**1. Introduction**

The extent of cancer at time of diagnosis is a key factor to assess the chance of successful treatment outcome [1]. In papillary thyroid cancer (PTC), there is general consensus that gross extrathyroidal extension (ETE) affects prognosis [2–4]. However, there has been considerable debate over the years regarding the most appropriate treatment for patients with minimal ETE (mETE), defined as extension to perithyroidal soft tissue or sternothyroid muscle [5–9]. Since January 2018, the new eighth edition of the TNM staging system (published in October 2016) has been used to classify patients with differentiated thyroid cancer (DTC) and predict disease mortality. Several substantial modifications were made to the seventh Union for International Cancer Control/American Joint Committee on Cancer (UICC/AJCC) tumor-node-metastasis (TNM) staging system to improve prognostic power, guide best treatment, and change towards the current trend of "personalized medicine" and risk adapted therapy concepts [10,11].

As a key change, mETE detected only on histological examination is no longer a determinant of the T-stage within the TNM classification [12]. The former T3 category included any tumor with >4 cm limited to the thyroid gland or tumors of any size with minimal ETE. In the updated eighth AJCC/TNM staging system, DTC with tumor size of ≤4 cm limited to thyroid gland is staged T1/2-disease regardless of the presence of mETE. Considering the initial risk stratification proposed by the 2015 American Thyroid Association (ATA), the presence of mETE alone upstages low-risk patients to the intermediate-risk group [11]. Therefore, the presence of mETE has a direct impact on clinical patient management.

The impact of mETE on the clinical outcome is still a matter of debate. Some studies suggest that mETE has no impact on the disease-free survival [6,8,13,14], whereas others could not confirm this data [15–17] and report a worse outcome in these patients. These studies focused on long-term outcomes. Undoubtedly, long-term outcomes remain the most crucial endpoint, but on a day to day basis, initial presentation of patients influences clinical decisions, and early response to therapy determines the clinical follow-up examinations (e.g., dynamic risk stratification according to the latest ATA guidelines).

Therefore, the aim of the present study was to investigate the initial clinical presentation and the effect on early response rates in PTC patients treated at our institute, which were reclassified as PTC T1/2 with mETE (+mETE) according to the updated eighth TNM staging system and compared to a control group consisting of PTC T1/2 without mETE (−mETE).
