What is a Metastasis and Why, How, and Where Do They Happen

Definition and Importance of Metastases With Cancer

diagram of caner cells spreading through tissue
What is the meaning of the term metastasis?. istockphoto.com

What does the word metastasis mean?   How, why, and where do cancers spread?

Definition of Metastasis

A metastasis is defined as the spread of cancer cells from their primary location to another region of the body.  Cancer that has spread in this way is called metastatic cancer.  

Metastatic cancer is named based on the site where the cancer began.  For example, if lung cancer spreads to the bones, it would not be called “bone cancer” but rather  “lung cancer metastatic to the bones.”  In this case, when the metastatic cells are looked at under the microscope they would be cancerous lung cells, not bone cells.

Some cancers are metastatic at the time of diagnosis, while others become metastatic after the cancer has progressed, or recurs.  When a cancer is gone and then later recurs at a site away from the original cancer, it is termed a "distant recurrence."  In staging cancers, a tumor that has metastasized is usually considered stage 4.

Importance of Metastases

The ability to metastasize is one major characteristic that distinguishes malignant (cancerous) tumors from benign (non-cancerous) tumors.  Some benign tumors can grow to be quite large, and cause significant problems, especially if they are in an enclosed space such as the brain.  Yet these tumors do not spread to other regions of the body. 

Metastases are responsible for 90 percent of cancer deaths.                                                                                                                                

How Do Cancers Metastasize (Spread)?

Cancer cells differ from normal cells in several ways, one of which is that cancer cells are able to detach from nearby cells in order to invade and spread to other tissues.

 Normal cells make adhesion molecules which act like glue, holding similar cells together.  Cancer cells lack these adhesion molecules allowing them to become "loose" and travel.  Another difference is that normal cells communicate with other nearby cells -- in essence, being reminded of their boundaries.

Cancer cells have devised ways to ignore these communication signals  Once they are “loose” and mobile, cancer cells can metastasize to other regions by a few methods:

  • Locally (regionally)  When benign tumors grow they do so as a solid mass, as if there is a clear boundary containing them.  In contrast, cancer cells invade neighboring tissues in an invasive manner which can appear like tentacles.  It is, in fact, the claw-like extension of cancer into other tissues from which the name originates; cancer being derived from the Greek word for claw.
  • Through the bloodstream - Cancer cells can enter the bloodstream and travel hence to other regions of the body.
  • Through the lymphatic system - The lymphatic system is another network through which cancer cells can travel.
  • In addition to these methods, recent studies suggest that lung cancer, likely spreads through the airway of the lungs (aerogenous metastasis) as well, and this may be even more important than spread through the bloodstream for people with lung adenocarcinoma.

Once a cancer has spread, further steps are needed to ensure the cancer cells can continue to grow.  One necessity is the formation of new blood vessels to feed the new tumor, a process called angiogenesis.

 Medications called angiogenesis inhibitors work to such as interrupt this process, making if difficult for tumors to establish themselves in new areas.

Where do Cancers Spread?

Most cancers have the ability to spread to any region of the body, but some sites of metastases are more common than others.

The most common sites of metastasis overall include the bones, liver, and lung.

The most common sites for breast cancer to metastasize are the bones, the brain, the liver, and the lungs.

The most common sites for lung cancer to spread are the adrenal glands, the bones, the brain, the liver, and elsewhere in the lungs.

 This article discusses the question, where does lung cancer spread?

The most common sites for colon cancer to metastasize are the liver, the lungs, and the peritoneum.

The most common sites to which prostate cancer spreads are the adrenal glands, the bone, the liver, and the lung.

Symptoms of Metastases

Symptoms of metastatic cancer can include those related to the presence of tumor in a particular area of the body, as well as non-specific symptoms such as weight loss and fatigue. Some symptoms may include:

  • Lung metastases may cause a persistent cough, shortness of breath, or chest pain.
  • Brain metastases may cause headaches, vision loss, numbness or weakness of the arms or legs, and loss of balance.
  • Bone metastases may cause pain in the region where the affected bone is located, as well as an elevated calcium level in the blood.  When cancer is present in a bone it can be more likely to fracture, and the first sign of a bone metastasis may be a pathologic fracture (fracture through a bone damaged by tumor.)  When cancer spreads to the spine, it can rarely cause spinal cord compression with resulting weakness of the legs and bowel or bladder dysfunction.
  • Liver metastases may cause jaundice (a yellow discoloration to the skin,) bloating, abdominal pain, and weight loss.
  • Metastases to the adrenal gland are often asymptomatic, but important with regard to treatment.

Treatment of Metastatic Cancer

The treatment of metastatic will depend upon the location of the primary tumor.  Metastatic cancer cannot usually be cured, but it is treatable.  Newer medications such as targeted therapies, and immunotherapy, are improving the survival rates for some people with metastatic cancer, and several medications are being studied in clinical trials which brings hope that further improvements in metastatic cancer treatment are near.

For some people who have one or only a few sites of metastasis (oligometastases,) removing the metastasis with surgery or radiation may improve survival.


American Cancer Society. What is Metastasis? Updated 05/02/16. http://www.cancer.org/treatment/understandingyourdiagnosis/bonemetastasis/bone-metastasis-what-is-bone-mets

Gaikwad, C. et al. Aerogenous Metastases: A Potential Game Changer in the Diagnosis and Management of Primary Lung Adenocarcinoma. American Journal of Roentgenology. 2014. 203(6):570-82.

National Cancer Institute. Metastatic Cancer. Updated 07/11/16. https://www.cancer.gov/types/metastatic-cancer

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