If a patient has been diagnosed with melanoma, a sentinel lymph node biopsy may be performed to see if the disease has spread to nearby lymph nodes at a microscopic level (i.e., the lymph nodes are not enlarged on the exam but contain small numbers of melanoma cells that have spread from nearby skin). Sentinel lymph nodes are the first nodes within a lymph node group in a region that drain fluid from the cancer site. The purpose of a sentinel lymph node biopsy is to find these nodes, remove them and check them for cancer cells. The surgeon injects a radioactive liquid and a blue dye into the melanoma site. The surgeon examines the lymph nodes for radioactivity to identify which are the first to drain fluid from the cancer site.

The surgeon makes a small incision in the area where the sentinel lymph nodes have been identified. The ones that have absorbed radiation and turned blue are the sentinel lymph nodes. They are removed and sent to the pathology lab for further examination under a microscope. If the sentinel nodes contain cancer, the surgeon will advise the removal of all other lymph nodes within the region. If there are no cancer cells present in the sentinel nodes, it is assumed there is no spread to the remainder of the lymph nodes in that region, and no further surgery is necessary. If a lymph node near the melanoma is enlarged, the test may be skipped altogether and the surgeon may simply remove the enlarged node (excisional lymph node biopsy).