Lymph nodes are present in a number of locations around the human body. They are a part of the lymphatic system, which is a mechanism for the body to collect potentially harmful substances within the body (bacteria for example) and drain them out via the lymph nodes, where these substances are filtered out and destroyed. The fluid that carries these substances is called 'lymph' and, once it has reached its destination lymph node, it passes back into main blood vessels. Lymph nodes are located in the armpit (axillary lymph nodes) as well as in the groin (inguinal), abdomen, chest and neck (cervical).
Sentinel lymph node biopsy is a procedure where lymph nodes in the armpit are checked to see if there is any evidence of cancer cells having travelled there from a tumour in the breast. One or several lymph nodes are removed in this procedure and are sent for pathological analysis.
Often the procedure forms a part of a lumpectomy or mastectomy, but it can also be carried out as a separate procedure. If there is no evidence of cancerous cells in the lymph nodes, it may be that the cancer has been caught at an early enough stage that cancer cells have not started travelling to other parts of the body. If there is evidence of cancer cells in the sentinel lymph node/s, there may be cancer in the other nodes and they may need to be removed by axillary lymph node clearance.
The term 'sentinel lymph node' refers to the most likely lymph node that cancer cells would have travelled to from a tumour in the breast. This can be identified by injecting blue dye and a mildly radioactive marker which indicates which is the primary lymph node the breast lymphatic system drains to. Most of the time the sentinel node is in the armpit, but it can also be located elsewhere, for example in the chest. Very occasionally the 'real' sentinel node is already blocked by cancer cells and the dye flows to another lymph node. Generally if a lymph node is blocked it swells slightly, and so it is general practice to remove swollen lymph nodes as well as the indicated 'sentinel' just in case the 'real' sentinel node is one of these and so not indicated by the dye or marker.
In practice the radioactive marker is injected before the operation, normally around the tumour or near the areola to check which lymph nodes receive lymphatic flow from the breast tissue, and then the blue dye is injected to assist the surgeon visually during the procedure.
Sentinel lymph node biopsy is generally considered a better option than removal of all lymph nodes in the armpit (Axillary Clearance - Resection) as clinical trials show fewer complications after this procedure, with a lower risk of after effects such as numbness, stiffness in the shoulder and lymphoedema.
Sentinel lymph node biopsy is standard practice in cases of early (operable) breast cancer, that is where tumours are no larger than 5cm in diameter and where there is no other evidence of cancer having travelled elsewhere in the body (metastasis).