Although it might remain hypothetical and elusive, the idea of a sentinel lymph node as the first place through which a metastasizing cancer cell must pass in order to infect other body sites is significantly influencing the diagnosis and treatment of this disease. Cancer’s deadliness multiplies greatly when it spreads, and the primary avenue for many types of cancer is the vascular lymphatic system. Most often, the first loose pathogenic cell will be trapped by the first lymph node it encounters. Identifying and treating this first tiny node has the potential to arrest a tumor’s ability to spread.
The human lymphatic system is a network of vessels, not unlike the circulatory system, that spreads throughout the body. It harbors and transports lymphocytes and phagocytes, cells that are specialized to seek and neutralize foreign matter, including cellular debris and alien organisms. Globular sinuses called lymph nodes are found along the network, and they filter the lymph fluid like terminal traps for unwelcome invaders. There are subdermal clusters of them in the neck, armpits and groin, but nodes are distributed throughout the vascular lymphatic system. It is the core system of the body’s functional design for pathogenic immunity.
Medically, when cancerous tissue is discovered, the overriding question is whether it has metastasized, or spread to other places in the body. If the primary tumor is isolated, it can be surgically removed and result in a favorable prognosis for the patient. Simultaneously treating multiple tumors and controlling their spread, however, calls for a different, much more difficult and aggressive, therapy. Oncology, the study and treatment of cancer, has definitively determined that many common cancers can and will quickly migrate to a secondary site via the lymphatic system. To this extent, with breast cancer for example, not only will affected tissue be surgically removed, its nearest lymph nodes might also be preemptively removed to prevent the cancer’s spread.
The concept of a sentinel lymph node was conceived in the 1990s as an alternative to radical lymphadenectomy. Given are the assumptions that metastasizing cancer cells will seek to infiltrate the lymphatic system and that lymph nodes function throughout it as guardhouses for potential pathogens. Conclusions that follow are that, if cancer’s most likely nearest place of infiltration can be surmised, it would lead to a lymph node that can be inspected or monitored for its presence. Not only would such a sentinel lymph node be a significant diagnostic aid toward the critical question of metastasis, it could dictate the course and prognosis of treatment.
Even if it ultimately remains theoretical, the search for sentinel lymph nodes has transformed oncology with advanced surgical procedures. In a sentinel lymph node biopsy, for example, a radioactive substance with a very brief half-life is injected near the tumor. The decaying substance, taken up by the lymphatic system, is easily tracked and mapped by a device called a scintigraph, which will reveal the path of vessels and location of nodes. This is followed by a second injection of dye to make the lymph nodes visible so that a surgeon can dissect the first one in line for immediate laboratory analysis for the presence of cancerous cells. It would seem reasonably so, that this is the sentinel lymph node, and if the biopsy returns a negative result, it could be a high indicator that the tumor has not metastasized and can be treated with less-aggressive therapies.
Questions will likely linger. There could be multiple sentinel nodes, and cancer can spread by means other than the lymphatic system. False negative test results from small biopsy samples might not be totally eliminated. It would seem impractical to follow positive test results from a lymph node to the next and to the next, until a terminal sentinel is found. Nevertheless, this underlying clinical framework of finely targeted identification of the principal path of metastasis has yielded greater knowledge and improved treatment of cancer.