Metastasis and Local Invasion: When Cancer Spreads

Metastasis and local invasion are mechanisms by which cancer infiltrates other parts of the body. Metastasis is when a particular type of cancer spreads from its initial site to more distant parts of the body while local invasion (or spread) is when a malignant tumor expands locally.

Key Takeaways

  • Local invasion is when a tumor gets so large that it infiltrates adjacent organs. Metastasis is when cancer cells break free from a tumor, travel to other parts of the body, establish themselves, and grow to create a new tumor.
  • A dog’s prognosis may worsen once metastasis occurs, because that means the cancer has already spread. Their lifespan will depend on the severity, aggressiveness, and type of cancer they have.
  • Higher grades of cancer tend to have local invasion, metastasis, or both.

Metastasis and Local Invasion: Two Different Ways for Cancer to Spread

Cancer spreads in two ways: metastasis, and local invasion. Let’s look at metastasis first.

Metastasis Is When Cancer Spreads to Distant Body Parts

Metastasis is when cells from an already established tumor break free and translocate, typically via the bloodstream or lymphatic system, to another part of the body, where they grow and divide, creating additional tumors.1

Metastasis is often referred to as the “spread” of cancer. Metastasis usually means the cancer is more aggressive.1

Some of the most common sites of metastasis in dogs are the bone, lymph nodes, spleen, and lung tissues.2

Doesn’t the Immune System Stop Metastasis?

The immune system is your dog’s main defense against metastasis, and for the most part, it does just that.

It’s inevitable that cancerous cells will break away from their tumor and enter your dog’s bloodstream or lymph nodes.3 The immune system is typically well-equipped to catch these stragglers and eliminate them.3

For those cells that do manage to by-pass the immune system, most of them will die due environmental factors in the part of the body they find themselves.3

On average, only about 0.2% of cancer cells break away and successfully establish tumors in other parts of the body.3

Those new tumors in other parts of the body are the cancer metastasis.

The Process of Metastasis Can Be Complicated

Researchers once thought that metastasis happened in a step-wise fashion, proceeding one process at a time.1

However, scientists are now learning that metastasis is more complex, and is the result of multiple overlapping processes.1

There are three steps in metastasis:

  1. Invasion is when changes in the ability of the tumor cell to adhere to other cells allow it to either “ratchet” forward or rapidly move along a path of least resistance.4 This may be triggered by environmental factors such as pH, low levels of oxygen, or other chemical signals in the body.4,5
  2. Survival in the bloodstream is the next hurdle a cancer cell must face. The bloodstream is an unfriendly place for cancer cells. It’s full of immune cells, moves rapidly, and has few places where cancer cells can attach. Regardless, it only takes one cell to be successful. Research suggests that some cancer cells contain signal receptors that make them more likely to survive in the bloodstream.4
  3. Metastatic colonization is the final step in metastasis. Just because a cell has survived the bloodstream and established itself doesn’t mean it will become a tumor. In a study on melanoma, only one in 100 cancer cells were able to persist longer than ten days.4 This is because of the role that environmental conditions and “assistant” proteins play. For example, a cancer cell needs to establish new blood vessels, a process called angiogenesis. This provides the oxygen and nutrients needed for growth.4 However, angiogenesis is dependent on several key players, such as a protein called VEGF.4

Tricky Dormant Cancer Cells: a Cause for Concern?

Some cells attach to tissue, and survive, but do not proliferate. These are called “dormant” cells.4 They remain viable but are quiet and inactive. Not only do scientists not understand what reactivates dormant cells, but they also don’t know much about how to kill them before they reactivate.4

Local Invasion is When Cancer Spread to Tissues Right Next to the Tumor

In addition to metastasis, cancerous tumors can infiltrate via local spread or local invasion.

In this case, tumors grow in place, spreading out. Some grow so big they impact other tissues, structures, or organs.6 For example, a tumor in the nasal cavity might invade the eye socket or the skull.

Both Local Invasion and Metastasis Can Happen at the Same Time

A single type of cancer can grow through local invasion, metastasis, or both.

For instance, an ovarian tumor can grow so large that it invades and spreads to the pelvic wall, kidneys, and abdominal organs while it also releases cells into the bloodstream that metastasize to the brain.

Once metastasis occurs, the prognosis worsens. A single tumor might be removed, but many in a location are more difficult to address. More locations mean a more aggressive cancer and more expensive and challenging treatments.

