7168 Papillary thyroid carcinoma co-existing with intrathyroidal metastasis from lung neuroendocrine tumor

Abstract Disclosure: P. Balasubramanian: None. A. Ladenheim: None. S. Gilani: None. C. Gibson: None. P. Kunz: None. Introduction: Papillary thyroid carcinoma is the most common differentiated thyroid carcinoma. Thyroid metastasis from neuroendocrine tumors (NETs) is extremely rare with an estimated prevalence of <1%. We report a rare case of 59 year old female with co-existence of papillary thyroid carcinoma and intrathyroidal metastasis from a lung NET. Case: A 59-year-old female with a metastatic, well-differentiated lung NET (Ki-67 15%, unable to distinguish between typical and atypical) was noted to have a suspicious hypermetabolic left thyroid nodule on both FDG PET CT and Ga68 DOTATATE PET CT during the initial work-up . Thyroid ultrasound showed a suspicious 1.9 cm TR5 left upper pole nodule and biopsy was consistent with papillary thyroid cancer. Final pathology following total thyroidectomy and central neck dissection demonstrated 1.6 cm left papillary thyroid carcinoma with no lympho-vascular invasion or extrathyroidal extension co-existing with an intrathyroidal metastasis from the lung NET. Calcitonin stain was negative in the NET. Post op, thyroglobulin level was 0.18 with negative thyroglobulin antibodies. Given her low risk thyroid cancer, RAI was deferred. She was started on everolimus for management of metastatic lung NET and has stable disease 12 months after initiation of treatment. Conclusion: With increased use of advanced imaging for diagnosis, staging and treatment response evaluation, incidental thyroid nodules have become a common finding. However, there is a paucity of guidelines from professional organizations regarding the management of these nodules. Malignancy rates of FDG and Ga68 DOTATATE avid nodules can be as high as 35% and 20% respectively. While a significant proportion of these will be papillary thyroid cancers, it is important to consider the possibility of metastasis in patients with diagnosed primary malignancy. Thyroid metastasis from NETs occurs rarely but in these cases, it is crucial to differentiate it from medullary thyroid carcinoma using immunohistochemistry for calcitonin. Accurate diagnosis is critical as medical and surgical treatment can vary significantly in primary versus secondary malignancies of the thyroid. Presentation: 6/2/2024