Procedures: Sentinel Node Biopsy

what is a sentinel lymph node biopsy?

A sentinel lymph node biopsy is a procedure in which one or more specific lymph nodes is removed and analyzed to check for the presence of cancer spread. The lymph node(s) removed are identified by a radiology mapping technique to be the one(s) most likely to have cancer spread if cancer spread to lymph nodes has occurred. This technique is useful for early identification of small amounts of spread to a lymph node, which would not be detectable on the basis of size.

Background and basis of sentinel lymph node biopsy

Fluid within body tissues collects and routes through the lymphatic system of channels with interspaced lymph nodes on its way prior to its return to the bloodstream. The lymphatic fluid contains a sampling of the proteins, other molecules, and even full cells from the upstream catchment area. A given area of skin or volume of tissue will preferentially route its lymphatic fluid to a specific lymph node for processing before passing through to the ongoing lymphatic channel as it courses through additional lymph nodes and eventually to the thoracic duct and back into the bloodstream.

The immune response involves sampling of the content of lymph in a lymph node, where immune cells that match a particular foreign molecule, such as a protein from a virus or bacteria, elicit a process of growth and expansion of cells that can fight this foreign entity. This is why upper neck lymph nodes, for example, become swollen and tender in response to a tonsillitis.

When a cancer spreads to lymph nodes, it does so with a cancerous cell breaking off from the cancer where it originated, the primary site, and enters a lymphatic channel. From there, it flows downstream to a lymph node. The cancer cell may stay in this lymph node and may divide into two cancer cells, which then become four, and so on. Eventually, a growth of cancer cells within a lymph node becomes large enough and with certain characteristic changes that can be identified on physical exam or various types of medical imaging. The purpose of a sentinel lymph node biopsy is to check for even a tiny cluster of cancerous cells within the lymph node(s) most likely to have spread from a given primary cancer site, and to do so with a minimal extent of surgery.

When is a sentinel node biopsy needed?

Not every cancer needs a sentinel lymph node biopsy. Cancers that have known spread to another location, or cancers with either low or high risk for spread to lymph nodes do not need a sentinel lymph node biopsy—as it would add little or no benefit in these situations. Instead, sentinel lymph node biopsy is useful for cancers that have intermediate risk of spread to lymph nodes. For example, a malignant melanoma with intermediate thickness invasion has intermediate risk of lymph node spread, and determining if spread to the sentinel lymph node has occurred is very informative and can guide additional therapy recommendations.

How is a sentinel lymph node biopsy performed?

1. The patient undergoes lymphatic mapping in the radiology/nuclear medicine department. This involves injecting a radioactive lymphatic tracer (such as technetium sulfur colloid) into the site of the primary tumor and then obtaining images of the tracer making its way to the sentinel lymph node(s). This is usually performed in the morning prior to afternoon surgery.

2. The patient is taken to the operating room where the sentinel lymph node or nodes are identified and removed. During this procedure, the primary tumor may be removed and reconstructed if not done previously.

3. The patient returns to the office after a week or so of healing to check the wounds and review the final pathology report, indicating whether the node(s) removed were found to contain cancer.

Here is A specific example of sentinel lymph node biopsy along with wide local excision and reconstruction of a scalp skin melanoma:

A patient with an intermediate thickness melanoma on the scalp underwent injection of a radio tracer followed by mapping based on detection of radiation. The patient goes to the operating room. In addition to the radio tracer placed in radiology/nuclear medicine, a blue dye may be injected to the primary site to facilitate identification of the sentinel lymph node(s). Based on the mapping images and the detection of radiation with a gamma probe, a small incision is made over the sentinel node(s). The node(s) are removed. The primary skin melanoma site is excised and reconstructed with a full thickness skin graft taken from the low neck.

The pathologist analyzes the lymph node specimen and the excision of the primary tumor, reporting on both. In the melanoma example above, the pathologic information gained from a sentinel lymph node biopsy is highly prognostic (predictive) for the behavior of the cancer and is used to base further treatment recommendations.