Home » Pet Health » Cancer 101 » What is a Tumor?


A tumor, also called a neoplasm, is an atypical solid mass of cells formed due to the rapid division of abnormal cells. Tumors can occur in any body area, including bones, muscles, and organs, but not in blood. There are two types of tumors: (1) benign (non-cancerous) and (2) malignant (cancerous). It is difficult to predict how a tumor’s development will progress; some non-cancerous tumors will remain benign, and others may become cancerous.  Therefore, it is crucial to monitor any abnormal growths.

Benign tumors

Benign tumors are non-cancerous tumors that can form anywhere in the body. They grow slowly and do not invade nearby healthy tissues or spread throughout the body. Benign tumors often require no treatment outside of surgery, and they are easily curable as they generally do not relapse after surgical removal.

Malignant tumors

In contrast to a benign tumor, a malignant tumor is a cancerous tumor comprised of abnormal cells that divide uncontrollably. Cancerous cells have the potential to overgrow and invade nearby normal tissues, where they compete with healthy cells for nutrition, oxygen availability, and blood supply. They may also spread to other parts of the body through the blood or lymphatic system to form new tumors in a process called metastasis. Although current cancer treatments have yielded significant results, some malignant tumors are resistant to treatment (chemotherapy or radiation) and may recur even after surgical removal.

Cancerous tumor types

The most common cancers found in our pets occur in the breast, skin, mouth, connective tissue, and lymphatic tissue. Cancer can also form in the blood and is referred to as leukemia, while cancer in blood-forming tissues is called lymphoma. However, blood-borne cancers do not form tumors; they remain as single cells.

A tumor on body surfaces, glands, and areas where the body produces hormones is called a carcinoma. Usually, cancers that occur in the skin, colon, lung, stomach, breast, prostate, and thyroid gland are carcinomas.

In contrast, a sarcoma is a tumor that develops in your pet’s muscles, bones, and connective tissues. A very common type of sarcoma is osteosarcoma, which is a cancer that occurs in the bones.

Although cancer is prevalent in pets, there are several treatments available. If you receive a cancer diagnosis for your pet, your vet will be able to tell you what type of cancer they have and provide you with the best treatment options so that you and your pet will have many quality years together.

The Pet Cancer Foundation’s Website Editorial team is comprised of veterinarians, veterinary oncologists, and veterinary technicians, as well as scientific writers and editors who have attained their PhD’s in the life sciences, along with general editors and research assistants. All content found in this section goes through an extensive process with multiple review stages, to ensure this extended resource provides pet families with the most up-to-date information publicly available.

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Last Updated: September 11, 2022

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The following sources were referenced to write the content on this page: 

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Gardner, HL, Fenger, JM & London, CA 2016, ‘Dogs as a model for cancer’, Annu Rev Anim Biosci, vol. 4, pp. 199-222.

Gupta, GP & Massagué, J 2006, ‘Cancer metastasis: building a framework’, Cell, vol. 127, no. 4, pp. 679-695.

Joyce, JA & Pollard, JW 2009, ‘Microenvironmental regulation of metastasis’, Nat Rev Cancer, vol. 9, no. 4, pp. 239-252.

Merck & Co., Inc., Merck Manual Veterinary Manual, 2022, viewed 11 September 2022, https://www.merckvetmanual.com/special-pet-topics/cancer-and-tumors/types-of-cancer

Sudhakar, A 2009, ‘History of Cancer, Ancient and Modern Treatment Methods’, J Cancer Sci Ther, vol. 1, no. 2, pp. 1-4.

The Pet Cancer Foundation’s medical resource for pet owners is protected by copyright.

For reprint requests, please see our Content Usage Policy.