What is metastatic (non-localized) kidney cancer?
Metastatic kidney cancer is cancer that has spread from the kidney where it started to a different organ or tissue. The spread of kidney cancer can occur in one of three ways: 1. There can be direct growth from the kidney to adjacent tissues, 2. Kidney cancer cells can leave the kidney and enter the bloodstream through arteries or veins, and 3. Kidney cancer can enter the lymphatic system (the network of vessels that contain most of our cells that fight infection and drain extra fluid and waste from organs in the body). Of these mechanisms, kidney cancer is most likely to spread through veins and by direct extension.

The idea behind cancer metastasis was originally proposed by James Ewing in 1928 as the “seed and soil” theory, which stated that cells must travel from the original tumor as a “seed” and find a proper host tissue (the “soil”) in which to implant.  In principle, this idea has held true through further investigation, and 2 steps are currently believed to be key to metastasis.  First, the kidney cancer cells must be able to break free from the kidney.  All kidney cells are held in place by anchoring tissue which is known as extracellular matrix which is made from proteins and carbohydrates.  The purpose of this matrix is to keep the cells in place as well as provide structural support to the organ.  In order for cancer cells to break free of the matrix, they have to make substances that break down the matrix components so that they can break free.  Once the kidney cancer cells break free, they then have to be able to survive on distant organs.  This process requires the kidney cancer cells to make other proteins that recognize and bind to distant organs. 

Once both of these processes have occurred, the kidney cancer becomes non-localized, or metastatic. The process by which cancer cells gain these properties is through mutation of DNA, which is a random process that can take a great deal of time. For this reason, metastatic disease occurs later in the course of cancer. (I think an image here of the process of metastasis would be great. This is a lot of information that I think of in picture form, so I'm sure others would like that as well.)

When kidney cancer metastasizes to a new location, it is referred to as metastatic kidney cancer in the new organ or tissue. Although it resides in a different location, the cancer cells continue to look and behave like kidney cells. This is why some people have "kidney cancer in the lungs," for example. This is true of all cancers. In fact, a cancer patient is much more likely to have cancer originating in a single organ spread throughout their body than they are to develop multiple different cancer types (or have multiple "primary cancers").

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