Follicular Variant of Papillary Thyroid Carcinoma is a Unique Clinical Entity: A Population-Based Study of 10,740 Cases

Background: Follicular variant of papillary thyroid carcinoma (FV-PTC) has been increasingly diagnosed in recent years. However, little is known about its clinical behavior. The purpose of this study was to determine the disease characteristics of FV-PTC, and to compare it with classical papillary thyroid carcinoma (C-PTC) and follicular thyroid carcinoma (FTC). Methods: All cases of C-PTC, FV-PTC, and FTC larger than 1cm in the Surveillance, Epidemiology and End Results (SEER) Cancer Database from 1988 to 2007 were identified. Tumor behavior and patient survival were compared among these three groups. Different risk factors for disease-specific mortality in each group were evaluated by multivariate analysis. Results: More than 36,000 surgical cases were identified, including 21,796 C-PTCs, 10,740 FV-PTCs, and 3958 FTCs. Extrathyroidal extension and lymph-node metastases were more common in FV-PTC than in FTC, but significantly less common than in C-PTC (p<0.0001). Distant metastasis rates were present in 2% of patients with FV-PTC, in 1% with C-PTC, and in 4% with FTC (p<0.0001). The 10-year disease-specific survival for patients with FV-PTC was 98%, similar to C-PTC (97%) but better than FTC (94%, p<0.0001). Being over the age of 45 years remained a strong risk factor for disease-specific mortality in both FV-PTC and C-PTC, while the presence of extrathyroidal extension and distant metastases were stronger predictors of disease-specific mortality in FV-PTC than in C-PTC. Conclusions: FV-PTC is a common variant of PTC. Its clinical behavior is unique and represents an intermediate entity with clinical features that are between C-PTC and FTC. Interestingly, despite the variations in clinical behavior, the long-term outcome of these patients remains excellent and similar to C-PTC.