Your Pet and Cancer

This section tells you about what cancer is, how it occurs and how it can be diagnosed.

Cancer is a scary word and once it enters a conversation about your pet you may not be able to concentrate on anything that is said after the ‘C’ word is spoken.

We hope that we can help you and your pet at a time when you will have lots of questions.

Featured on this page is a list of frequently asked questions (FAQs), please click on the question to view the answer.

If you have any further questions about any aspect of cancer in your pet, you should speak to your vet who will be able to discuss this with you more fully.

If you are concerned about the health of your pet you should contact your vet.

Early diagnosis is all important

The most important part of treatment is early and accurate diagnosis. A wait-and-see approach is best avoided as the majority of tumours do not disappear and indeed, they will continue to grow.

There are many different types of tumours and the treatment for each can differ. To ensure the most effective treatment is given and to get an idea of the likely outcome a biopsy will be needed to obtain a diagnosis. This 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.

Before making a decision on treatment, you may want another opinion about your pet’s diagnosis and treatment plan. See the FAQ about second opinions and referrals on the next page on treatment.


Frequently Asked Questions

Click on the heading to read more.

When we talk about lumps, growths or tumours we are usually talking about neoplasia. A neoplasm is a bunch of cells that forms a lump or tumour and it is the result of uncontrolled growth of cells. This may be due to a mutation or a lack of programmed cell death (apoptosis). A neoplastic tumour may be benign or malignant.

Benign tumours grow locally and do not tend to spread to other parts of the body. They can often be removed and usually do not grow back unless cells have invaded surrounding tissues. Benign tumours are rarely life threatening although they can cause serious problems as a result of their physical size or position. Benign neoplasia tends to be named in a similar fashion to cancer but with a simple ‘-oma’ as the ending such as in fibroma, lipoma or adenoma.

Cancer is the common name given to all the various forms of malignant tumours. A tumour is a mass of cells that are growing out of control. There are two main types of tumours: malignant tumours or cancers and benign tumours. As a general rule, malignant tumours (cancers) are more life threatening than benign tumours because of the risk of spread throughout the body.

Malignant tumours are called cancer. They are the most aggressive types of tumours as they tend to grow by invading into the surrounding tissues and may cause direct effects such as pain, inflammation or ulceration, depending on which part of the body is involved. They also have the ability to spread to other parts of the body (metastasis). It is this ability to produce secondary tumours that makes them so life threatening. Secondary tumours can occur anywhere in the body with the most common sites being the lymph glands and the lungs although they can also occur in the liver, kidney, brain and bones.

Most neoplasms are named for the cell type, tissue or body organ from where they start. Malignant neoplasia tends to be named by the site or location or organ affected and ends with either the word –‘sarcoma’ or ‘-carcinoma’. For example, bone cancer starts in a bone and is termed osteosarcoma, liver cancer starts in the liver and may be termed hepatic carcinoma or adenocarcinoma depending on the cell type. Lymphosarcoma is sometimes called lymphoma even though it is a malignant neoplasia that starts in the lymphatic system. Other malignant tumours include fibrosarcoma and liposarcoma.

Not all lumps turn out to be tumours but it is important to ask your vet to have a look when you first notice one. A small tumour, even a cancer, is more likely to be cured if it is treated early. A biopsy or fine needle aspirate is usually necessary to determine the exact nature of the lump or tumour type. “Let’s wait and see” is not a good option in most cases. 

This depends upon where the tumour is and what type it is. When small, many tumours do not cause a problem and it may be only when the tumour is large that it has an effect on an animal. There are many different types of cancer and each behaves in a slightly different way. It is therefore important to have a sample of the tumour sent for examination by a pathology laboratory.

Cancer cells can invade and damage nearby tissues and organs. Cancer cells can also break away from the original (primary) tumour, enter the bloodstream or the lymphatic system and form new tumours in other organs. This type of spread of cancer is called metastasis.

