal sized thyroid with a 10×8 mm isohyperechoic nodule in the left lobe, with a regular margin, halo, and peripheral vascularization. In addition, a 30×15 mm anechoic cystic lesion was detected anterior to the sternocleidomastoid muscle and was initially considered as BCC. Fine-needle aspiration biopsy was nondiagnostic. Contrast-enhanced MRI revealed a 17×22 mm well-circumscribed, dense-content lesion, again suggestive of BCC. The patient underwent surgery with this presumptive diagnosis. Histopathological examination demonstrated papillary thyroid carcinoma invasion in the cyst wall, consistent with metastatic PTC. Following multidisciplinary tumor board evaluation, total thyroidectomy with bilateral central and right lateral neck dissection was performed. Postoperative pathology revealed a 0.8 cm oncocytic variant papillary microcarcinoma in the right lobe, with capsular invasion within 0.1 cm, and metastasis in three central lymph nodes. The patient was classified as high-risk and received 150 mCi of radioactive iodine (RAI). Post-therapy I-131 scintigraphy showed uptake confined to the thyroid bed, with no evidence of distant metastasis. During follow-up, TSH was suppressed below 0.1 mIU/L. Stimulated thyroglobulin was 7 µg/L with negative anti-Tg antibodies. At the latest follow-up, thyroglobulin was 0.6 µg/L, consistent with a biochemical indeterminate response. Cystic lymph node metastases of PTC can easily be misdiagnosed as BCC, especially in adults. Several cases in the literature have reported lesions initially excised under the presumptive diagnosis of BCC that were later confirmed as metastatic PTC. Therefore, in adult patients with lateral cervical cystic lesions, careful evaluation is essential, and suspicious radiologic findings must be followed by thorough histopathological analysis. Clinical history, imaging features, and, when necessary, repeat biopsies are crucial for accurate diagnosis. Conclusion Papillary thyroid carcinoma can present as cystic lymph node metastases mimicking branchial cleft cysts; thus, this diagnosis must be considered in adults with lateral neck masses