Common phrased as “stage 4” in cancer diagnostic terms, invasion and metastasis of cancer is one of the main contributors of cancer mortality.
Invasion refers to the cancers ability to invade the bloodstream, while metastasis refers to the spreading of these cancer cells in the bloodstream to other organs where they land and begin to grow.
These terms can be better described looking at the steps of invasion and metastasis.
Step 1: Progression. Cancer begins and continues to proliferate at a certain contained location. This could be any organ that the cancer resides, such as the lung, or the liver for example.
Step 2: Invasion. These stationary cells switch to becoming migrant. This is called an EMT, or epidermal to mesenchymal transition. Epidermal refers to the stationary cells, while mesenchymal refers to in the lymphatics or blood.
Step 3: Intravasation. These are when the EMT cells from step 2 go into the blood stream.
Step 4: Transport. The cells move around the body through the circulatory system.
Step 5: Extravasation. Cells leave the circulatory system and become stationary. This is referred to as MET, or mesenchymal to epidermal transition (the opposite of EMT).
Step 6: Colonization. The cells that have become stationary begin to form a tumor, going from a little mass (micrometastasis) to a larger mass (macrometastasis).
Source: Figure 14.17b The Biology of Cancer (© Garland Science 2007)
Topics in Cancer Biology 