When cancer spreads and forms a new tumour in another part of the body, the new tumour has the same kind of abnormal cells and the same name as the primary tumour. For example, if bone cancer spreads to the lungs, the cancer cells in the lungs are actually bone cancer cells. The disease is called metastatic bone cancer and not lung cancer. The new or secondary tumour is called ‘distant’ or metastatic disease.

Metastasis is a characteristic of malignancy although not all malignant tumours undergo secondary spread. Some cancers, often referred to as low grade, have a low metastatic rate - that is to say only a small percentage actually spread. For example, tumours arising from fibrous tissue in the skin of dogs (known as fibrosarcomas) are generally low grade and <15% metastasise. At the other extreme there are cancers with a high metastatic rate where most will spread. Bone cancers in dogs (osteosarcoma) are ‘high grade’ and at least 95% will metastasise. However it is not always possible to predict which ones will spread.

Metastatic spread is the most difficult type of cancer to treat and, as they grow, they take over more of the patient’s normal organs until the patient dies. Secondaries growing in the lung, for example, will reduce the function of the lungs such that the patient cannot get enough oxygen and tires easily. As the tumours grow further coughing and general illness may also become apparent.

A secondary tumour is another mass of cancer cells that is a result of metastasis. Predicting which patient will develop secondary tumours, or secondaries, is often impossible unless they are of a size that can be seen with imaging studies (such as radiography, ultrasonography, CT or MRI scans). However, even if no secondaries are seen, this is no guarantee that they will not develop at a later date. A secondary tumour starts as a small ball of cells less than 100th of a millimetre and it must grow to a size where it can be detected. To be visible on a CT scan, a mass has to be about x mm or about 1 centimetre on an X-ray. The rate of growth to get to this size can be slow for some tumours or rapid for others. Most secondaries break away and grow when the original primary tumour is still small. Secondaries that appear after a primary tumour has been removed will have been present but were just not detected at that time. For tumours with a high likelihood of secondary spread, anticancer drugs (chemotherapy) may be recommended to delay or even prevent secondaries; however, not all tumours are sensitive to drugs.

Every cell of the body is normally under very strict control imposed by the body as a whole. Cancer begins in the basic unit of life for the body, cells. A group of cells forms a tissue and a group of tissues forms an organ in the body. Cells normally divide and grow to form new cells as the body needs them. When cells grow old and die, new cells take their place. This process may go wrong when the genetic material (DNA) of a cell becomes damaged or changed, producing mutations that affect normal cell growth and division. When this happens, new cells are formed that the body does not need or, alternatively, old cells do not die as they normally should through programmed cell death (apoptosis). These extra cells can form a mass of tissue called a growth or tumour.

Cancer results when changes occur in genes that control normal cell division and death (called mutations). Substances that cause DNA mutations are known as mutagens, and mutagens that cause cancers are known as carcinogens. These genetic changes may result from environmental or lifestyle factors, such as exposure to sunlight (ultraviolet radiation) or carcinogens (cancer-causing substances) or they may be inherited (from one or both parents). However, having an inherited gene alteration does not always mean that cancer will develop, just that the risk of developing cancer is increased. Most forms of cancer are sporadic, meaning that there is no inherited cause of the cancer.

Very occasionally, a cell acquires the ability to divide and grow outside this normal control and a cancer develops. This happens when a number of genes within a cell change or mutate. Mutations occur in all of us all the time; indeed, without mutations, evolution would not occur. Most of the time, these mutations come to nothing as they are repaired or do not change the function of a cell but occasionally they cause the cell to go wrong and develop into cancer. 

For an increasing number of human cancers the cause is known. Some of these risk factors are smoking (lung cancer), certain viruses (cervical cancer) and ultra-violet sunlight (skin melanoma). In addition, certain genes that are present in families have been identified. Cancer results when changes occur in genes that control normal cell division and death. Substances that cause DNA mutations are known as mutagens, and mutagens that cause cancers are known as carcinogens. These genetic changes may result from environmental or lifestyle factors, such as exposure to sunlight (ultraviolet radiation) or carcinogens (cancer-causing substances) or they may be inherited (from one or both parents). However, having an inherited gene alteration does not always mean that cancer will develop, just that the risk of developing cancer is increased (See What is the link between genes and cancer?). Most forms of cancer are sporadic, meaning that there is no inherited cause of the cancer.

