*2.1. Patient Specimen*

This retrospective cohort study was performed at Sinai Health System, Mount Sinai Hospital (MSH), a University of Toronto-affiliated hospital and a prime referral center for patients with thyroid disorders in Toronto, Canada. A total of 1859 patients who previously underwent thyroidectomy in MSH between 2010 and 2015 were identified following the approved guidelines of the Sinai Health System Research Ethics Board (REB #07-0212-E). The pathologist (OP) reviewed the patient clinical charts, surgical pathology reports and H&E stained sections [25]. The final diagnosis was made in accordance with the WHO's Classification of Tumors of Endocrine Organs [36], CAP protocol [7,37] and Endocrine Pathology Society Working Group [11]. After the histopathological evaluations based on strictly defined inclusion and exclusion criteria [11,23], a total of 165 archived formalin-fixed paraffin-embedded (FFPE) tissue blocks of thyroid tumors (≥1.0 cm) retrieved from the MSH Tumor Bank were classified into four specific diagnostic subgroups: 42 cases of benign nodules, 41 NIFTPs, 45 invasive EFVPTCs and 37 cases of noninvasive encapsulated FVPTC lesions with co-existing significant lymphocytic/Hashimoto's thyroiditis (LT) [25,37].
