Home » Pet Health » Cancer 101 » What Happens When Cancer Spreads?


When cancer cells spread, they separate from the original tumor site and migrate to other distant areas of the body. The cells will attach themselves to tissues at a new destination, start dividing and form new tumors. The spread of cancer to other body sites is metastasis, but you might also hear it referred to as advanced cancer or stage IV cancer. The most common areas where cancer spreads are bone, liver, and lung.

Reports show that cancer can spread in a few ways:

    1. Growing directly into the tissue that surrounds the tumor
    2. Using the bloodstream as a highway to get to distant areas in the body
    3. Traveling through the lymphatic system to get to nearby or distant lymph nodes

No matter where the metastatic cancer cells end up, they will have features that are the same as the cancer cells from the original tumor site, not from the place where the cancer cells have newly settled and developed. When a cancer specialist (oncologist) examines a tumor, the features of the cells can help determine that the cancer has spread from another body site. For example, cancer cells originating in the breast may spread to the lungs (known as metastatic breast cancer) but still exhibit features associated with the original breast cancer cells. These cells will continue to divide and spread until treated. When cancer grows to the point of spreading to other areas where a new tumor forms, the tumors are late-stage malignant tumors.

The cancers most likely to spread are breast, skin, urinary system, lung, and heart cancers. Additionally, the naturally occurring transmissible cancer in dogs, canine transmissible venereal tumor (CTVT), is metastatic.

Although it may be more difficult to treat cancer once it has spread, it is not impossible.  Vets and oncologists will base treatment of metastatic cancers according to the tumor’s original site. Treatments can vary, including surgical removal, radiation, or chemotherapy. Your vet will closely monitor your pet after treatment to ensure that cancer cells have not remained. If the cells evade treatment, they may cause new growth, resulting in relapse, at which point you and your vet can determine the best next steps.

If your vet detects cancer early, they may recommend surgery with an additional therapy post-surgery to help reduce the chances of cancer spreading. They will also do everything they can to keep you and your pet at ease when your furry friend is undergoing treatment and recovery. Furthermore, vets can provide excellent resources to help you plan for your pet’s health after cancer.

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 13, 2022

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

Boston, SE, Lu, X, Culp, WT, Montinaro, V, Romanelli, G, Dudley, RM, Liptak, JM, Mestrinho, LA & Buracco P 2014, ‘Efficacy of systemic adjuvant therapies administered to dogs after excision of oral malignant melanomas: 151 cases (2001-2012)’, J Am Vet Med Assoc, vol. 245, no. 4, pp. 401-407.

Chen, Z, Zhang, P, Xu, Y, Yan, J, Liu, Z, Lau, WB, Lau, B, Li, Y, Zhao, X, Wei, Y & Zhou, S 2019, ‘Surgical stress and cancer progression: the twisted tango’, Mol Cancer, vol. 18, no. 132.

Essner, R 2003, ‘Surgical treatment of malignant melanoma’, Surg Clin North Am, vol. 83, no. 1, pp. 109-156.

Merck & Co., Inc., Merck Manual Veterinary Manual, 2022, viewed 13 September 2022, https://www.merckvetmanual.com/special-pet-topics/cancer-and-tumors/types-of-cancer

Pizzoni, S, Sabattini, S, Stefanello, D, Dentini, A, Ferrari, R, Dacasto, M, Giantin, M, Laganga, P, Amati, M, Tortorella, G & Marconato L 2018, ‘Features and prognostic impact of distant metastases in 45 dogs with de novo stage IV cutaneous mast cell tumours: a prospective study’, Vet Comp Oncol, vol. 16, no. 1, pp. 28-36.

Rossi, F, Aresu, L, Vignoli, M, Buracco, P, Bettini, G, Ferro, S, Gattino, F, Ghiani, F, Costantino, R, Ressel, L, Bellei, E & Marconato L 2015, ‘Metastatic cancer of unknown primary in 21 dogs’, Vet Comp Oncol, vol. 13, no. 1, pp. 11-19.

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

For reprint requests, please see our Content Usage Policy.