SUN-297 - Metastatic Follicular Thyroid Carcinoma Mimicking Primary Pancreatic Cancer in RAI-Refractory Disease

Background: Follicular thyroid carcinoma (FTC) typically metastasizes to lung and bone; pancreatic metastases are extremely rare (1). We present a case of RAI-refractory FTC initially masquerading as a primary pancreatic malignancy.

Clinical

Case: An 82-year-old woman with FTC diagnosed in 2015 underwent total thyroidectomy and RAI ablation in 2015 and 2017. She was followed with thyroglobulin (Tg) and imaging; whole-body scan in 2020 and PET in 2021 were negative. Tg levels rose by 2022 (381 ng/mL), but the patient was lost to follow-up for three years. Upon reevaluation in 2025, Tg had risen further (2490 ng/mL) despite negative iodine-123 whole-body scan. PET/CT demonstrated a hypermetabolic pancreatic head lesion (12 mm) and a T4 vertebral lesion. MRI confirmed a well-circumscribed pancreatic mass concerning for neuroendocrine tumor or metastasis. EUS-guided fine-needle aspiration initially suggested poorly differentiated carcinoma of probable pancreato-biliary or upper GI origin. However, T4 bone biopsy and next-generation sequencing reclassified both lesions as metastatic FTC. Despite prior RAI therapy and negative imaging, the Tg rise and molecular data confirmed RAI-refractory disease. The patient was referred to radiation oncology for local therapy, advised denosumab for bone protection, and systemic therapy was considered for future progression.

Conclusion: In RAI-refractory FTC with markedly elevated Tg, atypical lesions may mimic primary malignancies. Pancreatic metastases, though rare, should be considered in diagnostic evaluation. Integration of clinical, imaging, pathology, and molecular data is critical to avoid misdiagnosis and guide management.

Reference: (1) Cristescu, Bogdan MD*; Cristescu, Ana; Cristescu, Tudor; Cristescu, Maria. S3409 A Rare Case of Follicular Thyroid Carcinoma Metastatic to the Pancreas. The American Journal of Gastroenterology 120(10S2):p S733, October 2025. | DOI: 10.14309/01.ajg.0001141096.59192.b0

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