How cancer is staged in pets: tests and imaging explained

Once a malignant tumour is confirmed, the next question is how far it has spread. The tests that answer it are called staging. Staging helps clarify the outlook and choose treatment — for example, whether surgery alone is likely to be enough or a combined approach is needed. Which tests make sense depends on the tumour type, your pet's overall health and your family's circumstances; not every pet needs every test.

The short answer

• Stage describes the tumour's size and whether it has spread to lymph nodes or other organs.
• Grade describes how the tumour cells look under the microscope. They are different things.
• Staging usually looks at the tumour itself, the nearby lymph nodes, general health and the organs this tumour type tends to spread to.
• A sensible testing plan is agreed together with your vet.

Stage versus grade

These terms are easily confused. Grade is assigned by a pathologist from histopathology: low grade means the cells look fairly normal and invade little, high grade means the cells are very abnormal. Stage comes from the combined test results: a small tumour without spread is early stage; a large, invasive tumour or one that has spread is advanced. Both matter for the outlook. To learn how tissue for histopathology is obtained, see Cytology or biopsy.

What staging involves

1. Assessing the tumour itself. The vet measures it and checks whether it moves freely or is fixed to surrounding tissue. If it's internal or may involve bone, ultrasound, X-rays, CT or MRI are used. Ideally, a board-certified veterinary radiologist reviews the images.
2. The regional lymph nodes. Feeling them is not enough: an enlarged node may just be reactive, and a normal-sized node can contain cancer cells. That's why a node is often sampled by aspirate or biopsy.
3. General health. A minimum database includes a complete blood count, biochemistry and urinalysis, plus testing for feline leukaemia and immunodeficiency viruses in cats. It reveals other conditions that affect treatment and anaesthesia choices.
4. Looking for distant spread. Most often chest X-rays in several views or CT, plus abdominal ultrasound. Which organs are checked depends on where that particular tumour usually spreads.

Why the set of tests varies

Oncology guidelines suggest choosing tests by three principles:

• Prognostic — does the test change the outlook or the treatment decision? If not, it may not be needed.
• Practical — if the budget is limited, avoid spending most of it on tests and leaving little for treatment. Prioritise tests that confirm the diagnosis and assess overall health.
• Pertinent — check the sites this tumour spreads to most often. For example, regional lymph nodes are especially important in canine mast cell tumours, and the lungs in osteosarcoma.

Staging systems

Many solid tumours are staged with the TNM system: T for the size and extent of the primary tumour, N for lymph node involvement and M for distant metastasis. Canine lymphoma uses stages I to V with a substage: "a" if the dog feels well, "b" if there are signs of illness. Your vet can explain which system applies and what it means for your pet.

Restaging

Tests are repeated during and after treatment to assess the response and catch any recurrence early. Your vet will advise on the timing of follow-up visits.

Questions to ask your vet

• Which tests are needed for this tumour type, and why?
• How would different results change the treatment plan?
• Does any test require anaesthesia, and how will that risk be assessed?
• Can we prioritise tests if the budget is limited?
• Who will interpret the imaging?

Frequently asked questions

Is staging needed if we choose palliative care?

Not always. Sometimes basic tests are enough to manage symptoms and comfort. Ask your vet which tests would genuinely help in your situation.

Does a benign tumour need staging?

Usually not — benign tumours don't spread. General health is still assessed before surgery.

What if spread is found?

It's important information for choosing treatment, but it isn't always the end of options. What can be done for treatment and quality of life depends on the tumour type and your pet's condition.

Important

This article is for general information and does not replace advice from a veterinarian. A testing plan is always individual. An online consultation with a veterinary oncologist can help you understand results and discuss next steps, but it does not replace an in-person examination.

Sources and further reading

• AAHA Section 3: Tumor Diagnostics and Staging (opens in a new tab)
• AAHA: 2026 Oncology Guidelines for Dogs and Cats (opens in a new tab)
• ACVS: Mast Cell Tumors (opens in a new tab)
• ACVS: Mammary Tumors (opens in a new tab)
• Merck Veterinary Manual: Cancer Treatment in Animals (opens in a new tab)
• Veterinary Cancer Society: Pet Owner Resources (opens in a new tab)