Endocrine Abstracts

Introduction: Intra-thyroid metastases are infrequent, comprising only 1-3% of all thyroid malignancies. Renal cell carcinoma, particularly of the clear cell type (CCRC), stands out as the predominant kidney malignancy and the most common primary source of intra-thyroid metastases, often manifesting several years post-diagnosis. This report highlights two cases of CCRC metastasis to the thyroid, both featuring deceptive cytological findings.

Case 1: A 60-year-old woman, post-right nephrectomy for a T1bN0M0 CCRC a year prior, exhibited a 3.5×2.4×6.4 cm nodule in the right thyroid lobe on ultrasound, characterized by hypoechoicity, internal vascularization, and irregular borders (TIRADS 5). Multiple isoechoic nodules, the largest measuring 1.6×1.4×2.3 cm (TIRADS 3), were evident in the left lobe. Fine needle aspiration revealed colloid, small epithelial cell clusters with round nuclei, and rare macrophages, indicative of follicular nodular disease. Incongruence between ultrasound and cytology, coupled with the lesion’s size, prompted total thyroidectomy. Macroscopical evaluation of the surgical specimen unveiled a 4×4×2 cm lesion with a "nodule in nodule" configuration and hemorrhagic areas. Histopathological analysis of the lesion showed a tumor with solid growth pattern composed of cells with clear cytoplasm, positive for CD10, racemase, and PAX8. The absence of TTF1, thyroglobulin, chromogranin, synaptophysin, and calcitonin confirmed CCRC metastasis. Left lobe lesions were due to thyroid follicular nodular disease. Case 2: A 74-year-old woman, eight years post-left nephrectomy for CCRC, was referred for an Endocrinology appointment following right adrenalectomy for a suspicious mass seven years post-diagnosis. Physical examination revealed a cervical mass. Initial ultrasound unveiled a predominantly cystic nodule (4.5×2.6 cm, ACR-TIRADS 3). A subsequent evaluation six months later showcased a transformation into a predominantly solid, isoechoic nodule, 4.6 cm in diameter, with localized vascularity. Cytology suggested benign follicular disease. Owing to the development of compressive symptoms, right lobectomy was pursued. Histological analysis of both thyroid and adrenal lesions was compatible with CCRC metastasis.

Conclusions: Identifying metastatic CCRC through fine needle aspiration proves challenging due to overlapping clear cell features with other lesions (thyroid follicular neoplasms and intrathyroidal parathyroid tumors) and to the potential for coexistence with primary thyroid neoplasms ("tumor to tumor metastasis"). A thyroid nodule in a patient with a history of RCC should evoke a high index of suspicion, demanding comprehensive judgment based on ultrasonographic and cytological findings. Surgical resection may be warranted for definitive characterization.