Lymph nodes are small, bean-shaped glands that are located along the lymphatic system (a system of vessels similar to arteries and veins that lymph fluid travels through). These nodes are located throughout the body, and they filter bacteria, cancer cells, and other foreign material that travel through the lymphatic system.
Lymph nodes are well-known as the "swollen glands" people may note in their neck when they are fighting a cold or sore throat.
With lung cancer, cancer cells can spread through the lymphatic system and become trapped in lymph nodes. Determining which, if any, of these nodes contain cancer cells help doctors determine the stage of a cancer -- that is, how far the cancer has spread. Subsequently, they can choose the best treatment.