*2.5. Local Recurrence and Metastatic Lymph Nodes*

The gold standard for locally recurrent DTC or nodal metastases was determined according to one of the following criteria: (1) histopathological diagnosis of recurrence or metastasis; (2) clinical serum Tg and/or TgAb levels increased continuously during the follow-up of more than 6 months and imaging (neck ultrasound, CT, and/or MR) revealing following malignant features simultaneously.

Ultrasound criteria: nodules or lymph nodes were considered malignant if the short axis diameter was ≥10 mm in levels I–II or ≥7 mm in levels III–VI, the volume increased more than 50%, or the diameter increased more than 20% or 2 mm. In addition, other signs of malignancy, including spherical or long-to-short axis ratio < 2, absence of an echogenic hilum, microcalcification and cystic changes, could classify the node as malignant, regardless of the size of the lymph node [28–31].

CT or MR criteria: recurrence was determined by size and abnormal density/signal with irregular edges or blurred boundaries on CT/MR; for nodules or lymph nodes, they were perceived as malignant if the maximum axial diameter was ≥8 mm in the retropharyngeal space, ≥15 mm in levels I–II or ≥10 mm in levels III–VII, or the volume increased more than 50%, or the diameter increased more than 20% or 2 mm [32,33]. In addition, other signs of malignancy, including central necrosis, contrast enhancement, intralesional calcifications and cystic changes, could classify the nodule as malignant, irrespective of nodal size [34].
