Sentinel node biopsy is a surgical procedure used to determine if cancer has spread beyond a primary tumor into your lymphatic system. Sentinel node biopsy is used most commonly in evaluating breast cancer and melanoma.The sentinel nodes are the first few lymph nodes into which a tumor drains. Sentinel node biopsy involves injecting a tracer material that helps the surgeon locate the sentinel nodes during surgery. The sentinel nodes are removed and analyzed in a laboratory. If the sentinel nodes are free of cancer, then cancer isn't likely to have spread and removing additional lymph nodes is unnecessary.If, after sentinel node biopsy, evaluation of the sentinel nodes reveals cancer, then you'll likely need additional lymph nodes removed for your doctor to determine how far the cancer has spread.