Dear Barks: A sentinel lymph node biopsy is a procedure that injects a dye and or radioactive substance into the area of the tumor. These substances will usually drain to the first lymph node(s). These are the sentinel lymph nodes. The theory is that if the cancer is going to spread to the lymph nodes, it will go to these nodes first. So if these first nodes are negative, then the rationale is that the nodes further down the chain should also be negative. There is a small incidence of "skip" metastasis where a sentinel node is negative and a node further down the chain is positive, so some surgeons like to remove a larger sample than just the sentinel node. Removing a sample of lymph nodes at the time of mastectomy is standard practice.
The reason to remove nodes is that it is an important part of the staging process and stage will help determine the treatment plan.
It's important in managing breast cancer to know if it has spread to the lymph nodes, so some are removed for testing. "Sentinal node" means the one or ones to which lymph first drains from the breast, as detected by injecting some sort of marker into the breast and seeing which nodes "light up" first. Theoretically, if cancer has spread, those are the nodes to which it will go first. So if they are ok, then presumably all others are as well. The false negative rate -- meaning the chance of the sentinal node being ok even though there's tumor in other nodes -- is around 5 - 10%. Most surgeons would not take just one node; they'd identify the sentinal node and take a few more in the area. Also, when the technique is in the early stages of a surgeon's experience, they tend to want to take more to confirm the accuracy of their technique. As to removing before a prior biopsy: there's no accurate way to biopsy a lymph node without removing it. Biopsy is removal, and vice versa.