**1. Introduction**

Differentiated thyroid cancers (DTC) account for over 90% of all thyroid cancers and are further divided into papillary thyroid carcinomas (PTC), follicular thyroid carcinomas (FTC), and Hürthle cell carcinomas. PTC is by far the most common subtype, accounting for 85% of all DTC [1].

### **1.1 Incidence of lymph node metastasis in DTC**

The incidence of lymph node metastasis is different between various types of DTC. PTC is associated with higher frequency of nodal metastasis [2]. On the contrary, FTC seldom metastasizes through the lymphatics [3]. The incidence of nodal metastasis in PTC depends on its detection method and definition. Preoperative imaging can detect nodal disease in up to 30% of patients [4] whereas pathological series reveal nodal metastasis in 20−50% of operated patients [5]. With the replacement of traditional hematoxylin and eosin staining by modern immunohistochemical or molecular genetic techniques, the incidence of nodal micro-metastasis (defined as tumor foci <2 mm) has been reported in up to 90% of patients [6]. Hence nodal metastasis is very common in DTC, particularly amongst patients with PTC and is often underestimated.