In dogs and cats, there are only a few known risk factors for cancer. In cats, the virus known as Feline Leukaemia Virus (FeLV) is a definitive trigger for certain cancers (feline lymphoma and leukaemia). However, not all cats that are infected by FeLV will develop cancer and, conversely, not all lymphomas in cats are induced by FeLV infection. 

Another example of a cancer, for which there is a known cause, occurs in white-faced cats that sit out in the full sun. The relatively hairless pink areas of the ear tips, eyelids and nose are prone to sunburn which can progress to skin cancer.

Some types of cancer can be found before they cause symptoms. Checking for cancer (or for conditions that may lead to cancer) in individuals who do not have signs is called screening. Screening can help your vet find and treat some types of cancer early. Generally, cancer treatment is more effective when the disease is found early. There is currently no effective genetic or blood test that is a reliable screening tool for the diagnosis of cancer in dogs. Regular physical examinations, laboratory testing and imaging studies are the best tools that vets have for the early diagnosis of cancer.

There are a number of recognised syndromes in people where there is an inherited predisposition to cancer, often due to a defect in a gene that protects against tumour formation. Examples include certain inherited mutations in the genes BRCA1 and BRCA2 that are associated with an elevated risk of breast cancer and ovarian cancer, and individuals with Down syndrome have an extra chromosome 21 and are known to develop malignancies such as leukaemia and testicular cancer.

Certain breeds of dog appear to be susceptible to specific tumours. This may be related to the characteristics of the breed or due to a genetic predisposition. Bone cancer is more common in giant and large breeds such as Great Danes, Irish Wolfhounds, Rottweilers, and St Bernards. This is thought to be related to the rapid growth characteristics of the adolescent pup although there is also evidence for a genetic susceptibility in some of the giant breeds. 

Boxers have a relatively high incidence of tumours of the skin and the brain. Bernese mountain dogs appear to suffer from a variety of malignant cancers that tend to affect a younger age group than other dogs as a whole. Similarly Flat-coated Retrievers tend to have a relatively high incidence of cancer which behaves more aggressively than in other breeds of dog. These and other breed related tumours are highly suggestive of a widespread genetic susceptibility for the specific cancer associated with the breed. However, truly hereditary cancers (i.e. that are passed down in certain lines) are rare in dogs.

Current research is looking at methods of identifying specific causes of cancers in dogs and cats, and this may lead to novel ways of treating them. More importantly, as the specific causes become known, specific ways of preventing that cancer may be possible.

Most cancers develop as a result of changes in genes (mutations). A normal cell may become a cancer cell after a series of gene changes occur. Some gene changes that increase the risk of cancer are passed from parent to offspring. These changes are present at birth in all cells of the body. While it is uncommon for cancer to run in a family, certain types of cancer do occur more often in some families than in the rest of the population. For example, cancers of the breast, ovary, prostate, and colon in people. Several cases of the same cancer type in a family or line of pedigree dogs may be linked to inherited gene changes, which may increase the chance of developing cancers. However, environmental factors may also be involved.

If you think your pet may have a pattern of a certain type of cancer in the line, you may want to talk to your veterinary surgeon about ways to detect cancer early should it develop in your pet. You may also want to ask about genetic testing. These tests can check for certain inherited gene changes that increase the chance of developing cancer. But inheriting a gene change does not mean that your pet will definitely develop cancer. It means that your pet has an increased chance of developing the disease.

Some breeds of dogs are more likely to be affected by certain types of cancer than other breeds of dogs. The 2004 purebred dog health survey showed that several breeds were more likely to be affected by certain cancers, including malignant histiocytosis in Bernese Mountain Dogs and mast cell tumours in Golden retrievers (


As in human medicine, we often cannot explain why one pet develops cancer and another does not. Research has shown that certain risk factors increase the chance that a person will develop cancer. It is likely that many of the most common risk factors for cancer in people also affect our pets, including age, sunlight (ultraviolet radiation), certain chemicals, some viruses, certain hormones, poor diet, lack of physical activity, being overweight or a family history of cancer.

