Sentinel lymph node biopsy, a procedure that is part of staging, determines whether cancer has spread to the lymph nodes during staging or how far the cancer has spread. Different cancer stages require different treatments. This test is mostly performed while removing the original (primary) tumor. However, sometimes specialists may perform a sentinel lymph node biopsy before or after removing the primary tumor. Sometimes it is used to assess axillary lymph nodes after neoadjuvant chemotherapy.