Lymph node, sentinel: The first lymph node to receive lymphatic drainage from a tumor. The sentinel node for a given tumor is found by injecting a tracer substance around the tumor. This substance then travels through the lymphatic system to the sentinel node. The tracer substance may be a blue dye that can be tracked visually or a radioactive colloid that can be followed radiologically. Biopsy of the sentinel lymph node can reveal whether cancer has spread through the lymphatic system. If the sentinel node contains tumor cells, removal of more nodes in the area may be warranted.