How does cancer spread in cats and dogs?
What is metastasis?
Metastasis refers to the spread of a malignant tumor from the site (organ) where the tumor originally started to a distant
organ in the body. Normal cells (with the exception of blood cells) typically adhere to each other, but malignant cells can
gain the ability to break away from the primary tumor (the very first tumor that formed), travel throughout the body via
bloodstream or
lymphatic system, and to initiate new growths at these distant sites. For example, melanoma tumors that
originally developed in a dog's mouth have a tendency to spread to the lungs where they form new tumors.

When cancer cells spread and form a new tumor in a different organ, the new tumor is referred to as a metastatic tumor.
The cells in the metastatic tumor come from the original tumor. This means that if, for example, stomach cancer in a cat
spreads to the liver, then the metastatic tumor in the liver is made up of cancerous stomach cells (not liver cells). In this
case, the disease in the liver would be called metastatic stomach cancer, and not liver cancer.

Where does cancer spread in cats and dogs?
Cancer cells can spread (metastasize) to almost any other organ in the pet, commonly the lungs, bones or liver. Some
cancers tend to spread to specific organs (e.g. oral melanoma in a dog tends to spread to the lungs), however, each
cancer has the ability to spread to any organ throughout the pet's body. Metastases are usually detected by imaging
techniques such as chest X-rays (to detect lung metastases), ultrasound (to detect liver metastases) or CT scans (to
detect brain metastases).

Are there symptoms of metastatic cancer?
Some pets with metastatic cancer may not have obvious symptoms but if they do, the type and frequency of the symptoms
will depend on the extent and location of the metastasis. For example, cancer that spreads to the bones is likely to cause
pain and can lead to bone fractures, or cancer that spreads to the brain can cause symptoms such as seizures.

Sometimes a pet’s primary cancer is discovered only after the metastatic tumor causes symptoms. For example, a pet
whose stomach cancer has spread to the liver may experience abdominal swelling or jaundice (yellowing of the skin) before
experiencing any symptoms from the primary tumor in the stomach.

How do we know whether the tumor is a primary or metastatic?
To determine whether a tumor is primary or metastatic, a veterinary pathologist (a doctor who specializes in disease
diagnosis based on collected samples) examines a tumor sample under the microscope and performs a variety of
specialized diagnostic tests. Cancer cells often look like abnormal versions of cells in the tissue from which they developed
(e.g. cancer cells from a dog's breast tumor may still resemble in appearance normal breast cells even though they were
found in the lung), so the tumor found in the lung is likely a metastatic tumor that came from the primary breast cancer.

If my pet is diagnosed with metastatic cancer, can the primary tumor be found?
When a metastatic tumor is found first, the primary tumor can be found in many cases using diagnostic procedures, such
as laboratory tests or
imaging studies. However, in some cases, the primary tumor cannot be found despite extensive tests.
The pathologist will know if the tumor is metastatic if the cells do not resemble the cells found in the organ or tissue from
which the tumor came. Veterinarians then refer to the primary tumor as unknown, and the pets are said to have cancer of
unknown primary origin.
© 2007 Pet Cancer Center. ALL RIGHTS RESERVED.
Last updated 2/22/2017
Comprehensive guide to cancer diagnosis and treatment in cats and dogs
How does cancer spread?