SUN-325 A Rare Case of Metastatic Thyroid Cancer with Lung Adenocarcinoma as Primary and Coexistence of Papillary Thyroid Cancer

Abstract Disclosure: H. Tabassum: None. G. Reddy: None. A.M. Garcia: None. K.L. Vinales: None. Background: Secondary tumors in the thyroid gland occur in approximately 1.9% of cases, with lung adenocarcinoma being one of the most common metastatic sources. The diagnosis of metastatic thyroid cancer can be challenging, often presenting with poor prognosis due to advanced disease. The impact of thyroidectomy on prognosis is unclear. Clinical Case: We present a 76-year-old male with a history of basal cell carcinoma, prostate cancer, and lung adenocarcinoma. In 2018, he underwent robotic-assisted middle lobe lobectomy with mediastinal and hilar lymph node dissection, followed by chemotherapy (carboplatin and pembrolizumab). Pathology confirmed poorly differentiated non-small cell carcinoma consistent with lung primary, with neoplastic cells positive for CK7 and TTF-1 and negative for p63, CK5/6, PSA, chromogranin, Synaptophysin, CD56, and Napsin A. The patient remained in remission until February 2022, when a thyroid nodule was incidentally identified on a PET scan, showing high tracer avidity and raising suspicion for primary thyroid carcinoma. Ultrasonography revealed a multinodular goiter, and fine-needle aspiration (FNA) of a 5 cm nodule confirmed malignant cells. A total thyroidectomy was performed, and pathological findings were consistent with poorly differentiated carcinoma, favoring metastatic lung cancer. The tumor cells were positive for CK7, TTF-1, and Napsin-A, but negative for CK20 and PAX-8, supporting a primary lung origin. Interestingly, an incidental 6-mm encapsulated papillary thyroid carcinoma was found in the right thyroid lobe. Following surgery, the patient has undergone regular follow-up testing and imaging, and, to date, there is no evidence of disease recurrence. Conclusion: The coexistence of primary thyroid cancer and metastatic lung cancer is exceedingly rare. Previous studies have shown that thyroidectomy was offered to very few patients given the poor prognosis associated with lung cancer metastasis to thyroid, and patient died within 24 months of thyroid metastasis diagnosis. However, early intervention through total thyroidectomy may improve prognosis and offer a chance for long-term remission. This case underscores the importance of close monitoring, early detection, diagnostic evaluation, and timely surgical intervention to optimize management and outcomes in patients with complex cancer histories. Presentation: Sunday, July 13, 2025