The follicular variant of papillary thyroid carcinoma (FV-PTC) represents a unique clinical entity with features that lie between classic papillary thyroid carcinoma (C-PTC) and follicular thyroid carcinoma (FTC), say the authors of a study published in Thyroid.

Despite an increase in the number of diagnoses of FV-PTC, the most common subtype of PTC, the clinical behaviour and outcome of this tumour type are not well known. “FV-PTC contains a mixed histology of both PTC and FTC; it was speculated that FV-PTC might therefore exhibit mixed clinical features and be more aggressive than C-PTC,” says lead investigator Xiao-Min Yu (University of Wisconsin). In a population-based, retrospective cohort study, Yu and co-workers assessed differences in tumor behaviour and patient survival in 21,796 patients with C-PTC, 10,740 with FV-PTC and 3,958 with FTC.

The investigators found that extrathyroidal extension and lymph-node metastases occurred with the highest frequency in patients with C-PTC, with intermediate frequency in those with FV-PTC, and were least common in FTC cases. The percentage of patients with distant metastases in FV-PTC (2%) was between that of C-PTC (1%) and of FTC (4%).

Overall and disease-specific survival did not differ significantly between patients with FV-PTC and those with C-PTC; both groups showed slightly higher overall survival rates than patients with FTC. Age >45 years was the strongest risk factor for disease-specific mortality in patients with FV-PTC and in those with C-PTC.

Yu et al. conclude that, despite variations in clinical behaviour, the long-term outcome of patients with FV-PTC remains excellent and similar to that of patients with C-PTC. However, extrathyroidal extension or distant metastases were associated with a higher risk of disease-specific mortality in individuals with FV-PTC than in patients with C-PTC after adjustment for surgical (total thyroidectomy) and radiological intervention (radioactive iodine ablation). “This finding suggests that FV-PTC is not biologically identical to C-PTC,” explains Yu. “Thus, for these high-risk FV-PTC patients, prophylactic lymph-node dissection, close monitoring after initial surgery and other interventions should be considered.”