A sentinel node is the first draining lymph node of a certain region. Of particular interest to surgeons who are removing a malignant growth (i.e. cancer) in any part of the body.

The concept of mapping the sentinel node was first reported in 1977 by a researcher studying cancer of the penis. The technique was later used to study drainage patterns of melanoma, and was first reported for breast cancer in 1993.

Sentinel nodes are found by injecting the tumour with some radioactive fluid and finding the area with the greatest count on a Geiger counter later. Pre-operatively, a blue dye (methylene blue) is also injected into the tumour and this can usually be seen in the sentinel node intra-operatively.

When a sentinel node is removed, it is sent straight off to the pathology laboratory for inspection by a pathologist, while the patient is still on the operating table. If the pathologist calls saying that the node was completely clear of malignant cells, it is highly likely that the tumour has not metastasized.

In practice, the surgeon will remove not only the node with the highest count on the Geiger counter but also one or two more that show up as being radioactive, and which therefore receive lymphatic drainage from the tumour.

Sentinel node biopsies are being trialled clinically for breast cancer operations. By only taking out the sentinel nodes, surgeons can avoid doing a full axillary lymph node clearance which usually leads to lymphedema of the arm on the same side. Successful sentinel lymph node biopsies will lead to less aggressive surgery for women with early breast cancer.

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