Prognostic Significance of the Proportion of Tall Cell Components in Papillary Thyroid Carcinoma

Tall cell variant (TCV) of papillary thyroid carcinoma (PTC) shows a poorer prognosis than conventional PTC. The World Health Organization (WHO) classification defines TCV as the tall cell component (TCC) in ≥50% of PTC lesions. We investigated whether and how the proportion of TCC affects the prognosis of patients with PTC with TCC.

Seventy patients with TCC in ≥30% of their PTC lesions and 210 age- and gender-matched controls with no TCC who underwent locally curative surgery at Kuma Hospital (2006-2014) were enrolled. The 70 PTC patients were divided into two categories: TCC ≥50% (TCC-major, n = 19) and TCC 30-49% (TCC-minor, n = 51). We performed univariate and multivariate analyses of the relationships between disease-free survival (DFS) and variables including the TCC proportion in 276 patients who had no distant metastases at surgery (median follow-up 64 months).

In the univariate analysis, TCC-major, TCC-minor, old age (≥65 years), clinical node metastasis, significant extrathyroid extension (Ex), and high Ki-67 labeling index (≥5%) significantly affected the DFS. In the multivariate analysis, TCC-major and Ex independently affected the DFS, but TCC-minor did not. In an analysis excluding TCC-major patients, TCC-minor was not an independent prognostic factor for DFS.

Studies or larger patient series with longer follow-ups are necessary, but we speculate that in PTC with TCC, TCC-major significantly and independently affects the DFS, whereas TCC-minor does not. Our findings indicate that the WHO definition of TCV is appropriate and that the prognostic impact of TCC-minor is limited.