Sentinel lymph node mapping refers to the investigation of which lymph node is the first node to drain from a cancer. There are potential advantages in knowing which lymph node is the sentinel one. What are these advantages?
1 - For some cancers, removing the sentinel node, if metastatic, has been shown to prolong survival time. Two examples are mast cell neoplasia and mammary gland neoplasia.
2 - Palpation is inaccurate in terms of determining if a lymph node is metastatic (has cancer cells present within). Around 50% of enlarged nodes have NO evidence of metastatic disease and 50% of normal sized nodes DO have metastasis.
3 - Cytology of a lymph node often misses metastasis. Therefore, extirpation (removal) of a lymph node for histopathology is preferable. In these cases, demonstration of which node is the sentinel node can help guide you on which node to extirpate. This is common practice in the human world.
So how do we map the sentinel lymph node?
Charlie is an example of where we used sentinel lymph node mapping. I map all my mast cell tumours now (if cost is not an object for owners).
Charlie presented with a mast cell tumour around the elbow. One of the local lymph nodes for this region, is the axillary lymph node, so it would have been tempting to remove this node for biopsy and staging. However, we wanted to be certain we had identified the sentinel lymph node.
A plain CT (computed tomography) scan was performed. We then injected 0.25mls of radiopaque iohexol contrast into the skin immediately adjacent to the mast cell tumour, with a 25 gauge needle. This was very lightly massaged, without disturbing the tumour itself. We then repeated the CT at 1, 3, 6, 9 and 12 minutes .The sentinel lymph node (the first LN with contrast uptake) was identified. In this case it was the pre-scapular lymph node, and not the axillary lymph node. This lymph node was extirpated at the time of surgery. Histopathology on this node revealed 'early metastatic MCT'. Extirpation of the primary tumour as well as the metastatic node should prolong Charlie's survival time, and this additional minor procedure added minimal time and morbidity.
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