When a patient is given the diagnosis of cancer, the first question to be answered is "has the cancer spread from the primary site (breast, colon, lung, etc.), or is it still contained in that area"? The answer to that question will have implications in the prognosis of the patient and the treatment prescribed. Determining whether a tumor has spread, or evaluating the extent of the cancer throughout the body, is called "staging". There are multiple ways to analyze whether the tumor has spread, or "metastasized" from the primary site. The size of the tumor, for example, might give clues as to the likelyhood of metastasis. But for most solid tumors, the most powerful indicator of metastatic spread is the presence or absence of tumor in the regional lymph nodes.

The lymphatic system can be thought of as a secondary circulatory system (the primary circulatory system being the arterial-venous system). The lymphatic system collects fluid which escapes from cells, arteries, and veins, and returns that fluid, called "lymph", back to the heart. The lymph nodes serve as filters in the lymphatic system, which filter any foreign bodies from the lymph before it returns to the heart. These foreign bodies may be bacteria, viruses, etc., or even cancer cells.

As tumors secrete fluid, and also shed cells (metastasize), the lymphatic system is a likely reservoir for those fluids and cancer cells to deposit. Thus, if the lymph nodes in the area of the tumor can be localized, surgically removed, and evaluated by pathology to determine if there are any cancer cells trapped there, the extent of the spread of the cancer can be determined.

First reported in 1977, a revised method of obtaining this staging information has been developed, and has gained broad acceptance in the surgical community. This method involves localization of the lymph node that is first in the lymph node chain to receive lymphatic drainage from a primary tumor site. This first draining node is referred to as the "sentinel" lymph node (SLN), and it has been found to be a highly accurate indicator of the metastatic involvement of the entire lymph node basin in which it is situated. In short, if the Sentinel Node doesn't contain cancer, then the rest of the lymph nodes are also negative for cancer. What this means is that instead of removing all (10-25) the lymph nodes in the lymphatic basin in the area of the tumor, only the Sentinel Node is necessary to remove.

The localization, excision, and pathologic evaluation of the sentinel node for the purpose of gaining cancer staging information is called "Sentinel Node Biopsy" (SNB), or interchangebly, "Intraoperative Lymphatic Mapping" (ILM). In theory, the histology of the SLN should be highly predictive of the remainder of the lymphatic basin, a theory which has been proven clinically for melanoma and breast cancer.

The first technique investigated to localize the SLN involved the injection of LYMPHAZURIN* 1% Blue Dye (a USS product). The dye flows into the lymphatic basin which receives lymphatic drainage from the tumor, and the nodes are stained blue. The surgeon then proceeds to carefully dissect the nodal basin, and visually identify blue-stained lymphatic vessels. These lymphatic vessels are followed to the SLN, the first blue node in the lymphatic chain from the tumor. The SLN is then excised, and its histologic status is determined by pathology.

The second technique introduced a radiocolloid (Tc99m Sulfur Colloid) as the agent injected around the tumor. This radioactive tracer flows into the lymphatics in the same manner as the blue dye, and deposits into the SLN. The advantage to this method is that a gamma detection probe is utilized to detect and mark the SLN at the skin (pre-incision) and also intra-operatively. This method allows a small incision to be used, since unlike the blue dye, no visual search for nodes is necessary. Once localized, the SLN is excised and it's histologic status determined.