The sentinel lymph node (SLN) concept is rooted in the body’s lymphatic system, a network of vessels and small, bean-shaped organs called lymph nodes. This system collects and filters fluid and waste products from tissues before returning the fluid to the bloodstream. Lymph nodes are a major component of the immune system, containing cells that fight infection.
The sentinel node is defined as the first lymph node, or group of nodes, to receive the lymphatic drainage directly from a primary tumor site. This node acts as the initial filter for the tumor’s region. Due to the directional flow of lymph, any cancer cells that break away from the main tumor are highly likely to travel to and become trapped in this specific node first.
The Role of Sentinel Nodes in Cancer Spread
The sentinel node concept relies on the predictable progression of cancer cells through the lymphatic system. When cancer begins to spread, it usually follows the lymphatic drainage pathway from the original tumor site to the nearest lymph node basin. This first node in the chain is the SLN, making its status an important indicator of the disease’s overall extent.
The primary medical significance of the SLN is its use in cancer staging and treatment planning. If the sentinel node is biopsied and found to be free of cancer cells, there is a high probability that the cancer has not yet spread to other, more distant lymph nodes or organs. This finding can spare the patient from a more extensive surgical procedure, such as a complete lymph node dissection, which can lead to complications like lymphedema.
Conversely, finding cancer cells in the sentinel node indicates that the disease has begun to spread and may have advanced to other regional nodes. This positive result provides crucial information for determining the patient’s prognosis and for planning subsequent treatments. These treatments may include radiation therapy, chemotherapy, or the removal of additional lymph nodes.
Primary Anatomical Locations
The location of the sentinel lymph node is not fixed, but rather is determined by the lymphatic drainage pattern of the primary tumor. For common cancers where sentinel node biopsy is a standard procedure, the primary anatomical sites are predictable and linked to the region of the body where the tumor originated.
For breast cancer, the most frequent location for the sentinel node is the axillary basin, or the lymph nodes located in the armpit. Approximately 75% of the lymphatic fluid from the breast drains toward this group of nodes. However, a sentinel node may also be found in the internal mammary chain, which runs along the breastbone inside the chest wall.
The internal mammary nodes are more frequently involved when the tumor is located near the center of the chest or in the inner quadrants of the breast.
The location of the sentinel node for melanoma is entirely dependent on the tumor site on the skin. For other cancers, the drainage patterns are also predictable:
- Melanoma on the torso or upper arm drains to the axillary lymph nodes.
- Melanoma on the leg or lower trunk drains to the inguinal or femoral nodes in the groin area.
- Melanoma on the head or neck drains to the cervical or parotid nodes.
- Cancers originating in the pelvic region (e.g., cervical, vulvar, or penile cancer) drain to the pelvic and inguinal regions.
- Head and neck cancers primarily drain to the cervical lymph nodes.
The Sentinel Lymph Node Mapping Procedure
The process of locating the specific sentinel node is known as lymphatic mapping, which is usually performed just before or during the surgical removal of the primary tumor. This procedure relies on injecting tracking agents that mimic the path cancer cells would take through the lymphatic vessels. The most common agents used are a radioisotope and/or a specialized blue dye.
The radioisotope, often technetium-99m, is injected into the tissue around the tumor site hours before surgery. This substance travels through the lymphatic vessels and accumulates in the sentinel node. A specialized imaging scan called lymphoscintigraphy is then performed to create a visual “map” of the drainage pathway and to precisely identify the node’s location.
During the operation, the surgeon uses a handheld device known as a gamma probe to detect the node that has concentrated the most radioactivity, identifying it as the sentinel node. If a blue dye is also used, it is injected closer to the time of surgery and stains the sentinel node blue, providing a visual landmark for the surgeon. The combination of a radioactive tracer and blue dye maximizes the chances of accurately identifying the SLN.
Once identified, the stained and/or radioactive sentinel node is surgically removed in a procedure called a sentinel lymph node biopsy. This small sample is then sent to a pathology lab for analysis to determine if any cancer cells are present.