Definitions
Tracheal invasion stage I (Shin I) was defined as disease invading through the capsule of the thyroid gland and abutting but not invading the external perichondrium of the trachea.[14] Tracheal invasion stage II (Shin II) was defined as invasion of the disease between the rings of the cartilage or as invasion such as to cause minor cartilage destruction.[14] Minimal ETE was defined as peri-thyroidal fat involvement diagnosed on final pathology without strap muscle or neck structure invasion according to the latest revision (8th edition) of the AJCC staging manual.[15] Gross ETE was defined according to the updated AJCC as including any strap muscle, subcutaneous soft tissue, larynx, trachea, esophagus, and RLN involvement.[15] Margins were considered R0 if frozen section analysis and/or final pathology confirmed that the soft tissue or cartilaginous margins were tumor-free.[16] Margins were considered microscopically invaded (R1) if histological analysis showed tumor at the margin of the operating specimen or <1 mm from the margin.[16] Dosimetry parameters of the delivered RAI were retrieved from treatment plans.
Local control was defined according to the 2015 American Thyroid Association Management Guidelines for Adult Patients with Thyroid Nodules and Differentiated Thyroid Cancer:[17] An “excellent” response was defined as no clinical, biochemical, or structural evidence of disease. A “biochemical incomplete” response was defined as abnormal thyroglobulin or rising anti-thyroglobulin antibody levels in the absence of localizable disease. A “structural incomplete” response was defined as persistent or newly identified loco-regional or distant metastases. Event-free survival was defined as survival without macroscopic local, regional, or distant disease visible on imaging, with or without persistent stable biochemical disease. Disease-free survival (DFS) was defined as survival without any evidence of disease at any site. OS was defined as death due to any cause. The treatment termination date was used as time point zero.