What is the role of a gamma probe during breast cancer lymphoscintigraphy (sentinel lymph node mapping)?

Updated: Dec 13, 2018
  • Author: Durre Sabih, MBBS, MSc, FRCP(Edin); Chief Editor: Mahan Mathur, MD  more...
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Answer

Answer

Imaging the sentinel node allows the surgeon to have an idea of where to explore to retrieve the sentinel node. However, the addition of radioguidance using a gamma probe is the true strength of the technique. This also introduces a multidisciplinary element to the procedure, with breast surgeons and nuclear medicine physicians operating in close cooperation.

By using sentinel node imaging, the surgeon can determine the area where the sentinel node lies. Complete excision can be confirmed by counting the area after removing the purported sentinel node to see if the counts have fallen to background levels. The probe also guides the surgeon into the right direction and allows removal of invisible non-blue nodes that are recognizable due to their radioactive signature.

The gamma probe is covered in sterile sheath that extends to cover the connecting wire up to the display console. The display console is placed where the surgeon can easily see the display. (See the image below.)

The sentinel node is located using the gamma count The sentinel node is located using the gamma counter probe in the area with the highest count rate. This is done after sterilization; note that the probe is covered with a sterile sheath.

The skin mark is confirmed to actually overlie the hottest node by placing the probe tip over the area and moving it to the surrounding areas. The hottest area is identified by the pitch of the audible signal as well as the count rate on the digital display. (See the images below.)

With the probe in the proper location, the area is With the probe in the proper location, the area is marked on the skin.
The incision is made over the area identified by t The incision is made over the area identified by the mark on the skin.
The incision leads to a blue lymph node. Note the The incision leads to a blue lymph node. Note the blue channel at 1 o'-clock position indicating that this is the sentinel node (to be corroborated by radiation counting).
The blue node is confirmed to be the sentinel node The blue node is confirmed to be the sentinel node by counting over it. It should be the "hottest" node.

Background is acquired. One practice is to acquire the background over the suprasternal notch.

The line of sight [23] is established by angling the probe at the site of maximum count to determine the angle that allows the shortest distance from the skin to the sentinel node. The node is approached along the line of sight, correcting the direction with periodic input from the probe. If a blue channel is seen, this reinforces the direction.

Once the node is exposed, the probe is applied to it again to ensure that this has a high count rate (in vivo counting). If a blue dye is used and the same node is colored, this corroborates the probe data.

Once the node is removed, it is placed on the probe surface to confirm the counts (ex vivo counting). (See the image below.)

The blue and hottest node is excised and ex vivo c The blue and hottest node is excised and ex vivo counts are checked. Note that the probe is directed towards the ceiling to avoid getting extraneous counts from the patient. Note the high counts (acquired over 2 seconds) at the top of the screen.

The probe is reapplied to the surgical field to confirm that the radioactive node(s) have been removed. If all radioactive nodes have been removed, the activity in the surgical bed should fall to 10% of the most active node. [23]  (See the images below.)

After the sentinel node is excised, the nodal bed After the sentinel node is excised, the nodal bed is checked again for residual counts. If the sentinel node has been removed, the count rate should fall to the background level or 10% of the in vivo count rate.
Postexcision counts are acquired. In this image, t Postexcision counts are acquired. In this image, the counts have fallen to very low background rates.

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