Papillary Thyroid Cancers of the Thyroid Isthmus: The Pattern of Nodal Metastasis and the Significance of Extrathyroidal Extension

Background Cancers of the thyroid isthmus are less frequent compared with the lobar cancers yet have their own unique clinicopathological characteristics. Herein, we sought to examine the isthmic papillary thyroid carcinomas (PTCs) for the pattern of and risk factors for nodal metastasis and the significance of extrathyroidal extension (ETE). Methods The medical records of 3138 patients diagnosed with solitary PTC who had undergone surgery were retrospectively reviewed. Of these, 122 isthmic PTCs were matched to common lobar PTCs at a ratio of 1:3 for age, sex, and nodule size. Patient demographics, surgical findings, and pathology reports were analyzed. Results Isthmic PTCs comprised 4.6% of all PTCs and had more lymphatic invasion (22.1% vs. 13.4%, p = 0.021), ETE (73.0% vs. 57.1%, p = 0.002), and perithyroidal and prelaryngeal node metastasis (18.0% vs. 9.0%, p = 0.006) compared with lobar PTCs. However, there were no significant differences in the rate of central and lateral node metastasis between the two groups. ETE was identified not to be a risk factor for isthmic PTCs for central and lateral node metastasis, in contrast to lobar PTCs in which ETE was a significant risk factor (odds ratio 3.18, 95% confidence interval [CI] 1.89-5.34; and 4.72, 95% CI 1.04-21.41). Conclusion The rates of central and lateral node metastasis of isthmic PTCs are comparable with that of lobar PTCs despite a higher rate of ETE in the isthmic counterpart. Although the extent of surgery for isthmic PTCs remains to be investigated, careful dissection of perithyroidal and prelaryngeal nodes may be necessary for isthmic PTCs.