Objective  This study aims to analyze the occurrence rate, positive rate, and other related factors influencing interpectoral lymph nodes (IPNs) in breast cancer patients. This work further aims to explore the significance and indications of the surgical dissection of IPNs.

  Methods  Clinical and pathological data from 1673 breast cancer patients were retrospectively analyzed. All patients were subjected to modified radical mastectomy, and IPNs were pathologically examined. The occurrence rate and metastasis of IPNs were recorded, and the relationship between the IPN positive rate and tumor size, axillary nodes, clinical stages, neo-adjuvant chemotherapy, hormone receptors, Her-2 expression, and molecular subtypes of breast carcinoma was determined.

  Results  The occurrence rate, overall metastasis rate, and the positive rate of IPNs in patients with axillary lymph node metastasis were 13.39%, 4.30%, and 10.01%, respectively. IPN metastasis was significantly correlated with axillary node metastasis and the tumor, node and metastasis (TNM) stage of tumors (P < 0.05). However, IPN metastasis was not significantly related with hormone receptor and Her-2 expressions. IPN metastasis rate may be unaffected by neo-adjuvant chemotherapy. Patients with IPNs metastasis were characterized by larger tumors, more positive axillary lymph nodes, and later TNM stages.

  Conclusion  IPN metastasis usually occurs in patients with larger tumors, more positive axillary lymph nodes, later TNM stages, as well as those with locally advanced cancer that meet the standard of neo-adjuvant chemotherapy. These indications suggest that the surgical dissection and pathological examination of IPNs should be routinely performed.