Cancer in dogs and cats is not one disease. It is a group of diseases that can start in many different tissues, and the types seen most often differ somewhat between the two species. In dogs, lymphoma, mast cell tumors, and osteosarcoma are among the commonly discussed cancers. In cats, lymphoma is common, and oral squamous cell carcinoma is a well-known site-specific example. No lump, weight change, or other sign identifies a particular cancer on its own. Each change needs a veterinary examination, and in most cases a sample of the affected tissue, before anyone can say what it is.
How common cancer is in pets
The American Animal Hospital Association (AAHA), in its 2026 overview of common cancers, states that 50% of dogs and approximately 30% of cats over age 10 are affected by cancer. Read this as a population-level figure. It describes how many animals in a group have cancer. It is not a lifetime risk estimate for your pet, and the summary does not give a separate age range for the dog figure. Only a veterinarian who examines your animal can talk about its individual situation.
What the common-cancer lists do and do not tell you
AAHA’s January 2026 overview is organized into separate canine and feline tables. These tables are quick references for starting a conversation with your veterinarian. They are not a complete ranking of cancers in either species, they do not capture all the variation in how tumors behave, and they cannot predict how an individual animal will do. They are also not clinical decision tools. For the primary sites, staging, and treatment detail of any tumor type, read the full AAHA tables and ask your veterinarian to apply them to your pet.
Cancer can arise in many tissues and organs. Dogs and cats share several common cancers, but their typical profiles overlap without being identical.
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Common cancers in dogs
AAHA’s canine table includes lymphoma, mast cell tumors, osteosarcoma, soft-tissue sarcoma, and other tumor categories. Lymphoma also affects cats, so it is covered in its own section below. The other three are described here.
Mast cell tumors
Cornell University College of Veterinary Medicine describes mast cell tumors as the most common malignant skin tumor in dogs. They can look like benign lumps, so appearance alone cannot tell an owner or a clinician what a lump is. Fine-needle aspiration, which collects cells with a thin needle for examination under a microscope, is a common first diagnostic step. Tumor grade and any evidence of spread help guide care.
Osteosarcoma
Osteosarcoma is a cancer of bone. Cornell describes pain and progressive lameness as common clinical signs, and notes that osteosarcoma commonly spreads to the lungs. Diagnosis starts with the history and bone radiographs (X-rays). Confirmation comes from a tissue sample or from removal of the tumor for examination.
Soft-tissue sarcoma
AAHA’s canine table lists soft-tissue sarcoma as a separate category. The tumor’s location and how far it extends into surrounding tissue shape the workup and the options, so ask your veterinarian to explain what the type and location mean for your dog.
Common cancers in cats
AAHA’s feline table includes lymphoma and several site-specific cancers. Lymphoma is covered in its own section below. The site-specific example covered here is oral squamous cell carcinoma.
Oral squamous cell carcinoma
Cornell describes feline oral squamous cell carcinoma as the most common cancer of the cat’s mouth. Reduced appetite or difficulty eating, drooling, bad breath, and jaw swelling can occur. Diagnosis is by biopsy. Cornell notes that the tumor’s location and its invasive growth can limit treatment options.
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Lymphoma in dogs and cats
Lymphoma is a cancer of white blood cells, and Cornell lists it among the most common cancers of both dogs and cats. It can involve lymph nodes, the spleen, liver, intestinal tract, skin, and other sites. Which organs are affected determines which signs appear. The main differences between the species are shown below.
| Feature | Dogs | Cats |
|---|---|---|
| Typical presentation (Cornell) | Enlarged, non-painful peripheral lymph nodes | Gastrointestinal tract is the most common site |
| Signs that can occur (Cornell) | Reduced appetite or energy, vomiting, diarrhea, weight loss, and increased drinking or urination. Which signs appear depends on the organs involved. | |
| Remission with multidrug chemotherapy (Cornell, accessed 2026; lymphoma protocols only) | Up to 90% of dogs | Up to 65% of cats |
| Median survival with multidrug chemotherapy (Cornell, accessed 2026; lymphoma protocols only) | One year | Seven months |
These figures apply only to lymphoma treated with multidrug chemotherapy protocols. Remission is not a cure rate, and median survival is specific to the disease and protocol studied. Neither figure should be applied to other cancers or other treatments, and neither predicts how any one animal will respond.
