Lymphoseek (generic: technetium Tc 99m tilmanocept) injection is a radioactive diagnostic imaging agent that helps doctors locate lymph nodes in patients with breast cancer or melanoma (a type of skin cancer) who are undergoing surgery to remove tumor-draining lymph nodes.

This medication comes in an injectable form to be given in the area of the tumor. Using a handheld radiation detector, your doctor can find lymph nodes that have taken up Lymphoseek's radioactivity. A surgeon removes these lymph nodes to examine them for cancer cells.

Common side effects of Lymphoseek include pain and irritation at the injection site.