Answer:
The sentinel lymph node (SLN) is identified using both preoperative and intraoperative techniques. Preoperative lymphoscintigraphy is performed using technetium-labeled colloid injected intradermally into the periphery of the primary lesion or the scar if it has already been excised. The patient is immediately placed under the scanner and images are obtained identifying any localization within lymph nodes, either in the nodal basin or within in-transit ones between the primary lesion and the nodal basin. These are usually marked with indelible ink. Intraoperatively, a supravital blue dye and/or technetium-labeled colloid will be injected intradermally around the lesion's perimeter. Usually, they will localize in about 10-20 minutes though the radionuclide may take longer. Using a gamma-probe, the area of greatest activity within the nodal basin is identified and any lymph nodes associated with that activity will be removed. Often, these nodes will also be stained with the blue dye if it has also been injected. Reports indicate that a negative SLN has only about 2-5% chance of being falsely negative. The pathologist takes the SLN and sagitally cuts it in half through the hilum. Cochran et al found that this cutting technique yields the greatest number of metastases. Each half is sectioned and stained with hematoxylin and eosin and the entire surface is examined for metastases. Immunohistochemical staining is also performed, especially if the H&E sections are negative. Because of the greater ability to perform more thorough and accurate pathologic examinations with permanent sections, frozen sections are no longer recommended in SLN procedures.