6921 Almost a Thyroid Malignancy but Not Quite! A Rare Case of Follicular Neoplasm with Uncertain Malignant Potential

Abstract Disclosure: G.P. Thota: None. C. Dampier: None. T. Jin: None. K. Luberice: None. S. Jhawar: None. A. Grover: None. P. Veeraraghavan: None. A. Gharib: None. S. Sadowski: None. M. Roth: None. A. Filie: None. S. Hewitt: None. M. A. Zeiger: None. J. Klubo-Gwiezdzinska: None. S. Gubbi: None. Background: Follicular Neoplasm with Uncertain Malignant Potential (FT-UMP) represents an exceedingly rare group of encapsulated follicular thyroid neoplasms demonstrating architectural or cellular atypia that are suspicious but do not entirely fulfil the histopathologic criteria for malignancy. The 2022 World Health Organization classification has categorized FT-UMP as a distinct entity within the realm of low-risk follicular cell-derived thyroid neoplasms. Case Presentation: A 56-year-old male with a history of ileocecal valve neuroendocrine tumor (NET) and Hashimoto’s hypothyroidism on levothyroxine therapy was found to have an incidental 3 cm right-sided thyroid nodule during the evaluation of NET via computerized tomography. The patient denied any hypo/hyperthyroid or local compressive symptoms, nor any appetite or weight changes. Physical examination of the neck was unremarkable. Laboratory studies revealed a normal thyrotropin level (3.86 mIU/L; range: 0.35 – 4.94 mIU/L). Neck ultrasonography revealed a 3 cm x 2.7 cm hypoechoic right thyroid lobe nodule with cystic regions, smooth margins, wider than tall shape, and punctate echogenic foci. The cervical lymph nodes were normal. A fine-needle aspiration biopsy (FNAB) yielded a cytopathologic diagnosis of follicular lesion of undetermined significance (Bethesda III), with TTF-1 positivity on immunohistochemistry. The patient opted for serial sonographic monitoring. One year later, the patient noted mild dysphagia. Neck sonogram revealed an increase in nodule size (4.1 cm x 2.5 cm), but with similar sonographic characteristics and absence of cervical lymphadenopathy. The cytopathology on a repeat FNAB sample showed atypia of undetermined significance (Bethesda III). Total thyroidectomy was performed. No post-surgical complications were observed. Histopathologic examination revealed a 2.6 cm encapsulated tumor with eosinophilic, cuboidal neoplastic cells protruding into the capsule but without complete capsular invasion and no vascular involvement, and with background lymphocytic thyroiditis. Next-generation sequencing (TruSight Oncology 500 Gene Panel v3) of the tumor was negative for pathogenic variants. Currently, the patient remains symptom-free and is under surveillance. Conclusion: FT-UMP poses a diagnostic challenge as its malignancy potential cannot be definitively ascertained even after histopathologic evaluation. A meticulous microscopic assessment is required to confirm the absence of invasion at the tumor-capsule and tumor-vascular interfaces. Although predominantly an indolent neoplasm, there have been rare instances of recurrence or distant metastases [1]. Therefore, an individualized approach to patient management involving regular monitoring may be warranted. Reference: 1. Ito et al. Endocrine Journal 69.7 (2022): 757-761. Presentation: 6/1/2024