*4.1. Study Population*

We retrospectively reviewed consecutive patients with pT1-T3 PTC from our institutional data base who underwent total thyroidectomy followed by RAI therapy at the Department of Nuclear Medicine (University Hospital, LMU Munich) between January 2010 and June 2015. Epidemiological and clinical features of these patients were assessed (age at diagnosis, gender, TNM stage, tumor size, presence of ETE, resection margins). Patients initially classified as PTC T3 solely due to mETE (according to the seventh UICC/AJCC TNM staging system) were reclassified according to the updated eighth UICC/AJCC TNM staging system into the subgroup of T1/2 +mETE. These patients were compared to a control group consisting of PTC patients classified as T1/2 −mETE according to former and updated TNM classifications. Pathological reports from the referring hospitals were reviewed and reclassified according to the updated TNM staging system. Since the majority of PTC cases comprise classical and follicular variants of PTC, we only investigated these histological subgroups in our study. Patients with aggressive histological subtypes of PTC, distant metastases diagnosed from clinical examination or imaging (cM1-stage), or unresectable carcinomas or positive resection margins (R1/R2-stage) were excluded.

For our study, a total of 1140 patients were screened, and 638 patients with PTC were evaluated. A total of 164 patients were retrospectively reclassified as T1/2 with mETE (T1/2 +mETE); 346 T1/2 patients without mETE (T1/2 −mETE) served as a control group. A total of 46 patients remained T3 according to former and updated TNM staging systems and were consequently excluded, and 24 patients were excluded due to aggressive histological subtypes of PTC. A total of 455 patients fulfilled the inclusion criteria. The majority of patients (300/455, 65.9%) showed classical variant of PTC. For subgroup analysis of N0/Nx patients, we defined N0- and Nx-status according to Robinson et al. Patients with T1-stage were staged as N0 after examination of ≥6 lymph nodes and patients with T2 after ≥9 lymph nodes [28]. All other patients were staged Nx.
