For someone diagnosed with breast cancer, the underarm lymph nodes may provide important information about whether the disease has spread to another part of the body. This is essential to determining both the appropriate treatment and the long-term outlook.

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The lymph nodes are part of the lymphatic system, which carries lymph fluid, nutrients, and waste material between body tissues and the bloodstream. It is possible for breast cancer cells that break away from the original tumor to get swept up into the lymph fluid that drains from the breast. They may then be filtered out of the fluid and trapped in the lymph nodes.

Sometimes the cancer cells go no further than the lymph nodes. But the presence of cancer cells in the lymph nodes signifies an increased risk that the malignancy in the breast has “metastasized” – spread beyond its initial site.

In every person, there is a specific path through the lymphatic system that particles of foreign material will traverse. The pattern is an orderly one, although unique to each person. It is therefore possible to locate the individual node or nodes that serve as the first way-station for metastases.

The idea is that if this first, or “sentinel” node shows no sign of cancer, then the cancer probably has not spread. Therefore, if the first node on the lymphatic path traveling away from a breast tumor site can be located and removed for analysis, further node removal might be unnecessary.

Because the location of the sentinel node may vary from person to person, the test begins with the injection of a weak radioactive dye, or a blue dye (or both) near the tumor site. The path of the injected material is traced, and one or a few key lymph nodes are removed. The advantages of sentinel node biopsy include less likelihood of swelling and faster recuperation.

Whereas previously it was necessary to remove a dozen or so lymph nodes in essentially everyone with breast cancer, sentinel node biopsy allows this more radical approach to be reserved for patients who show actual disease in the nodes. If no disease is detected in the sentinel nodes, there is a high likelihood that none of the others need to be removed.

“How often are cancer cells found to have migrated to the lymph nodes from a breast tumor?”

Cancer moves into the lymph nodes in about 30 percent of breast cancer cases. From there it can spread to other parts of the body.

“Is a separate surgical procedure required in order to perform a sentinel node biopsy?”

No. The sentinel node biopsy is done in conjunction with the mastectomy or lumpectomy in which the breast tumor itself is removed. The tumor removal and the biopsy are accomplished in a single procedure, minimizing the amount of surgery the patient must undergo.

“If no cancer cells are found in the sentinel node, does this prove beyond all doubt that the cancer has been confined to the breast tumor?”

No. While such a finding is a very hopeful sign, it does not conclusively prove that the cancer has been eradicated. Cancer cells can also spread to other parts of the body through the blood vessels, bypassing the lymphatic system.