Sentinel lymph node

The sentinel lymph node is the hypothetical first lymph node or group of nodes draining a cancer. In case of established cancerous dissemination it is postulated that the sentinel lymph node/s is/are the target organs primarily reached by metastasizing cancer cells from the tumor. Thus, sentinel lymph nodes can be totally void of cancer because they were detected prior to dissemination.

The sentinel node procedure is the identification, removal and analysis of the sentinel lymph nodes of a particular tumour.

The spread of some forms of cancer usually follows an orderly progression, spreading first to regional lymph nodes, then the next echelon of lymph nodes, and so on, since the flow of lymph is directional, meaning that some cancers spread in a predictable fashion from where the cancer started. In these cases, if the cancer spreads it will spread first to lymph nodes (lymph glands) close to the tumor before it spreads to other parts of the body. The concept of sentinel lymph node surgery is to determine if the cancer has spread to the very first draining lymph node (called the "sentinel lymph node") or not. If the sentinel lymph node does not contain cancer, then there is a high likelihood that the cancer has not spread to any other area of the body.

The concept of the sentinel lymph node is important because of the advent of the sentinel lymph node biopsy technique, also known as a sentinel node procedure. This technique is used in the staging of certain types of cancer to see if they have spread to any lymph nodes, since lymph node metastasis is one of the most important prognostic signs. It can also guide the surgeon to the appropriate therapy.

There are various procedures entailing the sentinel node detection:

In everyday clinical activity, entailing sentinel node detection and sentinel lymph node biopsy, it is not requested to include all different techniques listed above. In skilled hands and in a center with sound routines, one, two or three of listed methods can be considered sufficient.

To perform a sentinel lymph node biopsy, the physician performs a lymphoscintigraphy, wherein a low-activity radioactive substance is injected near the tumor. The injected substance, filtered sulfur colloid, is tagged with the radionuclide technetium-99m. The injection protocols differ by doctor but the most common is a 500 μCi dose divided among 5 tuberculin syringes with 1/2 inch, 24 gauge needles. In the UK 20 megabecquerels of nanocolloid is recommended. The sulphur colloid is slightly acidic and causes minor stinging. A gentle massage of the injection sites spreads the sulphur colloid, relieving the pain and speeding up the lymph uptake. Scintigraphic imaging is usually started within 5 minutes of injection and the node appears from 5 min to 1 hour. This is usually done several hours before the actual biopsy. About 15 minutes before the biopsy the physician injects a blue dye in the same manner. Then, during the biopsy, the physician visually inspects the lymph nodes for staining and uses a gamma probe or a Geiger counter to assess which lymph nodes have taken up the radionuclide. One or several nodes may take up the dye and radioactive tracer, and these nodes are designated the sentinel lymph nodes. The surgeon then removes these lymph nodes and sends them to a pathologist for rapid examination under a microscope to look for the presence of cancer.

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Wikipedia