*2.4. Pathologic Examination*

The surgical specimen was fixed with formaldehyde; sections were stained with hematoxylin-eosin (H&E) and analyzed by a dedicated pathologist. Immunohistochemical analysis was performed with the streptavidin-biotin technique on paraffin sections using anti-pan-cytokeratin antibody.

In cases of multifocal carcinoma, the tumor size was defined as the largest diameter among the malignant nodules. A microcarcinoma was defined as a tumor with larger diameter equal or inferior to 10 mm. Extrathyroidal extension was defined as the presence of gross infiltration of perithyroidal tissues found in pathological examination. Vascular invasion was defined as invasion of vessels in the tumor capsule or beyond it, with intravascular tumor cells attached to the vessel wall.

The lymph node yield was defined as the number of lymph nodes harvested after lymphectomy, and lymph node ratio was defined as the ratio between metastatic lymph nodes and total lymph nodes retrieved, calculated only in patients with metastatic lymph nodes.

Lymph node micrometastasis was defined as a metastasis with maximum dimension ≤0.5 cm.
