How do dogs get cancer?

As a veterinarian who has cared for many families facing a cancer diagnosis in their dog, I aim to give clear, practical information you can use now—what cancer is, how it may start, when to worry, and what to do next. The goal here is to make complex biology usable at the kitchen table.

The stakes: how cancer can affect your dog’s life

Cancer is one of the most common serious diseases affecting pet dogs, especially as they get older. Depending on age and breed, cancer may be the single leading cause of death in middle-aged and senior dogs, and even when it is not fatal it can reduce quality of life through pain, reduced mobility, or chronic side effects from the disease or its treatment.

Understanding cancer matters because early detection and thoughtful decision-making often change the options available. Detecting a suspicious lump when it is small may make surgery straightforward and curative; finding a problem late can limit choices to palliation. Knowing the likely outcomes, costs, and commitments ahead of time helps families make decisions that match their values and resources.

There are emotional and financial sides to consider. I routinely see families surprised by how demanding cancer care can be—multiple visits, tests, medications, and time for at-home care. Preparing emotionally and financially, and having a frank conversation about goals for quality of life, makes choices less chaotic if cancer is diagnosed.

At a glance — common pathways by which dogs develop cancer

At its simplest, cancer is the result of cells that acquire errors in their DNA and begin to grow and divide in an uncontrolled way. Those errors can come from genes a dog inherits, or from damage accumulated over a lifetime from inflammation, chemicals, sunlight, or random mistakes during cell division.

Some cancers are likely linked to inherited risk: certain breeds show higher odds for particular tumors. Other cancers are usually acquired during life. The cancers most commonly seen in dogs include skin tumors (like mast cell tumors and melanomas), lymphomas, bone cancers such as osteosarcoma, and mammary tumors in unspayed females. Which types are most likely depends on a dog’s age, sex, and breed.

From cell to tumor: the biology behind canine cancer

Cancer usually begins at the DNA level. Over time a cell accumulates changes that reduce its ability to repair DNA, to die when it should, or to control growth. When enough of these changes pile up, a single cell can start dividing more than it should and form a lump or tumor.

As that abnormal clone expands, its behavior can change: some tumors remain localized and grow slowly, while others invade nearby tissues or enter blood or lymphatic channels and seed new tumors in other organs—a process called metastasis. Tumors may also encourage growth of new blood vessels to feed themselves, which can speed growth in some types.

The immune system usually watches for and eliminates abnormal cells, but this surveillance can fail. Immune defenses may be overwhelmed if many mutated cells appear, or tumors may actively suppress immune responses. That’s why immune-based treatments are now part of veterinary care for certain cancers—when the immune response is coaxed back into recognizing tumor cells, outcomes can improve.

When the odds increase: key risk factors for dogs

Age is the strongest single factor: cancer risk climbs as dogs get older because mutations accumulate and repair systems wear down. Breed also matters—large and giant breeds tend to get osteosarcoma more often, Boxers and Bernese Mountain Dogs show higher rates of certain skin cancers and histiocytic tumors, and purebred lines with small gene pools can concentrate risk for specific cancers.

Chronic inflammation and obesity are likely linked to higher risk in many species, including dogs. Longstanding skin irritation, chronic infections, or persistent wounds can create an environment where abnormal cells are more likely to emerge and thrive. Hormonal influences matter, too: intact female dogs are more likely to develop mammary tumors, and spaying at a young age reduces that risk.

Environment is a contributor but usually not the sole cause. Exposure to tobacco smoke, certain lawn pesticides, industrial chemicals, prolonged ultraviolet light on thin or lightly pigmented skin, and prior therapeutic radiation can increase risk. In most cases these exposures are one of several factors that together push the process forward.

Spotting trouble: warning signs and early red flags in your pet

• Any new lump or mass, or a lump that changes size, shape, or texture; quick growth is particularly concerning.
• Sores that do not heal, persistent swellings, unexplained bleeding, or discharge from body openings.
• Unexplained weight loss, decreased appetite, lethargy, persistent lameness (especially in large breeds), difficulty breathing or a chronic cough, and changes in bathroom habits.

Some cancers produce few signs until they are advanced, so routine checks—feeling along your dog’s body, inspecting skin and mouth, and noting changes in behavior—are important. If you notice anything new or persistent for more than a week, it’s reasonable to arrange a veterinary exam sooner rather than later.

