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Metastasis is a medical term that can strike fear in the minds of cancer patients. It’s the process of cancer cells spreading to vital organs and then interfering with their normal function. This is a 2 minute animation that describes the complex process of cancer circulating to tissues, invading across the blood vessels, acquiring a new circulation (to get oxygen and nutrients), and then growing. The animation is oversimplified, but it may be a start to those who are trying to understand more about how cancer cells function.
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Cancer cells can spread by growing into the surrounding blood vessels or lymphatic channels and then circulating to the lymph nodes in the region or beyond to vital organs, such as the liver or lungs. Some experts have estimated that less than one cell in a million can survive long enough to help form a metastatic tumor. Cells that do survive begin the growth of another tumor. Sometimes, the tumor cells fall or move to a nearby structure. This occurs most commonly in certain abdominal cancers, such as ovarian cancer or cancer of the appendix. Cells that break off an ovarian tumor may form sheets of cells along the lining of the abdominal cavity and grow on another organ in the abdomen, such as the bladder or the outside of the colon. This process is known as seeding.
Cancer cells that detach from a tumor can also travel through the lymphatic system, a network of vessels connecting lymph nodes, small bean-shaped glands located throughout the body. Lymph nodes are often the first place to which many cancer cells travel. Because of this, during surgery to remove a primary tumor, the surgeon will often remove nearby lymph nodes. A pathologist will examine the nodes with a microscope to determine if cancer cells are present. The presence or absence of cancer in lymph nodes is a factor in determining the stage of cancer, or how advanced it is. When many cancer cells are found in one or more nearby lymph nodes, it is referred to as regional disease or stage III cancer. Cancer cells can also travel to lymph nodes far from the primary tumor or to other organs or tissues in the body, where they collect to form a metastatic tumor. This is sometimes referred to as distant disease or stage IV cancer.
Cancer cells shed directly from the original cancer or from metastases growing in the lymph nodes and enter the body’s main bloodstream, which means that cells can be taken anywhere in the body. However, in order to grow, these cells must be able to lodge in a new tissue, overcome the local defenses and acquire their own blood supply and nutrients. This blood supply is created when tumor cells release substances that attract vascular cells, or cells that form new blood vessels. The formation of a blood supply within the tumor, known as angiogenesis, not only offers a way for cancer cells to travel to the body’s main bloodstream but it also enables the tumor to grow more quickly. This is why some treatment approaches are designed to attract the tumor’s blood supply so it will starve.
Not all cancers spread, but any cancer can spread to any other part of the body. When it does take hold in an organ, it can grow in a disorderly fashion and interfere with the normal function of that organ. When cancer does spread, it more commonly metastasizes to vital organs such as the lungs, liver, bone, and brain. It may take months or even years for a microscopic metastasis to grow large enough to be seen on x-ray images or to cause symptoms. Also, some cancers that spread affect a particular organ or tissue more commonly than others. The cells of a metastatic tumor are derived from those originating in the primary tumor. For example, if colorectal cancer metastasizes to the liver, the tumor in the liver is made up of colorectal cancer cells, not liver cancer cells. This is how a pathologist can identify a tumor as metastatic cancer. Knowing whether a cancer is a metastatic tumor is important because treatment is based on the type of cancer.