Does macroscopic extrathyroidal extension to the strap muscles alone affect survival in papillary thyroid carcinoma?

Macroscopic extrathyroidal extension to structures adjacent to the thyroid gland is one of the most important predictors of survival in papillary thyroid carcinoma. However, the prognostic significance of macroscopic extrathyroidal extension to strap muscles alone is unknown. The aim of this study is to determine the impact on survival in patients with macroscopic extrathyroidal extension to strap muscles alone compared to those with no macroscopic extrathyroidal extension and macroscopic extrathyroidal extension involving other adjacent structures.

After institutional review board approval, adult papillary thyroid carcinoma patients were identified from an institutional database of 6,259 patients undergoing initial surgery for well-differentiated thyroid carcinoma from 1986 to 2015. Patients were classified as having no macroscopic extrathyroidal extension, macroscopic extrathyroidal extension to strap muscles alone, or macroscopic extrathyroidal extension to other adjacent structures. Disease-specific survival was calculated using the Kaplan-Meier method and groups were compared using the log-rank test. A P value < .05 was considered statistically significant and significant factors were used in a Cox proportional hazard model to predict disease-specific survival.

There were 5,880 patients included in the analysis; 5,485 patients (93.3%) in the no macroscopic extrathyroidal extension group, 179 (3.0%) in the macroscopic extrathyroidal extension to strap muscles alone group and 216 (3.7%) in the macroscopic extrathyroidal extension involving other adjacent structures group. With a median follow-up of 64 months, the estimated 10-year disease-specific survival for patients with no macroscopic extrathyroidal extension, macroscopic extrathyroidal extension to strap muscles alone, and macroscopic extrathyroidal extension involving other adjacent structures were 98.9%, 95.7%, and 83.7%, respectively (P < .0001). In the ≥55-year-old cohort, the estimated 10-year disease-specific survival for patients with no macroscopic extrathyroidal extension, macroscopic extrathyroidal extension to strap muscles alone, and macroscopic extrathyroidal extension involving other adjacent structures were 97.6%, 89.3%, and 68.1%, respectively (P <.0001). After controlling for pathological nodal stage and distant metastasis stage, extent of extrathyroidal extension remained an independent predictor of disease-specific survival; patients with macroscopic extrathyroidal extension to strap muscles alone had a 3.3-fold increased likelihood of a disease-specific death compared to no macroscopic extrathyroidal extension patients (hazard ratio 3.294; 95% confidence interval 1.076-10.086, P < .0368).

In our study, patients aged ≥55 years with papillary thyroid carcinoma and macroscopic extrathyroidal extension to strap muscles alone appear to have an increased likelihood of a disease-specific death compared to patients with no macroscopic extrathyroidal extension.