The sentinel lymph node (SLN) is the first lymph node or small group of nodes that receives lymphatic fluid drainage directly from a specific site in the body. This node acts as the initial filtering station for the area it drains. Since the lymphatic system has predictable flow patterns, the sentinel node is consistently the first stop for any substances, including fluid or cells, leaving a tissue region. Its location changes depending on the primary site it drains.
The Role of Sentinel Nodes in Lymphatic Drainage
The lymphatic system functions as a one-way circulatory network, collecting excess tissue fluid, known as lymph, and returning it to the bloodstream. Lymph nodes are small, bean-shaped organs positioned along this network, serving as biological filters that trap foreign particles and cellular debris. Lymph flow is highly organized, moving from smaller vessels into larger collecting ducts and into regional clusters of lymph nodes.
This directional flow dictates that material entering the lymphatic vessels must first pass through a single, distinct node or cluster before proceeding to any other nodes. This node acts as the primary biological barrier for that lymphatic basin. Because the flow is sequential, if the sentinel node is clear of abnormal material, it suggests that the rest of the downstream lymph nodes are also likely unaffected. This principle gives the sentinel node its significance as the earliest indicator of systemic spread.
Primary Anatomical Locations of Sentinel Nodes
The location of the sentinel node is defined by the anatomical region being drained, which is typically the site of a primary tumor.
For tissues in the chest wall and breast, lymphatic drainage commonly routes to the axillary lymph nodes, located in the armpit. The sentinel node for the breast is found within this axillary basin.
For cancers arising from the skin, such as melanoma, the sentinel node’s location depends entirely on the primary lesion’s position. A melanoma on the torso will drain to the nearest major lymphatic cluster, which might be the axilla, the neck, or the groin. A lesion on the leg typically drains to the inguinofemoral lymph nodes in the groin region.
Gynecological Cancers
In gynecological malignancies, the sentinel node sites reflect the deep pelvic lymphatic channels. For vulvar cancer, the sentinel nodes are consistently found in the inguinal and femoral regions of the groin. Cancers of the cervix and uterus often drain to the pelvic lymph nodes, specifically the external iliac, internal iliac, or obturator nodes deep within the pelvis. Drainage from uterine tumors can occasionally follow vessels higher up to the para-aortic lymph nodes near the large abdominal vessels.
The Mapping Procedure for Sentinel Node Identification
To precisely locate the sentinel node during a procedure, a technique called lymphatic mapping is used, which relies on injecting a traceable substance near the originating site.
One common method involves using a low-level radioactive tracer, such as Technetium-99, injected hours before surgery. This tracer travels through the lymphatic vessels and accumulates in the sentinel node, making it a “hot spot” that can be detected using a handheld gamma-detection probe during surgery.
Another established method involves injecting a visible blue dye, such as Patent Blue or Methylene Blue, directly into the tissue surrounding the area of interest. The lymphatic vessels quickly absorb this substance, carrying it to the sentinel node. The dye visually stains the lymphatic vessels and the sentinel node itself a bright color, allowing the surgeon to physically identify the first stained node.
Often, both the radioactive tracer and the blue dye techniques are used together to maximize the chance of accurate identification. The combination of a visible marker and a detectable radioactive signal provides the most reliable way to confirm the sentinel node. Once identified using these mapping agents, the node is removed for detailed examination.