Development and external validation of a nomogram to predict four or more positive nodes in breast cancer patients with one to three positive sentinel lymph nodes.

•Five predictors of four or more positive nodes in breast cancer patients were identified.•A nomogram was built using these five factors.•The nomogram was validated on an external cohort.•The proposed nomogram predicts four or more positive nodes with high accuracy.•The nomogram can help in decision making on use of regional nodal irradiation.