P-107 Incidental Renal Presentation of Papillary Thyroid Carcinoma: A Rare Case Report

Abstract Introduction Papillary thyroid carcinoma (PTC) is the most common thyroid malignancy, usually presenting as a thyroid nodule or cervical lymphadenopathy. Distant metastases at diagnosis are rare, and initial presentation with an extrathyroidal lesion is extremely uncommon. Here, we report a case of PTC that initially presented as a renal mass resected for presumed renal malignancy, with subsequent confirmation of multifocal thyroid carcinoma and nodal metastases. Clinical Case A 44-year-old woman underwent partial nephrectomy for a suspicious left renal mass detected during screening. Histopathological analysis revealed metastatic papillary thyroid carcinoma (PTC). Microscopy showed colloid-containing tissue with focal calcifications, nuclear grooves, and intranuclear pseudoinclusions, consistent with PTC. Immunohistochemistry confirmed thyroid origin with positive staining for TTF-1, thyroglobulin, and PAX8, while PAX2 staining was negative. Subsequent total thyroidectomy with central lymph node dissection demonstrated multifocal PTC involving both lobes (1.2 cm in the right lobe and 1.2 cm in the left lobe), composed of classical and follicular variants. Lymphovascular invasion was present. Of eight lymph nodes dissected, six contained metastatic carcinoma, with the largest measuring 1 cm, without evidence of extracapsular spread. Psammoma bodies and calcifications were observed, necrosis was absent, and the closest tumor margin was 0.1 cm. Postoperatively, the patient developed iatrogenic hypoparathyroidism and was maintained on levothyroxine (125 µg/day), calcitriol 0,5 mcg/day, and calcium/vitamin D supplementation. Family history was notable for an aunt with thyroid cancer. Biochemical results included thyroglobulin 0.7 ng/mL (anti-Tg negative), TSH 1.5 µIU/mL, free T4 1 ng/dL, calcium 8.9 mg/dL, phosphate 4.5 mg/dL, magnesium 2.2 mg/dL, parathyroid hormone 10.1 pg/mL, and albumin 3.9 g/dL. An 18F-FDG PET/CT demonstrated no FDG-avid lesions in the thyroid bed, cervical, mediastinal, pulmonary, abdominal, or skeletal regions. Whole-body scintigraphy revealed uptake confined to the thyroid bed. Based on multidisciplinary evaluation, 150 mCi radioactive iodine (RAI) therapy was planned with a target TSH suppression 0.1 µIU/mL. BRAF mutational analysis was requested, and results are pending. Conclusion This case illustrates an unusual presentation of PTC discovered incidentally in a renal lesion. The coexistence of renal and nodal metastases emphasizes the heterogeneous metastatic potential of PTC and the importance of detailed histopathological and immunohistochemical evaluation of atypical extrathyroidal lesions. Comprehensive management including surgery, RAI therapy, and long-term TSH suppression is essential to optimize outcomes in such rare cases.