MON-390 An unusual PET-avid thyroid nodule

Abstract Disclosure: H. Baig: None. F.A. Forouhar: None. D. Millar: None. U. Hegde: None. A. Martelli: None. G.Q. Phan: None. Introduction: 77 year-old man was found to have incidental hypermetabolic uptake in the thyroid on a staging PET scan done for workup of recurrent stage III metastatic melanoma. He had a history of a right thigh malignant melanoma treated in 2015 and presented 9 years later with a new palpable nodule in the right groin. Fine needle aspiration (FNA) biopsy of the right groin nodule confirmed metastatic melanoma with BRAF V600E mutation. Methods: Thyroid ultrasound showed normal right thyroid with no nodules and 3 nodules in the left thyroid, measuring 1.1 cm, 1.3 cm, and 1.8 cm. The 1.8 cm nodule was solid and had microcalcifications with TI-RADS 4. FNA biopsy of the 1.8 cm nodule showed anaplastic cells with marked pleomorphism with prominent nucleoli, with some cells showing intracytoplasmic pigment. The cytology matched the prior melanoma specimens, thus confirming metastatic melanoma. Results: Since the presence of metastatic melanoma to the thyroid upstaged the patient to have stage IV disease, he underwent systemic therapy with immunotherapy (ipilimumab and nivolumab) for which he had a near-complete anti-tumor response. Follow-up imaging showed resolution of his inguinal lymphadenopathy and decreasing size of the biopsied thyroid nodule from 1.8 cm to 0.9 cm. His baseline TSH was normal at 1.9 mIU/mL, but he later developed drug-related immune thyroiditis and subsequent hypothyroidism which was treated with levothyroxine. He remains on maintenance nivolumab with plans to continue for a total of 2 years of therapy for stage IV disease. Discussion: Although uncommon, metastases to the thyroid gland do occur, with the most likely primary source being from malignancy of the kidney, breast, and lung. Clinically apparent metastatic melanoma to the thyroid is rarer and comprises less than 5% of all metastases to the thyroid. In a patient with a history of cancer, metastases to the thyroid can change a patient’s stage and subsequent therapy. A new or suspicious thyroid nodule in a patient with history of malignancy, including melanoma, should warrant a workup to rule out a metastatic lesion as well as to evaluate for a thyroid malignancy. Presentation: Monday, July 14, 2025