Which lymph node is the sentinel node for a given tumor is determined by injecting around the tumor a tracer substance that will travel through the lymphatic system to the first draining (sentinel) node and identify it. The tracer substance may be blue dye that can be visually tracked or a radioactive colloid that can be radiologically followed.
Biopsy of the sentinel lymph node can reveal whether there are lymphatic metastases. If the sentinel node contains tumor cells, removal of more nodes in the area may be warranted. If the sentinel node is normal, this obviates the need for extensive dissection of the regional lymph-node basin.