Prognostic value of the 8 tumor-node-metastasis classification for follicular carcinoma and poorly differentiated carcinoma of the thyroid in Japan.

Follicular thyroid carcinoma (FTC), a form of differentiated thyroid carcinoma, is the second most common malignancy arising from thyroid follicular cells. Recently, the tumor-node-metastasis (TNM) classification for differentiated thyroid carcinoma was revised from the 7 to the 8 edition. The diagnostic criteria for poorly differentiated carcinoma (PDC) were also updated in the latest World Health Organization (WHO) classification. In this study, we investigated whether these changes are appropriate for accurately predicting prognosis. Three hundred and twenty-nine patients diagnosed with postoperative pathologically confirmed FTC, who underwent initial surgery at our hospital between 1984 and 2004, were enrolled. For this study, patients were re-evaluated and diagnosed with FTC (N = 285) or PDC (N = 44) without typical nuclear findings of papillary thyroid carcinoma. For FTC, the 8 TNM classification was a more accurate predictor of prognosis than the 7 TNM classification. In the 8 TNM classification, cause-specific survival became significantly poorer from Stage I to IVB. The cause-specific survival of PDC based on the latest WHO classification was worse than, but did not significantly differ from, that of PDC based only on the former WHO classification. For PDC, neither of the TNM classifications could accurately predict prognosis. Taken together, we conclude that (1) the 8 TNM classification more accurately reflects the prognosis of FTC than the 7 TNM classification; (2) PDC based on the former WHO classification should be retained, at least in Japan; and (3) the TNM classification may not be suitable for predicting the prognosis of PDC.