THU516 An Unusual Neck Mass

Disclosure: P. Rios: None. N. Kalara: None. T. Shyam: None. J. Velez: None. C. Gomez: None. S. Gra Menendez: None. Background: Parathyroid carcinoma is a rare tumor which presents with a neck mass and hypercalcemia. Non-functional parathyroid tumors account for 10% of all parathyroid carcinomas and are associated with decrease or impaired secretion of PTH, having normal calcium, being difficult to diagnose. We present a case of a patient who presented with a neck mass and normocalcemia, who was diagnosed of non-functional parathyroid tumor, were immunohistochemistry played a useful role in differentiating an unsuspected parathyroid lesion from a thyroid neoplasm. Clinical Case: 49-year-old woman presented with a 6-month history of a firm mass on the left side of her neck that was initially stable but then started to grow and was painful. On examination, she was noted to have a 5 cm firm thyroid mass with bilateral level 2 lymphadenopathy. Patient had a calcium of 9.9 mg/dL (8.6-10.3 mg/dL), normal kidney function and was euthyroid. An ultrasound of the thyroid revealed a left mass near the thyroid lobe, measuring 3.1 x 4 x 2.5 cm. Fine-needle aspiration of the mass revealed a poorly differentiated carcinoma possible originating from the thyroid. The patient underwent total thyroidectomy with central compartment dissection, bilateral neck dissection and radical resection of the tumor. Frozen section revealed malignancy highly concerning for anaplastic thyroid. Immunohistochemistry (IHC) showed that the differentiated component was positive for PTH, chromogranin, PAX8 and GATA-3, while negative for TTF-1, thyroglobulin, BRAF and p53. The higher-grade component was positive for GATA-3, p53, TTF1 (weak focal), PAX-8 (weak focal) and negative for thyroglobulin, chromogranin, PTH and BRAF which was suggestive of parathyroid tissue. The final pathology report was a differentiated parathyroid carcinoma transitioning to a high-grade carcinoma with anaplastic features, 5.6 cm in the largest dimension with invasion to thyroid tissue, skeletal muscle and angiolymphatic invasion. The tumor was a pT3N0Mn/a. Next-Generation Sequencing (NGS) of the tumor showed a high tumor mutational burden as well as high microsatellite instability being beneficial Pembrolizumab for treatment if required. IHC showed PD-L1, were Nivolumab and Pembrolizumab could work in the future if needed. Also, TP53 was detected on the NGS. She completed 32 Gy/16 fractions of adjuvant radiotherapy and continues to be on active surveillance 8 months after diagnosis without evidence of recurrence of the cancer. Conclusion: Non-functional parathyroid carcinoma is a very rare tumor that should be on the differential diagnosis in a patient with normocalcemia and neck mass. Immunohistochemistry would help in the diagnosis between thyroid or parathyroid lesion as described in this case. Presentation: Thursday, June 15, 2023