The Lymphatic System
The lymphatic system plays an important role in controlling the movement of fluid throughout the body. Specifically the lymphatic system controls the flow of lymph, a colorless fluid containing oxygen, proteins, sugar (glucose) and lymphocytes (cyte=cell). There are some similarities and differences between the (more well known) circulatory system and the lymphatic system.
Small lymphatic vessels merge into larger ones and these large vessels eventually empty into lymph nodes. Lymph nodes are kidney bean shaped tissues that are found in grape-like clusters in several locations around the body. Lymph nodes are sites of immune system activation and immune cell proliferation (growth). The fluid in this extensive network flows throughout the body, much like the blood supply. It is the movement of cancer cells into the lymphatic system, specifically the lymph nodes, that is used in the detection of metastatic disease. The staging of cancer is discussed in more detail in the Diagnosis and Detection section.
Spread of Cancer Through the Lymphatic System
When a cancer cell has moved through the blood or lymphatic systems or via direct contact to another location, it may divide and form a tumor at the new site. Metastatic tumors often interfere with the functions of the organs involved and lead to the morbidity and mortality seen in cancer.
The lymphatic system plays a crucial role in the metastasis of certain cancers. Lymphatic vessels are designed for entry and exit of immune cells, and are therefore easy for tumor cells to enter. In addition, the flow of lymph is quite slow, so there is little stress to harm cells.1 Researchers originally believed tumor cells invaded the lymphatic system by eroding the vessel walls as the tumor advanced and metastasis would then occur by passive drainage. However, current evidence suggests the interactions between metastasizing cells and lymph vessels are much more active and complex, and specific interactions between the two are required.
The presence of metastases in lymph nodes near the primary tumor often indicates metastasis to distant organs, and is a significant prognostic indicator in many cancers. To assess the presence of metastasis to surrounding lymph nodes, physicians perform a lymph node biopsy. In this procedure, the lymph nodes are removed by surgery and are checked for the presence of cancer cells. Nodes are determined either positive or negative for cancer.
Because lymph drainage pathways from a tumor vary greatly between patients, even for the same area, up to 30% of tumors cannot be accurately predicted to migrate to specific lymph nodes. Improvement in lymphatic imaging and mapping are needed to ensure that metastasizing cancers are not accidentally missed. 2
The diagram below shows the lymphatic system
- 1. Kopfstein, L., and G. Christofori. 2006. Metastasis: cell-autonomous mechanisms versus contributions by the tumor microenvironment. Cell Mol Life Sci. 63:449-68. [PUBMED]
- 2. Shayan, R., M.G. Achen, and S.A. Stacker. 2006. Lymphatic vessels in cancer metastasis: bridging the gaps. Carcinogenesis. 27:1729-38. [PUBMED]