Most primary melanomas will grow and spread horizontally along the top layer of the skin or epidermis before they begin to penetrate deeper into the layers of the dermis of the skin. When they do grow deeper into the skin, the melanoma cells can reach the lymph and blood vessels in the dermis.

Sometimes melanoma cells break away from the primary tumor and enter the lymph vessels. They may then spread along the lymph vessels, forming in-transit (satellite) metastases, and eventually spread to the regional lymph nodes. 

In any area of skin where there is a deeply penetrating melanoma, there will be a nearby sentinel node (or less frequently, 2 or more nodes) to which it drains. The sentinel lymph node is the very first lymph node to receive drainage from that area. When lymph does its job to filter the skin, and drains an area of skin that contains melanoma, the sentinel lymph node is the one most likely to contain melanoma cells if any lymph nodes are involved.

Techniques are available that help doctors accurately locate sentinel lymph nodes and enable the surgeon to remove them through better-directed and smaller incisions.

For more on sentinel lymph node biopsy click here.