Tumors and Cancer In Dogs

mammary tumor in a dog: courtesy of Dr Joel Mills 
A tumor (or tumour) is abnormal growth of cells that causes a mass that is not normally present. Neoplasm is another word that is sometimes used. Tumors can occur anywhere in the body.

A tumor may or not be cancer. Tumors are classed as benign, precancerous or cancerous.

Dogs of any age can get tumors, benign or cancerous. The majority of tumors occur in middle aged and old dogs.


Get any new lump, one that is growing rapidly, one that is irritated in any way or looks red, or one that changes in any way checked by your vet.

Types of tumors

Benign tumors

Benign tumors are not able to spread to adjacent areas or other organs in the body.

A lipoma is an example of a common benign tumor in dogs. Lipomas are composed on mature fat cells and are most frequently found under the surface of the skin.

Though a benign tumor such a lipoma is not cancerous, they can occasionally cause health problems. This can be due to their location and/or size. As examples, a benign tumor that presses on an organ or nerves, or grows to block the throat, major blood vessel etc. can cause local damage or affect body function.

In the case of lipomas, most are not removed, however those that are in locations such as the mouth, or are in the armpit and get large enough to affect movement will usually be surgically excised.

Precancerous tumors

Precancerous, sometimes called premalignant, tumors are those that have the potential to become cancerous but are currently not a cancer.

For example about half of mast cell tumors at the time of diagnosis are classed as precancerous (Grade 1). Some of these tumors will remain at this stage, others will at some time become cancerous. There are currently few ways to tell whether and how fast a tumor may progress to being malignant.

Cancerous tumors

Cancerous, or malignant, tumors are those that can invade bordering areas and often (but not in every case) into other areas. The term metastasis means spread to a non-adjacent area.

The growth of “normal” cells is tightly controlled by the body; whereas cancer cells have unregulated growth.

Classification of cancers

Cancers are classified according the cell type from which they are thought be derived. Common examples are carcinoma which are derived from epithelial cells (the cell type that lines many the inside cavities of many organs and vessels and forms many glands), sarcomas derived from either connective tissue cells and mesenchymal cells (the cell types that form the structure of bones, cartilage etc.) and lymphoma and leukemia derived from blood cells.

A cancer will start in one tissue, which gives it the simple common name used. Examples are bone cancer or liver cancer.

Since many tissues have more than one cell type, then a more descriptive name can include which of the cell types the cancer originated in. For instance for liver cancers, the most common cancer type is hepatocarcinoma – liver (hepato) cancer derived from an epithelial cell type (carcinoma).

Treatment and outcomes of cancer

The treatment of any cancer depends on many things, the type of cancer, the location and size of the primary cancer, whether the cancer has a distinct boundary, whether the cancer is invasive locally, whether the cancer has metastasized and the location of any secondary cancers, the age and overall health of the dog, and a range of other factors.

Obviously, the best treatment, if possible, is to remove all the cancer surgically. Unfortunately, this is often not possible.

Outcomes depend on how early or late the cancer was diagnosed, all the factors listed above, plus how advanced the cancer is, how fast the cancer is growing and generally how aggressive the cancer is.


Tumors can be benign, precancerous or cancerous.

There are a range of types of cancer. Treatment and outcomes depend on a wide range of factors. There is no one size fits all for cancer.

Get any new lump, one that is growing rapidly, one that is irritated in any way, one that looks red, or one that changes in any way checked by your vet. The sooner a cancer is diagnosed the better the chance that treatment will be successful.

Photo courtesy of Dr Joel Mills: Mammary tumor in a dog