Many of these risk factors can be avoided. Others, such as family history, cannot be avoided. Several factors may act together over time to cause normal cells to become cancerous. When thinking about the risk of cancer remember that not everything causes cancer. Having one or more risk factors does not mean that an individual will get cancer. Some individuals are more sensitive than others to the known risk factors.

One of the most important risk factors for cancer is growing older. Most cancers occur in older pets although pets of all ages can get cancer. This means that as our pets are living longer, the incidence of cancer is rising. It has been reported that about 1 in 4 dogs are affected by cancer and about 1 in 6 cats are affected by cancer in their lifetime. In the 2004 purebred dog health survey, 1 in 4 dogs died due to cancer from 1994 to 2004 ( The median age of diagnosis of cancer was 8 years in a survey of 36,006 live dogs with 22,504 conditions in total. The median age at death due to cancer was 10½ years in a survey of cause of death for 15,881 dogs from 1994 to 2004. In a survey of cause of death in UK cats, cancer was the second most common cause of death after trauma overall and most common reason for euthanasia ( 

A risk factor is something that is associated with an increased risk of disease. Risk factors are not necessarily causal, because association does not imply causation. For example, being old cannot be said to cause cancer, but older pets are more at risk of developing cancer because they have experienced more cell damage and exposure to potential carcinogens during their lifetime. Having a risk factor does not mean that your pet will get cancer just as not having a risk factor does not mean that your pet will not get a cancer.

The effect of a risk factor is often expressed as a relative risk (RR). In epidemiology, relative risk is the risk of developing a disease relative to exposure to a risk factor. Relative risk is a ratio of the probability of disease occurring in a group exposed to the risk factor compared to the probability of disease occurring in a non-exposed group. Another term for the relative risk is the risk ratio because it is the ratio of the risk in the exposed divided by the risk in the unexposed. For example, a RR of 3.0 means that the relative risk of cancer associated with smoking was 3 and that smokers were 3 times as likely as non-smokers to develop lung cancer. While a RR of 3 sounds high, the absolute risk is 1 case of lung cancer for every 10,000 smokers and this is very small.

In survival analysis, the hazard ratio (HR) is used instead of RR and median survival times can be compared between those individuals exposed to one treatment compared to another treatment. One study that used survival analysis for cats with oral squamous cell carcinoma (OSCC) showed that whether the cat received non-steroidal anti-inflammatory (NSAID) therapy after surgery was associated with survival. The hazard ratio was 2.2 (95% confidence interval:  4.8 – 10.2), meaning that a cat that did not receive post-operative NSAID therapy was more than twice as likely to die due to its tumour than cats that did receive NSAIDs. This HR is an important estimate of the prognosis for a cat with an OSCC. Also, the median survival time for cats that did not receive post-operative NSAID therapy was 38 days from the date of diagnosis, which was significantly less than the median survival time of 84 days for cats that did receive NSAIDs, controlling for breed and COX-1 tumour cell staining (link to REF Hayes et al.). The median survival time (MST) is the time, often expressed as the number of days, months or years, when 50% of the patients are still alive.

When we talk about cancer we often use big words such as prevalence, incidence and survival.

Prevalence is the total number of cases at any one time.

Incidence is the number of newly diagnosed cases during a specific time period.

Mortality is the number of deaths during a specific time period.

Survival is the proportion of patients alive at some point after the diagnosis of their cancer.

Lifetime risk is the probability of developing or dying from cancer, in the course of one’s lifespan.

Median survival time (MST) is the period of time that has passed by which 50% of the affected individuals have died due to their disease.  

Researchers describe risk as the estimated chance of developing a disease during a certain time period, such as within the next 5 years or during a lifetime. People perceive risk as the probability of something happening, such as their pet developing cancer.

Most people are concerned about the chances of their pet developing cancer as an individual or personal risk. What you consider your pet’s personal risk may be influenced by what you have heard from your veterinary surgeon, the media and from your family and friends. Many people view their pet’s risk of developing cancer as being higher or lower than it actually is due to the influence of these various sources of information.

