Secondary tumors involving the thyroid gland: A multi-institutional analysis of 28 cases diagnosed on fine-needle aspiration.

BACKGROUND: Fine-needle aspiration (FNA) is routinely used to evaluate primary thyroid lesions (PTLs), however, its role in diagnosing secondary thyroid neoplasms (STNs) has not been extensively studied. The goal was to examine the clinical and cytopathologic features of STNs diagnosed on FNA.

METHODS: The clinico-pathologic features of 28 STNs were analyzed. All PTLs, lymphomas, and locally invasive tumors were excluded.

RESULTS: There were 28 STNs (0.18% incidence) out of 15,800 thyroid FNAs (12 males, 16 females, 32 - 85 years), all occurring metachronously (3 weeks-20 years, average 78.3 months) comprising 24 (85.7%) metastatic carcinomas (14 [50%] renal; 4 [14.3%] head and neck squamous cell carcinomas, 3 [10.7%] breast, and 1 [3.6%] colorectal, uterine serous carcinoma, and lung adenosquamous carcinoma, respectively), 3 sarcomas (10.7%) and 1 melanoma (3.6%).

CONCLUSIONS: STNs are rare and diverse tumors which may occur decades after primary malignancy. Renal carcinomas are the most common. Prior history of malignancy, high index of suspicion, and attention to key distinguishing cytologic clues are critical for accurate diagnosis.