SUN-322 Anaplastic Thyroid Cancer Arising from a Previously Benign Thyroid Nodule

Abstract Disclosure: D.H. Tran: None. C. Urnes: None. A.N. Lim: None. C.R. Hurtado: None. Background: Anaplastic thyroid carcinoma (ATC) is a rare, aggressive cancer with a high mortality rate. It typically presents as a rapidly growing neck mass in a patient with a history of thyroid nodules and may coexist with or develop from differentiated thyroid cancers. Early detection and treatment are crucial given the high mortality associated with ATC. We present a case of ATC in a patient with a previously benign fine needle aspiration (FNA) result from the same nodule less than a year prior. Clinical Case: A 68-year-old female with history of thyroid nodules, hyperthyroidism, stroke, and severe mitral valve stenosis presented to endocrinology clinic following a thyroid ultrasound (US) demonstrating nodules in both the right lower (3.2 cm, TIRADS 5) and left lower (1.3 cm, TIRADS 5) poles with multiple other TIRADS 3 nodules throughout. Radionuclide uptake scan demonstrated an autonomous functioning hot nodule in the left lobe, suspected to be the etiology for her hyperthyroidism, and decreased uptake in the right lobe. She was biochemically euthyroid on methimazole 5 mg daily and FNA was deferred. At age 73, she reported concern over a palpable neck mass and underwent a repeat thyroid US demonstrating enlarged right lower (4.9x3.5x3.9 cm, TIRADS 4) and left lower (4.9x2.7x3.3 cm, TIRADS 4) nodules. FNA of right lower nodule was favored benign and left lobe nodule consistent with benign colloid. Less than one year after the FNA, she presented to endocrinology with concerns of enlargement of the neck mass and associated dysphagia. Repeat thyroid US showed increased size of the right lobe nodule (5.5x4.3x5.0 cm, TIRADS 5) and stable left lobe nodule (4.9x3.1x3.4 cm, TIRADS 5). Repeat FNA was planned, however, prior to completion she presented to the hospital with worsening dyspnea and dysphagia. Thoracic computed tomography revealed a large right thyroid mass with tracheal invasion and extensive pulmonary metastases. Total thyroidectomy was performed with pathology revealing anaplastic thyroid carcinoma. She subsequently passed due to progressive respiratory failure in the setting of metastatic disease. Conclusion: This is an unusual case of ATC diagnosed in a patient with a previously benign FNA from the same nodule less than one year prior. While a Bethesda II (benign) FNA carries a 0-3% risk of malignancy, this case highlights the need for reconsideration of diagnosis when clinical findings conflict with recent biopsy results. Presentation: Sunday, July 13, 2025