Methods
Subgroup analysis was performed on three distinct groups (staging in brackets is according to the AJCC 8th edition on thyroid cancer):
  1. Gross ETE involving only strap muscles (T3b)
  2. Gross ETE with minimal tracheal invasion with or without strap muscle involvement (T4a)
  3. Gross ETE with involvement of neck structures (RLN, larynx, esophagus, subcutaneous tissue) with or without tracheal invasion (T4a)