Using the example above, the ovaries, pelvic wall, and brain, would have ovarian tumor cells present, but treatment would likely focus on the original type of cancer diagnosed while attempting to alleviate symptoms from the primary tumor and in the brain.7,8

How Veterinarians Find Metastasis and Local Invasion

Most of the tests and images that veterinarians run when they stage cancer are looking for evidence of metastasis and local invasion.

Staging your dog’s tumor helps you and your veterinarian understand what you are up against, and helps your veterinarian or oncologist make treatment plans.

The tests needed will vary depending upon the tumor type and location, but some of these tests could include:

  • Blood work can reveal a great deal about your dog’s general health, including white and red blood cell count, kidney, and liver function.
  • Imaging techniques such as ultrasound, x-ray, MRI, or CT scan could reveal where the tumors are, and how big they are.
  • A biopsy, which is a small surgery to get a tumor sample to send out to a pathologist.
  • Local lymph nodes may be sampled, if they weren’t already, to look for metastasis.

Why So Many Tests, Doc?

One of the reasons veterinarians want to run so many staging tests is because they are looking for both metastasis and local invasion, and each test might reveal clues.

For example, comparing measurements from previous appointments might tell your veterinarian that a dog’s mammary gland tumor has grown, which is a local invasion.

But other tests that look for tumors elsewhere in the body will detect whether the breast tumor has metastasized (spread to distant locations). Since breast cancer can metastasize to the liver, your veterinarian might look for a liver mass. If one is found, you now know mammary cancer cells migrated from the primary tumor and are now creating a tumor in the liver.

You don’t have to do every staging test, of course. The value of the information gained has to be balanced against the cost.

For example, if you know you will not do chemotherapy, which treats metastasis in many cancer types, you might not need to look for metastasis. Or, if you know surgery is out of the question, you might not need to get detailed images.

“Metastasis of Unknown Origin”

Occasionally, veterinarians are unable to determine the original location of the cancer cells tested. This is called “metastasis of unknown origin” (MUO) or “metastatic cancer of unknown primary” (MCUP).2,9 Either of these designations generally entails a poor prognosis.2,9

Is My Dog Dying If They Have Metastasis and Local Invasion?

Many dog lovers worry when their veterinarians find evidence of local invasion and/or metastasis. They worry that their dog is very close to death.

Cancers that have grown and spread are indeed harder to treat. It’s also true that they have a generally worse prognosis.

It’s not true, however, that cancer is likely to kill your dog immediately, right away, or even soon. In most cases, especially when you catch it early, there is time to think about your options, read and learn, and make good decisions.

Every case is different, and different cancers have different treatments and protocols, but in every case, there are ways to increase your dog’s quality of life for however much time they have left with you.

If your veterinarian thinks your dog is very close to natural death, especially if they think she is suffering and can’t relieve the pain or discomfort, they will tell you. Hospice, and perhaps eventually euthanasia, may be appropriate.

  1. Suhail Y, Cain MP, Vanaja K, et al. Systems Biology of Cancer Metastasis. Cell Syst. 2019;9(2):109-127. doi:10.1016/j.cels.2019.07.003
  2. Rossi F, Aresu L, Vignoli M, et al. Metastatic cancer of unknown primary in 21 dogs. Vet Comp Oncol. 2015;13(1):11-19. doi:10.1111/vco.12011
  3. Chambers AF, Groom AC, MacDonald IC. Dissemination and growth of cancer cells in metastatic sites. Nat Rev Cancer. 2002;2(8):563-572. doi:10.1038/nrc865
  4. Steeg PS. Tumor metastasis: mechanistic insights and clinical challenges. Nat Med. 2006;12(8):895-904. doi:10.1038/nm1469
  5. Hofschröer V, Koch A, Ludwig FT, et al. Extracellular protonation modulates cell-cell interaction mechanics and tissue invasion in human melanoma cells. Sci Rep. 2017;7:42369. Published 2017 Feb 13. doi:10.1038/srep42369
  6. Australian Government – Cancer Australia. Invasion and metastasis. EdCaN Learning Resources. No publication date available. Accessed on January 30, 2023.
  7. Cleveland Clinic. Metastasis (Metastatic Cancer). Diseases & Conditions. December 20, 2021. Accessed January 31, 2023.
  8. No author. Metastatic Cancer in Dogs. Vet Info. No publication date available. Accessed on January 31, 2023.
  9. Haskell CM, Cochran AJ, Barsky SH, Steckel RJ. Metastasis of unknown origin. Curr Probl Cancer. 1988;12(1):5-58. doi:10.1016/s0147-0272(88)80007-2


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