FAQ - Diagnosis of Cancer

How is cancer diagnosed?

If your pet shows one or more clinical signs that may suggest cancer, your veterinary surgeon will need to find out whether it is due to cancer or to some other cause.

Your vet will take a history of your pets’ illness and perform a thorough physical examination. They may order laboratory tests, imaging studies and/or recommend a biopsy.

At this point, your vet will have a list of possible conditions in mind (differential diagnosis) that could be causing your pets illness.

In most cases, a biopsy is the only way to know for certain whether cancer is present.

What is a differential diagnosis?

A differential diagnosis is often a list of two or more conditions. Creating a list of differential diagnoses involves a process of differentiating between conditions which share similar history and clinical signs.

This process results in a list of the possible conditions that might produce the observed clinical signs. Generating a differential diagnosis is an important part of clinical reasoning. It enables appropriate testing to rule out possibilities and confirm a final or definitive diagnosis.

Note: When lumps, bumps and enlarged lymph nodes (glands) are found in a cat, there are more non-cancerous disease possibilities or differential diagnoses than for dogs. As a result of infection or reactive inflammation, cats can develop significantly enlarged lymph nodes which can make the diagnosis of cancer more difficult.

Which laboratory tests?

Tests of the blood, urine, or other fluids can help veterinary surgeons make a diagnosis.

Examples include:

  • CBC – a complete blood cell count of the red blood cells, white blood cells and platelets.
  • Chemistry screen – a panel of tests that includes electrolyte levels (sodium, potassium, chloride, calcium and phosphorus), urea, creatinine and liver enzymes (ALT, AST, ALKP)
  • Urinalysis – an analysis of the specific gravity (concentration) of urine as well as presence of any cells, blood and protein.


These tests can also give an indication of how healthy your pet is by demonstrating how well the body organs (such as the kidneys or the liver) are performing their functions.

Although high amounts of some substances (such as calcium in the blood) may be a sign of cancer abnormal lab results are not a sure sign of cancer and veterinary surgeons cannot rely solely on laboratory tests to diagnose cancer.

Sometimes the tests will show the presence of cancerous cells.

What about imaging studies?

Imaging procedures create ‘pictures’ of areas inside the body that help the veterinary surgeon see whether a tumour is present. These images can be obtained in several ways:

  • Radiographs (x-rays): The most common way to view bones and organs inside the body. They allow us to see the outline of an organ but may not tell us what is going on inside it. A radiograph creates a 2-dimensional picture so several radiographs may need to be taken from different directions to know where a lesion is located.
  • Ultrasound: An ultrasound device sends out sound waves that people cannot hear. The waves bounce off tissues inside your body like an echo. A computer uses these echoes to create a picture called a sonogram. This allows to see inside an organ.
  • MRI: A strong magnet linked to a computer is used to make detailed pictures of areas in your body. Your veterinary surgeon can view these pictures on a monitor and can print them on film. MRI or magnetic resonance imaging is most useful for the brain and spine.
  • CT scan: An x-ray machine that takes 3-dimensional image by taking many pictures from many different directions feeds these images into a computer to produce a more detailed picture of the body in cross section. CT or computed tomography scans are useful in detecting cancer, diagnosing lung and chest problems and identifying bone injuries. They can detect smaller lesions that radiographs but are significantly more expensive.


Your pet may receive a contrast material (usually by injection) to make x-rays, MRI images and CT scans easier to read.

It is important to remember that not every case needs to undergo every type of imaging available and there is no point in having some very good images of a tumour with no finances left to pay for treatment.

The type of imaging that is appropriate for diagnosing cancer depends on the likely differential diagnoses that your vet is considering.

What is a biopsy?

In most cases, your veterinary surgeon will need to perform a biopsy to make a diagnosis of cancer. During a biopsy, a sample of the lump or tumour tissue is removed and sent to a diagnostic laboratory where a vet with specialist training in cytology and/or histopathology will look at the tissue under a microscope to identify cancer cells.

A sample of cells or tissue may be obtained in several ways:

  • With a needle: The veterinary surgeon uses a needle to withdraw tissue or fluid. This is often referred to as a fine needle aspirate or cytology sample.
  • With an endoscope: The veterinary surgeon uses a camera in a thin, lighted tube (an endoscope) to look at areas inside the body. The veterinary surgeon can remove tissue or cells through the tube.
  • With surgery: Surgery may be excisional or incisional.
    • In an excisional biopsy, the veterinary surgeon removes the entire tumour, often with some of the normal tissue around the tumour (called a margin).
    • An incisional biopsy involves removing a small part of the tumour and is more common in tumours which are large or in a difficult location.


The type of biopsy taken will depend on the location and suspected type of tumour. A biopsy must be planned in advance and is usually best performed before any attempt is made to remove the tumour.

Contrary to popular belief, taking a biopsy does not increase the risk of spread if performed correctly.