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Diagnosis usually begins with needle sampling of an affected site. A biopsy is sometimes needed to determine histologic grade and whether the lymphoma is B-cell or T-cell.
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Warning signs and when to call your veterinarian
No single sign establishes cancer. A lump, weight loss, appetite change, non-healing wound, persistent cough, change in toileting, or pain can each have many causes, and many of those causes are not tumors. What makes a sign worth acting on is that it is new, persistent, or changing. AAHA’s owner guidance (updated May 5, 2026) and Cornell’s cancer management FAQ list the following as reasons to talk with a veterinarian:
- New or rapidly growing lumps, or persistent swellings
- Unexplained weight loss
- Reduced appetite or difficulty eating or swallowing
- Lethargy or reduced stamina
- Persistent cough or any change in breathing
- Non-healing sores, or bleeding or discharge from a site
- Unusual odor
- Persistent lameness
- Changes in urination or defecation, or difficulty with either
- Pain or discomfort
What to record before the visit
- When you first noticed the change.
- How it has changed since then. For a lump, note its size, texture, and whether it is growing; for other signs, note how often they occur or how severe they are.
- Appetite and weight changes, including whether your pet is eating less or having trouble eating.
- Any related symptoms, such as coughing, limping, bleeding, or changes in drinking, urination, or bowel habits.
When not to wait
Do not delay veterinary assessment for a rapidly growing mass, significant pain, collapse, or breathing difficulty. Contact your veterinarian promptly. If your regular clinic is closed, an emergency veterinary hospital can assess an urgent problem. A photo or description cannot establish what a growth is, and home observation does not replace an examination.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How a veterinarian works up a suspected tumor
AAHA’s 2026 oncology guideline says treatment planning depends on identifying the tumor’s type, grade, and stage. That requires cytologic or histologic evaluation, meaning examination of cells or tissue under a microscope, along with staging tests such as imaging and lymph node sampling. The exact workup depends on the suspected tumor. A typical sequence looks like this:
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- Examination and history. The veterinarian reviews your observations and examines the animal, including the lymph nodes and the area of concern.
- Sampling the suspicious tissue. Fine-needle aspiration collects cells for cytology. Some tumors need a biopsy, a larger tissue sample, to determine grade or other features. For suspected bone tumors, bone radiographs come early, and a tissue sample or removal of the tumor is used for examination.
- Staging tests. These assess how far the disease has spread, how the organs are functioning, and whether the animal is a candidate for treatment. For lymphoma, Cornell lists blood and chemistry testing, urinalysis, chest radiographs, abdominal ultrasound, and bone-marrow aspirate.
- Classification and planning. The results together establish the tumor type, grade, and stage that treatment planning depends on.
Treatment options and how they are chosen
The AAHA 2026 guideline abstract, published January 1, 2026, describes cancer therapy as follows:
“Therapy is most often multimodal and may include chemotherapy, immunotherapy, radiation therapy, and surgery, along with nutritional support and pain management.”
— 2026 AAHA Oncology Guidelines for Dogs and Cats
Specialist consultation or referral can be part of care, especially for complex cases. The choice of approach is individualized and made with your veterinary team. The main factors are:
- Tumor type
- Grade and stage
- Your pet’s health and needs
- The goals of care that you and your veterinary team agree on
Planning for costs
Cornell’s cancer management FAQ includes illustrative cost ranges, but these are time- and case-sensitive. Ask for an itemized estimate for the specific workup and treatment proposed, and check current figures with the provider, because costs vary between clinics and cases.
Questions to ask when comparing two diagnoses
No simple ranking of treatments applies across all cancers. If you are weighing two named diagnoses, such as lymphoma and a mast cell tumor in a dog, compare them along the following lines and bring the answers to your veterinarian:
Quick Recap
- Species: whether the cancer is typical in dogs, cats, or both
- Tissue and site: where the tumor is and what it involves
- Tumor type and grade
- Extent or stage, and which staging tests were done
- Diagnostic test: what sample was taken and what it showed
- Treatment goal and the options that match it
- Prognosis evidence: which population the figures describe
- Likely impact on comfort and daily life
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