The sentinel node is the first of a chain lymphatic draining into a certain tissue territory. This means that before continuing along the chain, lymph coming from that territory must first pass through the sentinel node. The sentinel node is often related to oncology. In the same way that the lymph of a territory must progress through the lymphatic chain ganglia, tumor cells that might be released in that territory must circulate and spread through the lymph chain, the first being the sentry. When cancer cells break away from the primary tumor and circulate through the lymph or blood to other places in the body could form a secondary tumor, a process called metastasis.