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August 2001. Review Article. To the updating
The sentinel node hypothesis states that the histopathologic status of the first node on the lymphatic drainage pathway from a primary tumor reflects the tumour status of the entire lymphatic drainage basin.
The anatomical definition of the sentinel node as the lymph node closest to the primary lesion does not take into consideration the physiology of lymph drainage: the node closest to the primary tumour is the first one to be involved only when it receives direct drainage from the injection site.
Some investigators in the field of the nuclear medicine define the sentinel lymph node as the first lymph node that becomes visible on the lymphoscintigraphic images. But sometimes there are two or more lymphatic channels originating in the region of the primary tumour and running to different lymph nodes. All this first tier nodes should be harvested and examined by the pathologist. One may be depicted on the scintigraphy images before the other, this does not imply that the other nodes are not sentinel nodes.
There are some reasons not to use the brightness of a node on the scintigram to decide whether or not a lymph node is a sentinel node. The amount of tracer that is accumulated by a node not only depends on its position in the drainage order but also on the number of lymphatic channels that enter the node, on the size of lymph node, on macrophage avidity for the tracer, and on parameters such as lymphatic flow rate. Lymph flow depends on factors such as physical exercise, medication, massaging of injection site and hydratation of the patient.
Moreover the brightness of a node depends on the amount of radionuclide in that node but also on its distance to the gamma camera. And some of the tracer may pass through the first tier lymph node and lodge in secondary nodes that are not at risk of harbouring metastatic disease.
Similarly if we assume that a sentinel lymph node is a blue node, (definition based on use of the vital dye), we must remember that some of the tracer may pass through the first tier lymph node and stain secondary nodes. The ingress of lymph may be obstructed or the lymphatic flow may have stopped because the duct has suffered damage upstream.
Although good results have been achieved using blue dye or a probe alone, the most practical approach is probably to use all the available detection techniques
Techniques of intraoperative lymphatic mapping and SLND using dye, tracer, or both have high success rates in the hands of experienced investigators, but their routine and widespread use awaits resolution of questions about the timing, dose, and type of radioactive tracer; the optimal lymphatic mapping technique; indications and contraindications for SLND; and certification of qualified surgeon, pathologists, and nuclear medicine physicians ( 2 ).
1) Omgo E Nieweg, Pieter J Tanis, and Bin B R Kroon. The definition of a sentinel node. Annals of Surgical Oncology 2001, 8 (6): 538-541.
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