Your surgeon will inject a radioactive substance and possibly a blue dye into the site of your melanoma. Then he or she will wait for about an hour or so while the substance and dye spread. Using a tool that detects radioactivity, your surgeon will follow the path of the dye to one or more lymph nodes, which are then considered most probable to have cancer cells (if your cancer has spread). This is how the surgeon finds the lymph nodes that could be biopsied as part of your cancer diagnosis and treatment. If there are any lymph nodes in the area that are unusually large, they may be biopsied without the dye and imaging procedure. Risks of this procedure include false negative results in about 2 percent of patients. This means that, in these rare cases, lymph nodes that have cancer are missed. The most common complications are wound infection, lymphedema, or a hematoma, which is a localized swelling filled with blood. But these complications are far more likely in people undergoing removal of many or all lymph nodes in the tumor area.