Background: The recently published eighth edition of the American Joint Committee on Cancer (AJCC) staging system has emphasized the importance of gross extrathyroidal extension (gETE) while classifying the tumor (T) stage in differentiated thyroid carcinoma (DTC). However, the clinical impact of gETE invading only the strap muscles or the recurrent laryngeal nerve (RLN) remains unclear due to scarce and conflicting data.
Methods: A retrospective cohort study was carried out in patients with DTC who underwent thyroid surgery from 1996 to 2005. In total, 3104 patients were included, and disease-specific survival (DSS) was compared according to the degree of gETE, with a median follow-up duration of 10 years.
Results: Patients with gETE invading only the strap muscles and with a tumor size ≤4 cm (T3b [≤4 cm]) showed no difference in DSS compared to patients with T2 stage disease (hazard ratio [HR] = 0.81 [confidence interval (CI) 0.24-2.77]; p = 0.737) but rather showed a better DSS than patients with T3a disease (HR = 0.19 [CI 0.05-0.72]; p = 0.014). Conversely, patients with gETE invading to the posterior direction showed significantly poorer DSS than patients with T3 stage disease, even when only the RLN was invaded (HR = 7.78 [CI 3.41-17.75]; p < 0.001). However, there was no difference in DSS between gETE invading only the RLN and that invading other posterior organs beyond the RLN (p = 0.563). A modified T classification was suggested to downgrade patients with T3b (≤4 cm) disease to the T2 stage, which revealed higher predictability of survival than the T classification according to the eighth edition of the American Joint Committee on Cancer tumor-node-metastasis staging system (proportion of variation explained: 3.6% vs. 2.65%).
Conclusions: gETE invading only the strap muscles did not significantly affect DSS, while that invading the posterior organs significantly affected DSS, even when only the RLN was invaded. The data support the applicability of downgrading patients with T3b (≤4 cm) disease to the T2 stage for a better predictability of survival.
Keywords: AJCC; TNM-8; differentiated thyroid carcinoma; gross extrathyroidal extension; survival.