How, Why, When, and Where Does Cancer Spread?

Importance and Mechanism of Metastases

Pancreatic cancer cells can spread to other areas of the body
Pancreatic Cancer Cell. STEVE GSCHMEISSNER/Science Photo Library/Getty Images

Cancer is different from benign tumors in that it can spread (metastasize) to different parts of the body from where it began, and can do so in different ways. Cancer can spread locally, via the lymphatic vessels, through the bloodstream, and in the case of lung cancer, through the airways. Unlike normal cells, cancer cells lack substances called adhesion chemicals that make them stick to other similar cells, and can thus break off and travel.

Importance of Spread (Metastasis)

Cancer, when it stays where it begins, can be very treatable with surgery or other treatments. The reason physicians and cancer patients alike dread this is because the spread of cancer (metastases) are responsible for at least 90 percent of cancer deaths due to solid tumors such as lung cancer, breast cancer, colon cancer, and more. For this reason, and understanding of how and why cancer spreads, and where it is most likely to spread, are critical in reducing the risk of deaths related to cancer.

Why Cancer Spreads

Cancer begins after a series of mutations lead to cells that divide as if they are immortal (never die). This is followed by the growth of a tumor that can invade nearby tissues and organs. But while normal (benign) cells may grow locally, cancer cells are able to detach from the tumor and spread to regions far from the original tumor. A simplistic way to look at it is that normal cells are “sticky.” Heart cells stick to other heart cells and don't travel to the foot for example. Scientifically normal cells make substances called adhesions molecules that make them stick together. Adhesion molecules can be viewed as similar to glue. Cancer cells, in contrast, lack these adhesion molecules making it easier for them to break off and thus travel.

This is only one of the differences between cancer cells and normal cells. Normal cells also understand their "boundaries" better than cancer cells, by communicating and sending signals that essentially say "this is my territory."

How Cancer Spreads

Once a cancer cell is loose (having escaped due to a lack of adhesion chemicals binding it to the tumor as well as other mechanisms) it is free to travel. There are three primary routes in can then take:

Locally (Regionally)

Often the first place cancer spreads is nearby where the tumor starts. Doctors use the words "locally" or "regionally" to describe this kind of spread. In some ways, this is like the growth of a benign (non-cancerous) tumor, but in other ways it is different. While benign tumors can cause problems (especially if they are in an enclosed space like the brain) they tend to be more "polite" and respectful of bordering tissues and cause damage when they push on nearby tissues. Cancers, in contrast, ignore those boundaries and may invade neighboring tissues and organs.regionally, or locally near the original tumor. This claw-like extension into surrounding tissue is responsible for the name cancer, which comes from the Greek word for crab.

Through the Bloodstream

Cancer cells may also enter the bloodstream through which they are carried around the body. When tumors grow, cancer cells may detach and enter blood vessels near the tumor. In fact, part of the requirements of tumor growth is the formation of new blood vessels (angiogenesis) to supply nutrients to the tumor. This provides yet further access to the bloodstream through which tumor cells can break off and spread.

Through the Lymphatic System

The lymphatic system plays an important role in defending the body from cancer, but can also serve as a conduit through which cancer spreads. After expanding locally, cancers tend to spread to nearby lymph nodes via the lymph channels first.

Through the Airways (Lung Cancer)

Recently,

especially adenocarcinoma

may spread through the airways (aerodynamic spread) to nearby or distant lung tissue

studies, it's thought that

intrapulmonary aerogenous metastases

When Cancer Spreads

Once cancer cells spread to another part of the body and begin to form a tumor, it's necessary for the tumor to have a source of nutrition. They accomplish this by a process of forming new blood vessels to bring nutrients to cancer. Researchers have named this angiogenesis. Medications called angiogenesis inhibitors have been used for some cancers, and further work is in progress to find drugs to stop the process of blood vessel formation by tumors so they cannot grow.

Where Cancer Spreads

(and consequences)

As noted above, spread (metastasis) of tumors is responsible for most cancer deaths. But these consequences can vary depending on where cancer spreads. The most common sites of metastases overall occur to the lungs, the bones, the brain, and the liver, though different cancers (and subtypes of cancer) may be more likely to spread to different regions.

Brain Metastases

brain metastases from

other cancers

Lung Metastases

(cancers that spread to the lungs)metastatic cancer tot he lungs

Bone Metastases

by changing the microenvironment of bones so that cancer cells can't "stick" as well

Liver Metastases

Spread to the brain can cause seizures, paralysis, and death. Spread to the lungs can compromise lung function. Spread to bones can cause fractures and the disability associated with fractures.

Cancer Cells in the Blood in Diagnosis

The fact that cancer cells spread via the bloodstream is important in cancer metastases, but can also has the potential to be used to diagnose and define cancers. In other words, researchers are finding ways to use the spread of cancer cells in the bloodstream to detect cancers. Cancer cells, like normal cells, continually go through a process of death and re-growth. In the past, studies looked for the presence of complete cancer cells in the blood, but the yield with this approach is low. Now, due to progress with the human genome progress and more, scientists can check blood tests to look for portions of DNA from cancer cells in the blood or cell-free DNA.

Liquid biopsies, or blood tests looking for these cells or cell-free DNA have become a reality in monitoring some cancers. For example, some lung cancers that have EGFR mutations can now be identified with a simple blood test. It's hoped, however, that blood tests looking for particular genetic abnormalities unique to cancer cells may help to identify the presence of a previously undetectable cancer (as a screening test) in the future.

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