When veterinary surgeons talk about risk they are thinking about numbers. Cancer risk in people is based on numbers of reports of cancer from medical facilities into a central registry. These reports count the total number of people who are diagnosed with a cancer in the United Kingdom. They also provide information about how many men or women are diagnosed, and their age and race. The risk numbers that come from these types of information may be presented as relative risk (RR) or absolute risk (AR). In human medicine, clinical trials are used to test the effectiveness of treatments and there is much more information available than in veterinary medicine.

Other information about risk comes from medical studies. In veterinary medicine, there are very few sources of accurate information about cancer in pets. Using observational studies, veterinary surgeons can look at exposure factors that may affect cancer risk. They draw conclusions about how these and other specific factors affect cancer risk by looking at groups of animals who have a certain risk factor and those who do not. Veterinary surgeons then look at how many individuals in the two groups are diagnosed with cancer. By comparing how many animals in each of these groups actually develop cancer, veterinary surgeons are able to estimate relative risk – that is the risk that one group will get cancer compared to another group. This type of research is becoming increasingly more available for pets with more and more peer-reviewed scientific publications coming out all the time.

Prognosis means to make a prediction of the probable course and outcome of a disease. The likelihood (chance) of recovery from a disease, based on the available evidence.

Certain factors can affect the prognosis and treatment options, including age, general health, presence of concurrent conditions, whether this is the first diagnosis or recurrence, etc.

Prognosis is usually expressed as a probability of survival sometime after diagnosis, such as the one-year survival rate being the probability of surviving for 1year from the time of diagnosis. Alternatively, it can be expressed as the period of time that has passed until 50% of the patients are still alive, and this is the median survival time (MST). 

Very few prospective studies have been published about pets with cancer. Therefore, most of the information available on how well animals do after being diagnosed with cancer comes from studies that follow a series of cases.

The clinical signs of cancer in pets are often vague and non-specific. It is often just a concern of the owner that their pet is just not doing well. The most common signs of cancer in small animals include the following:

  • Abnormal swelling or lump that grows
  • Sores that do not heal
  • Loss of appetite
  • Offensive breath 
  • Unexplained loss of weight
  • Difficulty eating or swallowing
  • Persistent lameness or stiffness
  • Difficulty breathing, urinating, or defecating
  • Breathlessness or loss of stamina
  • Bleeding or discharge

When these or other signs occur, they are not always caused by cancer. They can be caused by inflammation, infections, benign tumours or other problems. It is important to take your pet to see a veterinary surgeon in order to obtain a diagnosis. Your veterinary surgeon will perform a physical examination and may order blood tests and/or recommend a biopsy. In most cases, a biopsy is the only way to know for certain whether cancer is present. During a biopsy, the veterinary surgeon removes a sample of tissue from the abnormal area. A pathologist then studies the tissue under a microscope to identify cancer cells.

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 perform a physical examination and may order laboratory tests, imaging studies and/or recommend a biopsy (See additional questions below). In most cases, a biopsy is the only way to know for certain whether cancer is present.

Tests of the blood, urine, or other fluids can help veterinary surgeons make a diagnosis. 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 doing their jobs. Also, high amounts of some substances may be a sign of cancer. These substances are often called tumour markers. However, abnormal lab results are not a sure sign of cancer. Veterinary surgeons cannot rely solely on laboratory tests to diagnose cancer.

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

Radiographs:  Radiographs (x-rays) are the most common way to view organs and bones inside the body.

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.

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.

CT scan:  An x-ray machine linked to a computer takes a series of detailed pictures of your organs. Your pet may receive a contrast material (usually by injection) to make these pictures easier to read.

In most cases, your veterinary surgeon will need to perform a biopsy to make a diagnosis of cancer. During a biopsy, a sample of tissue is removed and sent to a diagnostic laboratory where a pathologist will look at the tissue under a microscope to identify cancer cells.

The sample of 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 some of the normal tissue around the tumour also is removed. In an incisional biopsy, the veterinary surgeon removes just part of the tumour.