RESUMO
TNM 8th edition introduces changes in the staging of patients with differentiated thyroid carcinoma (DTC). This study aims at assessing the value of TNM 8th edition in predicting response to therapy and structural recurrence of DTC. Four hundred and eighty DTC patients were retrospectively evaluated by 7th and 8th editions of TNM staging system in relationship with risk stratification, response to therapy and recurrence of disease as defined by 2015 ATA guidelines. As compared to the 7th edition, TNM 8th led to downstage 136 patients (28.3%), with 97.5% of patients falling into lower stages (I-II) and only 2.5% remaining in higher stages (III-IV) (P < 0.001). Patients who were downstaged in stages I-II by TNM 8th were classified more frequently at intermediate-high risk (P < 0.001), had more frequently structural incomplete response to therapy (P = 0.009) and had higher risk of structural recurrence (P = 0.002) as compared to patients who were in the same TNM stages but were not downstaged. Specifically, the risk of structural recurrence was significantly higher in patients in whom the downstaging was induced by changes in tumour classification (hazard ratio (HR) 6.18, 95% CI 2.20-17.40; P = 0.001) but not in those who were downstaged for the increase in age cut-off (HR 2.80, 95% CI 0.86-9.19; P = 0.09). In conclusion, TNM 8th edition did not show reliability in predicting aggressiveness of DTC. In fact, the downstaging of DTC patients especially when performed due to changes in tumour classification may overlook patients predisposed to structural recurrence, potentially causing uncertainty in the therapeutic decision-making at the time of disease's diagnosis.