What is a sentinel lymph node?

A sentinel lymph node is the first lymph node or group of lymph nodes that cancer cells are most likely to reach if they begin to spread from the primary tumour. Lymph nodes are small, bean-shaped structures that are part of the lymphatic system, a network that helps the body fight infections and remove waste.

The sentinel lymph node acts like the first checkpoint in the drainage system surrounding a tumour. If cancer cells start to move away from the original tumour, they usually travel through lymphatic vessels and enter the sentinel lymph node before reaching other nodes. For this reason, the status of the sentinel lymph node provides important information about whether the cancer has begun to spread.

Where are sentinel lymph nodes found?

Sentinel lymph nodes can be located in different parts of the body depending on where the cancer started. The lymphatic system follows natural drainage pathways, so the sentinel lymph node is always the first stop along that pathway.

Examples include:

• Breast cancers: The sentinel lymph nodes are usually found in the armpit (axilla).

• Melanoma on the arm or leg: The sentinel lymph nodes may be found in the armpit or groin.

• Cancers of the trunk or abdomen: Sentinel lymph nodes may be located along lymphatic channels in the chest, abdomen, or groin.

Your surgical team identifies the exact location of the sentinel lymph node based on how lymphatic fluid drains from the tumour.

Why are sentinel lymph nodes important?

Sentinel lymph nodes are important because they help doctors determine whether cancer has spread beyond its original site. If cancer cells are found in the sentinel lymph node, it means the cancer has begun to travel through the lymphatic system. If no cancer cells are found, it is less likely that the cancer has spread to other lymph nodes or distant organs.

This information is essential for:

• Staging the cancer.

• Planning treatment, including whether more lymph nodes need to be removed.

• Prognosis or what to expect going forward.

How do doctors identify sentinel lymph nodes?

Doctors identify sentinel lymph nodes using a procedure called sentinel lymph node biopsy. This procedure is usually done at the same time the tumour is surgically removed.

The process includes:

1. Injection of a tracer near the tumour. A small amount of radioactive substance, blue dye, or both is injected into the tissue around the tumour.

2. Movement of the tracer through lymphatic channels. The tracer travels the same path that cancer cells would take.

3. Identification of the sentinel lymph nodes.

   • A handheld device detects radioactivity to locate the sentinel nodes.

   • If blue dye is used, the sentinel nodes turn blue and can be seen easily.

4. Removal of the sentinel lymph nodes. The nodes are removed through a small incision and sent to a pathologist for examination.

This procedure removes only the nodes most likely to contain cancer, avoiding unnecessary removal of many lymph nodes.

How do pathologists examine sentinel lymph nodes?

Once the sentinel lymph nodes arrive in the pathology laboratory, they are carefully examined under the microscope. Pathologists often examine multiple levels of each lymph node and may use special tests, such as immunohistochemistry (IHC), to highlight any cancer cells that may be present.

Pathologists assess whether cancer has spread and, if so, how much cancer is present. The results are usually described in one of the following ways:

• Negative: No cancer cells are found in the sentinel lymph nodes. A negative result suggests that the cancer has not spread to the lymph nodes and is less likely to have spread elsewhere in the body. This is generally associated with a more favourable prognosis.
• Positive: Cancer cells are found in the sentinel lymph nodes. Cancer cells in a lymph node are called a metastasis.

For positive lymph nodes, your report may also describe:

• Isolated tumour cells: Very small clusters of cancer cells.

• Micrometastasis: A small area of cancer spread.

• Macrometastasis: A larger, clearly visible area of cancer.

The amount of cancer in the sentinel lymph node helps guide treatment. For example, finding only isolated tumour cells may require less aggressive treatment than seeing a large macrometastasis.

Why do sentinel lymph node results matter?

The presence or absence of cancer in the sentinel lymph node can influence:

• Whether additional lymph nodes should be removed.

• Whether radiation therapy is needed.

• Whether systemic therapies such as chemotherapy, immunotherapy, or hormone therapy are recommended.

• How closely a patient should be monitored after treatment.

Sentinel lymph node evaluation provides some of the most important information for understanding how a cancer is behaving.

Questions to ask your doctor

• Where is the sentinel lymph node located for my type of cancer?

• Did my sentinel lymph node contain any cancer cells?

• If the node was positive, how much cancer was present?

• Do I need additional surgery to remove more lymph nodes?

• How does the sentinel lymph node result affect my treatment plan?

• What follow-up care or testing will I need?