Hi there. The lymphatic system is the bodies drainage system - the way of getting excess fluid to where it needs to be. Therefore, if you have a cancerous area, excess fluid from that area will pass into the lymphatic system, or which the lymph nodes are a part. If the cancer is contained within the original area then the lymph nodes will not be swollen. If however the cancer has been left untreated it may have spread into the lymphatic system and therefore further around the body. Therefore the lymph nodes are checked to see if they are swollen, as this may be an indication that the cancer has spread. Often some lymph nodes are removed to be checked for cancer. This isn't a treatment but gives an indication of the severity of the cancer. Take care and stay safe.