6925 A Rare Case of Metastatic Papillary Thyroid Carcinoma Confined to Cervical Lymph Node with No Identifiable Primary Tumor In The Thyroid Gland. A Diagnostic Conundrum

Abstract Disclosure: R. Abdelmasih: None. W. Pan: None. Introduction: Papillary thyroid carcinoma (PTC) is the most common treatable thyroid cancer (TC) with very good prognosis. Metastatic PTC either in the lymph node LN or extra-nodal with no primary thyroid lesion identified is a rare phenomenon referred to as occult thyroid carcinoma (OTC). Despite its rarity, it is reported and constitute a diagnostic conundrum. herein we present a case of metastatic PTC identified in cervical lymph nodes (LN) without a primary tumor in the thyroid gland confirmed with surgical pathology. Case Presentation: A 43-year-old female with history of post-ablative hypothyroidism after treatment of toxic nodular goiter for 10 years, with no family history of thyroid cancer presented with incidentally found left 1.8 Heterogeneous hypoechoic solid nodule with lobulated borders and internal calcifications along with left-sided 0.8 LN without normal fatty hilum on Ultrasound Head and Neck. Fine needle aspiration of the left thyroid nodule and suspicious LN revealed PTC in the left LN though the thyroid nodule sample was non-diagnostic. CT of head and neck showed partially calcified thyroid nodule, Hyperattenuating 8 mm left III LN without fatty hilum, and 9 mm left II calcified LN. Patient underwent total thyroidectomy, central neck, and left lateral neck dissection. Surgical pathology showed no malignancy in thyroid gland or central LNs and 1 out 5 LN of the left neck dissection showed PTC with tumor grading of pT0 pN1b pMNA. patient underwent whole-body thyrogen-stimulated thyroid scan which showed iodine avid tissue in the thyroid bed followed by high-dose I-131 radioactive iodine ablation (RAI). Discussion: OTC refers to impalpable incidental TC less than 10 mm per the WHO. Boucek et al classify OTC into four categories, recently a fifth category was added, in which metastasis is confirmed either in regional LN or distant metastasis though thyroid tissue is confirmed to be benign per pathology and imaging evaluation, our patient falls in the fifth category. Hypothesis of this phenomenon is not well understood, one suggested explanation is missed diagnosis due to not enough sample or if lesion is less than 3 mm. another hypothesis is tumor spontaneous regression, or regression secondary to treatment that was inadequate to treat the systemic process of the tumor, which we believe our patient fits in this hypothesis, given that she underwent RAI ablation many years ago. Tumor regression is very rare and reported to be 1 in 140,000 cases. Another theory suggests that PTC can arise in ectopic thyroid tissue. This case is to shed light on a rare clinical and pathological entity to avoid missed diagnoses. Prognosis depends on the extent of metastasis, with favorable one when it is limited to regional LN and worse with distant metastasis. More cases reports are warranted to help guide further management of such cases due to the lack of available guidance in use of RAI surveillance and ablation. Presentation: 6/1/2024