The sentinel lymph node is the first node(s) draining the tumour: tumour cells spread to lymph nodes in an orderly way, so the first draining node is identified (blue dye and/or radioisotope with a gamma probe) and biopsied. If it is negative, axillary dissection is avoided. It is not simply any axillary node or the node physically nearest the cancer, and it is defined before (not after) axillary dissection.