PSAT289 Extranodal Marginal Zone B-Cell Lymphoma of the Mucosa-Associated Lymphoid Tissue (MALT) Type of the Thyroid: A Rare Tumor That Requires a Second Look

BACKGROUND: Extranodal marginal zone B-cell lymphoma of the mucosa-associated lymphoid tissue (MALT) type of the thyroid, or MALT lymphoma of the thyroid, is a rare, indolent tumor that accounts for only 1-2% of extranodal lymphomas. It is often associated with lymphocytic thyroiditis - a benign condition that might not be given additional investigation. CLINICAL CASE: A 79-year-old male consulted due to a 3-year history of a gradually enlarging, painless anterior neck mass accompanied by hoarseness. He denied fever, weight loss, dysphagia, dyspnea, palpitations, heat or cold intolerance, or changes in bowel movement. On physical exam, a 4×3×1 cm firm, fixed, non-tender right anterior neck mass and a 2×2 cm right supraclavicular mass were palpated. Thyroid function tests showed: TSH 3.301 uIU/mL (0.55 - 4.78 uIU/ml), FT3 2.560 pg/mL (2.30 - 4.20 pg/mL) and FT4 0.720 ng/dL (0.89 - 1.76 ng/dL). Neck ultrasound revealed multiple cervical lymph nodes measuring 1-2 cm in the right anterior, posterior, submandibular, and left submandibular areas. A lobulated heterogeneous mass measuring 9.7×2.9×4.3 cm occupied almost the entire right thyroid lobe, with intralesional and peripheral vascularity. The left thyroid lobe had an irregular, solid, isoechoic nodule measuring 0.9×0.5×0.9 cm. Fine needle aspiration biopsy of the right thyroid mass showed lymphocytic thyroiditis (Bethesda category II). The patient underwent total thyroidectomy with frozen section and selective neck dissection on the right (levels II, III, IV, and V). Final histopathology and immunohistochemical stains revealed extranodal marginal zone lymphoma of the thyroid gland, with high grade transformation to diffuse large B-cell lymphoma in 10% of the tumor. Level II, III and V lymph nodes were positive for marginal zone lymphoma. Histopathology also showed an adenomatous goiter with lymphocytic thyroiditis. The patient was started on levothyroxine 75 mcg once daily. He completed 1 cycle of chemotherapy (R-CHOP). Two weeks after chemotherapy, he developed febrile neutropenia and expired due to sepsis. CONCLUSION: In thyroid nodules and masses where lymphocytic thyroiditis is identified on fine needle aspiration biopsy, particularly in large masses accompanied by cervical lymphadenopathy, the possibility of an existing MALT lymphoma of the thyroid should be considered, and further investigation may be warranted. Presentation: Saturday, June 11, 2022 1:00 p.m. - 3:00 p.m.