MON-348 Unmasking the Unexpected: When a Benign Nodule Hides a Rare Lymphoma

Abstract Disclosure: S. Roh: None. Y. Chen: None. S. Eliava: None. S. Rosales: None. C. Kim: None. Introduction: In the United States 40% of the population has thyroid nodules identified on ultrasound. These nodules range from benign cysts to malignancies. Workup involves measuring TSH levels and performing thyroid US. For normal or elevated TSH levels and concerning sonographic findings, a FNA is performed. Benign nodules require low-frequency surveillance, given the low risk of malignancy. We present a case of a 79-year-old female who, despite having an unremarkable workup for a left sided nodule for a year, was found to have extranodal marginal zone lymphoma (MZL), a relatively rare cancer that can evolve into an aggressive course. Clinical Case: Our patient is a 79-year-old female with a past medical history of hypothyroidism on 50 micrograms of levothyroxine daily and thyroid nodules, who was incidentally found to have a new 1.69 cm thyroid nodule in her left middle lobe. At the time of discovery, her TSH was 2 milliunits per liter (mU/L), and a FNA demonstrated atypical lymphocytes. A repeat FNA of the now 1.94 cm nodule two months later showed benign Bethesda II thyroiditis, consistent with Hashimoto’s thyroiditis in one area, and was nondiagnostic in another. A follow-up thyroid US 6 months later showed an enlarged hypoechoic area in the left thyroid and a left neck 1.35 cm lymph node. Her TSH was 1.88 mU/L. The FNA of the lymph node demonstrated a reactive lymph node and thyroglobulin washout of 5.6 ng/mL. Computer tomography scan of the neck redemonstrated the enlarged left thyroid lobe. Given the concerning features of the thyroid, she underwent a left thyroid lobectomy and isthmusectomy. Biopsy of the left lobe and isthmus demonstrated MZL with plasma cell differentiation involving the thyroid and surrounding lymph node. Immunostains in the neoplastic lymphocytes were positive for CD20, CD43, PAX5, and negative for CD3, CD5, BCL6, and BCL1. Thyroid positron emission tomography showed intense FDG (Fluorodeoxyglucose) activity related to thyroiditis in the right and left thyroid lobes, along with a FDG-avid cervical lymph node, measuring 1.5x1.9 cm and consistent with lymphomatous disease. Conclusion: Thyroid lymphomas can be easily missed. For a year our patient demonstrated relatively unremarkable ultrasound and FNA results with normal TSH levels. Finally after a thyroid biopsy, she was found to have extranodal MZL, which carries a modest 1.4-fold increase in relative mortality. This case highlights the importance of active surveillance in benign thyroid nodules. Presentation: Monday, July 14, 2025