What Is a Sentinel Lymph Node?
When cancer spreads from its original site, it typically travels through the lymphatic system before reaching distant organs. The sentinel lymph node is the first lymph node (or small group of nodes) that receives lymphatic drainage from the tumour. Think of it as the first checkpoint on the lymphatic highway leading away from the cancer.
If cancer cells have started to spread through the lymphatic system, the sentinel lymph node is the first place they would arrive. Examining this node tells doctors whether the cancer has begun to spread or is still confined to its original location.
Why This Matters for Surgery
Before sentinel lymph node mapping became standard practice, surgeons often performed extensive lymph node dissections. In breast cancer, for example, this meant removing many or all of the axillary (armpit) lymph nodes to check for cancer spread. While this provided thorough staging information, it came with significant side effects.
Extensive lymph node removal can cause:
- Lymphoedema: Chronic swelling of the arm (in breast cancer) or leg (in melanoma or gynaecological cancers) due to disrupted lymphatic drainage. This can be permanent and significantly affects quality of life.
- Numbness and nerve damage: Sensory changes in the affected area.
- Restricted movement: Particularly in the shoulder after axillary dissection.
- Increased infection risk: The affected limb becomes more vulnerable to infections.
Sentinel lymph node mapping changed this approach. If the sentinel node is negative (no cancer cells found), the chance that other lymph nodes contain cancer is very low. This means surgeons can spare patients the full lymph node dissection and its associated complications.
How Lymphoscintigraphy Maps the Sentinel Node
Lymphoscintigraphy is the nuclear medicine scan used to map sentinel lymph nodes before surgery. Here is how the process works:
The Injection
A small amount of radioactive tracer (typically Tc-99m labelled nanocolloid) is injected near the tumour site. For breast cancer, the injection is usually around the tumour or near the nipple. For melanoma, it is injected around the skin lesion. The injection may cause brief stinging, but the discomfort is temporary.
The Imaging
After the injection, the tracer travels through the lymphatic channels, following the same path that cancer cells would take. Images are acquired with a gamma camera at specific time intervals, showing the tracer’s journey through the lymphatic system and its arrival at the sentinel node(s).
The nuclear medicine physician identifies the sentinel node(s) on the images and marks their approximate location on the skin. This gives the surgeon a roadmap before entering the operating room.
In the Operating Room
During surgery, the surgeon uses a handheld gamma probe, a small device that detects radioactivity, to locate the sentinel node precisely. A blue dye is sometimes also injected to provide visual confirmation. The sentinel node is removed and sent for pathological examination, often while the patient is still under anaesthesia (frozen section analysis).
If the sentinel node is clear of cancer, no further lymph nodes are removed. If it contains cancer cells, the surgeon and oncology team discuss whether additional lymph node removal or other treatments are needed.
Where Sentinel Lymph Node Mapping Is Used
Breast Cancer
Sentinel lymph node biopsy is now the standard of care for breast cancer patients with clinically negative axillary lymph nodes (no obvious lymph node involvement on examination or imaging). It has dramatically reduced the number of women who undergo full axillary dissection, significantly lowering the incidence of lymphoedema and other complications.
Melanoma
For melanoma, sentinel lymph node mapping is particularly important because the melanoma’s lymphatic drainage pattern is not always predictable from its location on the skin. A melanoma on the back, for example, might drain to the axilla, the groin, or both. Lymphoscintigraphy reveals the actual drainage pattern, ensuring the correct sentinel node is identified and biopsied.
Other Cancers
The sentinel lymph node concept has been applied to other cancers as well, including vulvar cancer, penile cancer, and head and neck cancers (particularly oral cancers). The principles are the same: identify the first draining node, examine it, and spare the patient unnecessary surgery if it is clear.
What the Results Mean
A negative sentinel lymph node biopsy (no cancer found) is reassuring. It means the cancer has not spread to the regional lymph nodes, which is a favourable prognostic factor. Your oncologist will still discuss whether additional treatment (radiation, chemotherapy, or other therapy) is needed based on the characteristics of the primary tumour.
A positive sentinel lymph node (cancer cells found) means the cancer has started to spread. This changes the staging and may influence treatment recommendations. It does not automatically mean a full lymph node dissection is required. Current guidelines, particularly in breast cancer, have evolved significantly, and in many cases, radiation or systemic therapy may be used instead of additional surgery.
A Patient-Centred Approach to Staging
Sentinel lymph node mapping represents a shift toward less invasive cancer staging. By focusing on the node that matters most, it provides accurate staging information while reducing surgical complications and improving quality of life for patients.
If you are scheduled for cancer surgery and want to understand how lymph node assessment will be handled, ask your surgeon about sentinel lymph node mapping. Your surgical and oncology team will determine the approach that is right for your specific cancer type and stage.
Learn more about how we perform this procedure on our lymphoscintigraphy page.