PSAT283 Sclerosing Mucoepidermoid Cancer – A Unique Entity

INTRODUCTION: - Sclerosing mucoepidermoid thyroid cancer is a rare entity with approximately 60 cases reported in the literature. Initially described by Chan. Et al in 1991,(1) it was included as a separate entity by WHO in 2004.(2) Previously considered to have indolent nature, recent evidence has reported aggressive nature ranging from local invasion to metastasis. CASE: - 66-year-old female with complaints of difficulty in swallowing, breathing, and sensation of a lump in her throat for last 6 months, was noted to have left thyroid mass on ultrasound. Thyroid ultrasound showed solid hypoechoic nodule, 8.2×5.3×5.9 cm replacing left thyroid lobe with solid hypoechoic nodule 2.2×1.1x 1.6 cm at the right end of the isthmus. The neck ultrasound was negative for lymph nodes. Past medical history was significant for long-standing hypothyroidism, and recently diagnosed invasive ductal carcinoma of the breast. CT chest showed an enlarged thyroid gland with substernal extension and mass effect on the trachea. FNA thyroid showed Bethesda category 5, suspicious for papillary thyroid cancer. The patient underwent a total thyroidectomy. Pathology for left nodule showed sclerosing mucoepidermoid carcinoma with eosinophils, 8.6×6.9×5.8 cm, p T4a N×Mx, with positive margins for tumor, no angiolymphatic invasion with co-existing right-sided papillary microcarcinoma (follicular variant) with chronic lymphocytic thyroiditis. Levothyroxine was dosed to aim goal TSH <0.1, considering the high risk of recurrence. Post-op Thyroglobulin was 1 ng/ml (< 2 ng/ml) with elevated Thyroglobulin antibodies 6.4 U/ml (<0.4). The patient further had radioiodine ablation with 150 mCi I-131, 3 months later. Post therapy whole-body scan showed multiple foci of increased uptake in thyroid bed consistent with thyroid tissue/ residual disease. No distant metastasis was noted. DISCUSSION: – Sclerosing mucoepidermoid cancer is thought to be derived from the ultimobranchial body.(1) It is more common in females in age groups 3rd - 8th decade.(3) Most common presentation is unilateral painless thyroid mass. Coexistent chronic lymphocytic thyroiditis and papillary thyroid cancer are usually found. On literature review of 56 cases, the gross extrathyroidal extension was seen in 26% of cases with distant metastasis was most commonly seen in lungs.(3) No established treatment strategy exists due to a small number of reported cases, however, treatment ranges from only Thyroid surgery to Thyroid surgery with radiation +/- chemotherapy and radioactive iodine. REFERENCES: Shah AA, La Fortune K et al. Thyroid sclerosing mucoepidermoid carcinoma with eosinophilia: a clinicopathologic and molecular analysis of a distinct entity. Mod Pathol. 2017;30: 329–339. Pantola C, Kala S, Athar M, et al. Sclerosing mucoepidermoid carcinoma with eosinophilia of the thyroid: a cytological dilemma. J Cytol 2016; 33: 37. Chambers M, Nosé V et al. Salivary-Like Tumors of the Thyroid: A Comprehensive Review of Three Rare Carcinomas. Head Neck Pathol. 2021 Mar;15(1): 212-224. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.