Thyroid cancers with benign-looking sonographic features have different lymph node metastatic risk and histologic subtypes according to nodule size.

A decision to perform fine needle aspiration (FNA) on thyroid nodules mainly depends on sonographic features. We investigated if lymph node metastasis (LNM) risk differed by tumor size of thyroid cancers without suspicious sonographic features. Three hundred sixty patients with thyroid cancers with benign looking sonographic features were grouped by nodule size on ultrasonography (US) (≤ or >1 cm). The clinicopathologic parameters were compared between the groups. A multivariate analysis was performed to discover the independent factors predicting the presence of LNM. The nodules greater than 10 mm on US (n = 157) demonstrated a larger tumor size on histology (17.9 ± 14.5 vs. 5.6 ± 2.4 mm, P < 0.001), a lower frequency of classical papillary thyroid carcinoma (PTC) (58.6 vs. 87.2 %, P < 0.001), and a higher frequency of follicular variant PTC and follicular thyroid carcinoma (19.7 and 17.8 % vs. 9.4 and 1.5 %, respectively, P < 0.01). In subgroup analysis of 269 patients with classical PTC, the larger nodule size on US was associated with a higher prevalence of LNM (28.3 vs. 14.7 %, P = 0.007). A multivariate analysis revealed that classical PTC, extrathyroidal extension, and the US nodule size >10 mm were independent predictive factors of LNM after adjusting for age, sex, TSH level, and multifocality. Thyroid cancers larger than 10 mm with benign US features are more likely to be nonclassical PTC than those with smaller diameters. The larger ones also have an increased risk of LNM in classical PTC. These cases require a more aggressive approach to FNA.