When doctors talk about metastasis, they are referring to a cancer that was inactive and remaining in a specific location, which has now begun actively spreading to other locations.
The first tumor, known as the primary tumor, originated from a single normal cell that was genetically injured, resulting in a cancer stem cell that has genetic instructions to create malignant cells. One of the characteristics of these cancer stem cells is that they can divide uncontrollably, rapidly increasing the amount of cancer cells at that location. When enough cancer cells have been created in that area so that it can be detected by examination and testing, it is then called a tumor.
These cells can also spread to lymph nodes near the site where the primary tumor is located. This is referred to as nodal involvement. If the cells do spread to area lymph nodes that are adjacent to the primary tumor, that is not necessarily an indication that a second tumor will develop, but it is an indication of a worse prognosis.
However, cancer cells are capable of entering the surrounding normal tissues that are near the original tumor site to form another tumor. The newly formed tumor in the nearby site is called a local metastasis.
Malignant cells are also able to infiltrate the walls of fluid-carrying mechanism like the lymphatic system and blood vessels. This allows them to circulate through the bloodstream to other areas of the body, stopping at one or more sites along the way. When they stop, these cells enter new tissue and start dividing just as they did in the original tumor site. After enough cells have collected in the new site so as to be able to be found during examination and testing, this new tumor is called a metastatic or secondary tumor.
In order for cancer cells to survive and develop, they need a continuous flow of oxygen and nutrients. Cancer cells create a structure to ensure this flow through a process called angiogenesis. The process begins when the cancer cells in the tumor release molecules the signal the surrounding host tissue that the tumor needs its own network of blood vessels to carry nutrients and oxygen and to remove waste products.
This signaling triggers genes in the host tissue that make proteins called Vascular Endothelial Growth Factors (VEGF), that when secreted, stimulate the growth of the necessary blood vessels. Vascular Endothelial Growth Factors are overexpressed in most malignant mesothelioma cases, meaning they are present in abnormally high levels.
Mesothelioma is a very aggressive type of cancer that tends to spread throughout the body. According to the Mayo Clinic, the spread of pleural mesothelioma can be broken down in the following manner:
- Localized cancer, meaning it’s limited to one portion of the lining of the chest.
- The cancer has spread beyond the lining of the chest to the diaphragm or to a lung.
- The cancer has spread to other structures within the chest and may involve nearby lymph nodes.
- In advanced mesothelioma, cancer has spread more extensively within the chest. It may have spread to distant areas of the body, such as the brain and lymph nodes elsewhere in the chest.
Keep in mind that pleural mesothelioma is the only form of the disease that has been studied enough to understand how it spreads.