ODP510 Pseudolymphoma of the Thyroid Gland in Association with Hashimoto's Thyroiditis: A Benign Lesion in Whitish Thyroid Gland

BACKGROUND: Pseudolymphoma or reactive lymphoid hyperplasia is a benign localized tumor-forming lymphoid hyperplasia found in various organs. Its occurrence in the thyroid gland is rare and could mimic malignant lesion. Here, we present a case of pseudolymphoma of the thyroid in a patient with Hashimoto's Thyroiditis. CASE DESCRIPTION: A 24-year-old woman without underlying disease presented with incidentally found thyroid nodule. Ultrasound revealed mild thyroid enlargement with diffuse heterogeneous parenchymal echotexture. A 5×7×8.5-mm hypoechoic solid nodule with irregular margin was found in the left upper pole. No microcalcification was detected. Multiple subcentimeter lymph nodes at bilateral cervical group VI and left cervical group IV were also detected, measuring up to 5 mm in short axis. Fine-needle aspiration was performed and suggested malignancy (Bethesda class V). Laboratory testing showed euthyroid state with positive anti-thyroglobulin antibodies and anti-thyroid peroxidase antibodies. The blood test and ultrasound findings suggested the underlying of Hashimoto's thyroiditis. The patient underwent total thyroidectomy and lymph node dissection without postoperative complication. Gross examination of the thyroid showed multiple well-demarcated whitish nodular lesions, ranging from 2-10 mm, in the upper pole of both thyroid lobe and a well-circumscribed area of a 7-mm greyish nodule in left thyroid lobe. Microscopically, the whitish lesion in both lobe composed of hyperplastic lymphoid tissues with many follicular centers on the background of lymphocytic thyroiditis. The immunohistochemistry study revealed a polyclonal nature of the plasma cell infiltrates. No abnormal staining with AE1/AE3 or cyclin D1 was found. Pseudolymphoma was diagnosed based on these findings. Papillary thyroid microcarcinoma (6 mm) without capsular invasion was also diagnosed in the lesion of left mid lobe. The cervical lymph nodes showed metastatic papillary thyroid carcinoma (1.5 mm) in level III cervical lymph nodes. No perinodal extension was found. Postoperative course was unremarkable and the patient was in stable condition at 18 months after the operation. CONCLUSION: Although primary thyroid lymphoma is often considered in patients with Hashimoto's thyroiditis. The possibility of pseudolymphoma characterized by the presence of well-demarcated pearly white nodules and polyclonal cell populations should also be considered. Presentation: No date and time listed