Impact of location and size of minimal extrathyroidal extension on lymph node metastasis in papillary thyroid cancer: a retrospective analysis.

Background: The 8th edition of the American Joint Committee on Cancer (AJCC)'s T-stage for differentiated thyroid cancer (DTC) removes minimal extrathyroidal extension (mETE), while ignoring the risk of mETE would lead to overtreatment or inadequate treatment. The aim of this study was to investigate the impact of location and size of mETE on lymph node metastasis in papillary thyroid cancer (PTC).

Methods: A retrospective analysis of 267 patients who underwent unilateral radical surgery for PTC was conducted. According to the postoperative pathology, they were divided into mETE group (121 patients) and non-mETE group (146 patients). The number of lymph nodes dissected and the number of lymph nodes metastasized were compared between the two groups. The linear regression analysis and the receiver operating characteristic (ROC) curves were performed to evaluate the impact of the locations and sizes on lymph node metastasis.

Results: There was no significant difference in the number of lymph node dissected between the mETE group and the non-mETE group. The tumor located at the upper part and the size <1.0 cm in mETE group showed a higher number of lymph node metastasis (0.78±0.88 0.25±0.45, P=0.03). Meanwhile, in the mETE group, the number of patients with lymph node metastasis was higher than that in the non-mETE group. Further subgroup analysis revealed that for PTC patients with tumors at the upper part and size <1.0 cm, the number of those with lymph node metastasis in the mETE group was also greater than that in the non-mETE group. Furthermore, the Spearman correlation analysis showed a positive correlation between tumors located at the upper part with a size <1.0 cm and lymph node metastasis rate (R=0.647, P=0.004). Additionally, if the upper part tumor was within 1 cm, the tumor's size was able to identify the lymph node metastasis, with the optimal cut-off point of 0.45 cm (Youden index =0.650).

Conclusions: When tumors combine with mETE, the probability of lymph node metastasis increases in tumors located at the upper part with a size <1.0 cm. Especially, when the upper part tumor is within 1 cm, the tumors of size ≥0.45 cm are more likely to have lymph node metastasis.