SUN-333 Parotid Gland Mucoepidermoid Carcinoma After Radioactive Iodine Treatment For Papillary Thyroid Cancer

Abstract Disclosure: D.J. Gomez D' Aza: None. A. Fnu: None. T. Zahra: None. J.E. Vargas-Jerez: None. Background: Thyroid cancer is the most common endocrine cancer. It is estimated that approximately 44,000 people in USA will receive a new diagnosis of thyroid cancer in 2025. Differentiated Thyroid Cancer (DTC) include papillary thyroid cancer (PTC), Follicular Thyroid cancer (FTC) and Hürthle cell carcinoma (HTC). Iodine-131(131I) is a radioactive isotope which is utilized as a diagnostic imaging modality and treatment option for differentiated thyroid cancer post thyroidectomy. It enters the cells through sodium-iodide symporter and works by emitting β and γ rays that destroys cancer cells. The sodium-iodide symporter is mainly located in thyroid epithelial cells but also present in extra thyroidal tissue including salivary glands which accounts for uptake and accumulation of 131I in salivary glands that can lead to radiation induced side effects including but not limited to development of malignancies. Here we present a case treated with high dose 131I for papillary thyroid cancer later developing mucoepidermoid carcinoma of parotid gland.Case Report: This is a report on a 58-year-old female who developed a high grade T4aN1 mucoepidermoid carcinoma (MEC) Left parathyroid tumor involving the facial nerve eighteen years after the post-operative treatment of 150 mCi of radioactive iodine (131I) for a papillary thyroid carcinoma (PTC) diagnosed back in her home country. The parotid tumor appeared during the patient's surveillance Whole Body Scan showing significant radiotracer uptake between the nose and the chin concerning for residual PTC. Subsequent CT neck revealed 2.4 cm soft tissue mass in the left parotic gland, with Fine Needle Aspiration histology revealing to be suspicious for malignant carcinoma. PET scan demonstrated the left parotid tumor with Standardized Uptake Value (SUV) of 5.1 as well as left supraclavicular lymph node measuring 1.3 x 1.0 cm (SUV 3.5). Patient underwent radical total parathyroidectomy, node dissection, and mastoidectomy with facial nerve reconstruction for high grade MEC and then underwent radiation therapy. Discussion: Salivary gland cancers have low incidence rates and complex histological patterns. There has been rarely reported after radioiodine therapy for thyroid. Prevention of salivary gland side effects and secondary malignancies might be achieved by consistent salivary gland’s protection during iodine therapy. Clinical awareness of this uncommon complication in patients treated with high doses of radioactive iodine (131I) is important in order to detect these malignancies as early as possible to direct necessary surgical intervention. Presentation: Sunday, July 13, 2025