If you suspect cancer: practical next steps for owners

1. Document what you see: take clear photos of lumps or wounds from multiple angles, note when you first noticed the change and whether it’s growing or bleeding, and keep a simple timeline of symptoms.
2. Call your veterinarian and describe the findings; request an appointment for evaluation rather than waiting. For masses, ask whether a fine needle aspirate (a quick cytology test) is appropriate that day.
3. Expect basic diagnostics: physical exam, bloodwork to check overall health, cytology or biopsy to identify the tumor type, and imaging (X-rays, ultrasound, sometimes CT) to determine if cancer has spread.
4. Discuss treatment options openly: surgery, chemotherapy, radiation, targeted or immunotherapies where available, and palliative care to manage symptoms. Ask about goals—curative, control, or comfort—and likely timelines and costs.
5. If the diagnosis or plan is complex, consider asking for a referral to a veterinary oncologist or a second opinion from a specialty center. Bring your records, images, and biopsy slides if possible to speed the process.

Throughout, be your dog’s advocate: keep organized records, ask for written estimates, and balance realistic expectations about outcomes with what you can provide at home. If treatment is chosen, regular reassessments are important—both for effectiveness and for your dog’s comfort.

Home, routines and training: reducing risk and supporting recovery

You can reduce some risks and make treatment easier by managing the home environment. Minimize exposure to known carcinogens where practical: avoid smoking around your dog, limit unnecessary use of lawn pesticides, and store household chemicals safely. For dogs with thin or lightly pigmented noses and ears, minimize prolonged sun exposure or use vet-approved sunblock to reduce risk of UV-related skin cancers.

Maintaining a healthy weight and consistent exercise routine helps overall immune function and may reduce cancer risk. Nutrition that meets life-stage needs without excess calories is sensible; extreme diets are rarely helpful unless guided by a veterinarian for a specific condition.

Regular grooming and hands-on body checks are one of the most effective habits for early detection. I recommend weekly quick checks: run palms along the body to feel for lumps, inspect ears, eyes, mouth, and between toes, and note any areas of redness, swelling, or discharge. The more you know your dog’s normal, the quicker you’ll notice change.

Practical equipment and helpful support tools for dogs with cancer

When a dog is undergoing treatment or recovering from surgery, a few practical items make a big difference. Soft recovery collars and inflatable alternatives reduce stress and protect wounds. Wound-care supplies—non-stick gauze, antiseptic wipes recommended by your vet, and medical adhesive tape—are useful, as are protective garments or T-shirts to cover shaved or sore areas.

Comfort and mobility aids can preserve quality of life: low ramps for cars and furniture, non-slip mats, supportive orthopedic bedding, and slings or harnesses for dogs with hind-end weakness. For medication management, a simple pill organizer, a written medication log, and a digital thermometer help prevent dosing errors and make follow-up calls to your clinic clearer. A small refrigerator thermometer for refrigerated meds and syringes for liquid medicines can also be practical.

Who to contact: vets, specialists and trusted support organizations

Your primary care veterinarian is the first contact for evaluation and initial diagnostic steps. When the diagnosis or treatment is beyond general practice, a veterinary oncologist or a surgical oncologist at a specialty or teaching hospital can offer staging, specialized therapies, and access to clinical trials. Many university veterinary hospitals maintain oncology services and can accept referrals for advanced imaging and multidisciplinary care.

For reliable, owner-focused guidance and for help locating specialists or clinical trials, the Veterinary Cancer Society and university oncology pages are useful. The American Veterinary Medical Association provides consumer information about pets and cancer and links to professional resources. I often point owners to the AKC Canine Health Foundation for summaries of current canine cancer research and funding opportunities.

References, cited studies and further reading

• Merck Veterinary Manual: “Neoplasia (Cancer) in Dogs and Cats” — Merck & Co., Inc.; section on canine tumors and diagnostic approach.
• Withrow SJ, Vail DM. Small Animal Clinical Oncology, 6th Edition. Elsevier, 2019 — standard veterinary oncology textbook covering biology, staging, and therapies.
• Veterinary Cancer Society: Owner Resources and Tumor Guides — practical summaries for owners and lists of oncology specialists.
• Cornell University College of Veterinary Medicine: “Canine Lymphoma” and oncology client information pages — staging and treatment overviews.
• AKC Canine Health Foundation: “Cancer in Dogs” research summaries and breed-specific studies.
• Dobson JM. “Breed-predispositions to cancer in pedigree dogs” — ISRN Veterinary Science, review of breed-associated cancer risks.