Risk Factors for Distant Metastasis in Follicular Thyroid Carcinoma in Korea

Purpose: Follicular thyroid cancer (FTC) can frequently metastasize to distant organs, which worsens the prognosis. This study evaluated the risk factors of distant metastasis in FTC. Methods: We retrospectively reviewed 149 patients diagnosed with primary FTC and underwent operations between 2001 and 2017. Results: The mean age of all patients was 44.8±14.4 years and 82.6% were female. The mean tumor size was 3.44±1.77 cm and a total thyroidectomy was performed in 41.6% of cases. The follow-up duration was 65.4±43.5 months. Distant metastases occurred in 8 patients (5.4%): 5 patients had synchronous and 3 had metachronous distant metastasis. The most common metastatic site was the bone, with bone metastasis observed in 5 of 8 patients, followed by the lung (3 of 8). Bone metastases occurred in 4 of 5 patients with synchronous distant metastasis and in 1 of 3 with metachronous distant metastasis. Distant metastasis was associated with old age (≥45 years), the presence of vascular invasion, widely invasive FTC, and high thyroglobulin (Tg) levels in the first postoperative month. Multivariate analysis showed that old age and the presence of vascular invasion were predictive factors for distant metastasis (P=0.016 and P=0.013, respectively). Conclusion: Old age and vascular invasion were significant factors for distant metastasis. The possibility of bone and lung distant metastases should be assessed when a patient is old (≥45 years), FTC is widely invasive, and when Tg levels are high in the first postoperative month.