Lymph Node Dissection in Thymic Malignancies: Implication of the ITMIG Lymph Node Map, TNM Stage Classification, and Recommendations

Abstract

ObjectivesThe aim of this study is to investigate the pattern of lymphatic metastasis and suggest a lymph node dissection (LND) strategy for thymic malignancies. MethodsWe retrospectively reviewed 131 thymic malignancy patients who had undergone LND. The recently introduced nodal map of the International Thymic Malignancy Interest Group/International Association for the Study of Lung Cancer and the TNM (tumor, node, metastasis) stage classification were used for grouping and staging the lymph nodes. The pattern of lymphatic metastasis and factors in lymphatic metastasis were investigated. ResultsNode metastasis was detected in 13 patients (N1 in six and N2 in seven). Six N2 patients (86%) had right paratracheal node (RPN) metastases. The rates of node metastasis were 1% at T1 and 37.5% at T2/3 (p < 0.001). The rates of node metastasis were 8% in the M0 and 43% in the M1 (p = 0.03). The rate was higher for thymic carcinoma (25%) than for thymoma (5.1%, p = 0.01), and the rates also differed between the subtypes of thymoma. There was no node metastasis of the A, AB, or B1 types. Tumor size was also a significant factor in node metastasis. The optimal cutoff value for the node metastasis was 6 cm and the specificity was 62%. Only 16% of the patients had received a preoperative histologic diagnosis. All patients with node metastasis had TNM stage II or higher thymic malignancy. The freedom from recurrence rate of the pN1/2 was significantly worse than that of the pN0 (5-year rate 38.5% versus 87.9%, p < 0.001). ConclusionA status of stage II or higher was the most specific predictor of node metastasis, and the RPN was a crucial station for lymphatic metastasis in thymic malignancies. Thus, LND including RPN is recommended in stage II or higher thymic malignancies.