What is it and how is it diagnosed?

Cancer is a term used to describe disease that is caused by a tumour (or neoplasm) – a collection of abnormal cells within the body that continue to grow and divide without control. This usually results in the development of masses (growths or lumps), which are mainly composed of the abnormal dividing cells.
Some tumours do not spread to other parts of the body and tend not to invade other surrounding tissues – these are termed ’benign’ tumours.
In contrast to this, the term cancer is generally used to describe ’malignant’ tumours, which often do invade surrounding normal healthy tissue, and may spread to other sites in the body (or ’metastasise’), typically spreading via the blood stream or lymphatic system.
Because of their more aggressive and invasive nature, malignant tumours (cancers) are generally more serious than benign tumours, often causing more serious and extensive disease, and are generally more difficult to treat.
Overall, cats suffer with neoplasia (or development of tumour[s]) less frequently than dogs. Neoplasms may perhaps be seen less than half as frequently in cats compared with dogs. However, when cats do develop tumours they are much more likely to be malignant (3-4 times more likely than in dogs) and therefore much more likely to cause serious disease.
The most common sites of cancer in cats include the skin, the white blood cells (leukaemia and lymphoma), the mouth, the stomach and intestines and the mammary glands.

Types of cancer

There are many different types of cancer, and they are often classified according to the origin of the type of abnormal cell they contain. Thus cancers known as ’carcinomas’ and ’sarcomas’ are solid tumours that arise from various different tissues, whereas ‘leukaemias’ are cancers that affect the bone marrow where blood cells are produced and often cause large numbers of abnormal cells to appear in the blood stream. ‘Lymphoma’ is a solid cancer caused by the growth of abnormal lymphocytes – a type of white blood cell that can also be found in tissues and is part of the immune system.
Because of the enormous variety of cancers that can affect cats (as with any other animal), it is impossible to list all the different types and their common manifestations. However, some of the most commonly encountered cancers include the following:

What causes cancer?

As is often the case in human medicine, the cause of cancer in any individual cat is often unknown, and indeed many cancers are likely to arise for a number of different reasons.
Inherited (genetic) susceptibility to the development of certain tumours almost certainly occurs in cats, although relatively little is known about this at present. During a cat’s life they may potentially be exposed to a number of different things that can trigger abnormalities within cells that may ultimately lead to development of cancer – this may include exposure to sunlight or to a wide variety of different chemicals (carcinogens) – but still in most individuals, the underlying causes and triggers for the cancer remains unknown.
We do know that some viral infections in cats can cause cancer, and feline leukaemia virus (FeLV) is probably the best example of this. Fortunately, infection with this virus is now relatively uncommon in most places. However, when cats are exposed to this virus it can infect the blood-producing cells of the bone marrow, and can lead to the development of leukaemia or lymphoma. Infection with feline immunodeficiency virus (FIV), related to human immunodeficiency virus or (HIV) also on occasions can lead to the development of cancer. Fortunately it is easy for your vet to test for the presence of both of these viruses.
Studies suggest that compared with an uninfected cat, a cat that is infected with FeLV has an approximately 50-fold increase risk of developing lymphoma, and a cat infected with FIV has an approximately 5-fold increased risk.
When cancer is diagnosed, a natural and common reaction is ‘What have I done wrong?’ or ‘What could I have done to have prevented this from happening?’  While these are entirely natural responses when we first learn that our pet has cancer, it is important to remember that in the vast majority of cases we don’t know what will have led to the development of the cancer, and therefore it would have been impossible to prevent.

What are the clinical signs of cancer?

Because cancers can affect any tissues in the body, the clinical signs that cats develop are extremely diverse and there are no signs that automatically suggest cancer is the cause of disease.
In general, cancers affect older cats more commonly than younger cats. In many cases, cancers will grow over quite a long period of time, and initially there may just be vague signs of disease such as poor appetite, lack of energy and weight loss. In other cases there may be more obvious signs such as persistent lumps in or under the skin, changes in the eyes, vomiting, diarrhoea, unexplained bleeding or wounds that do not heal.
As the disease progresses, additional complications will usually develop that often relate to the tissues or organs mainly affected. Although cancer may be one of the potential causes of a variety of different signs (especially in older cats), it is important to remember that many other diseases commonly cause the same signs as cancer and that, even where cancer is diagnosed, there may well be treatment options that will enable control or management of the disease, at least for a period of time. However, as it is important to diagnose cancer early, it is vital to seek veterinary advice as soon as any abnormalities are noticed.

How is cancer diagnosed?

You or your vet may suspect cancer to be an underlying cause of the clinical signs your cat is showing. However, the clinical signs and examination by your vet alone are not sufficient to be able to diagnose the condition.
Additional investigations in the form of radiographs (X-rays) or ultrasound examination are often needed to identify the location and/or the extent of any tumour, but the diagnosis of cancer can only be made by the microscopic examination of tissues by an experienced pathologist. This will usually necessitate a biopsy (surgical removal of a small piece of affected tissue) by your vet, although in some cases it may be possible to make a diagnosis from either a ’fine needle aspirate’ (a small needle is inserted into a mass to remove or ‘suck out’ a few cells that can be smeared on a slide for examination) or a ‘needle biopsy’ (where a larger needle is inserted into a lump to remove a very small ’core’ of tissue).
Occasionally other techniques are also used to obtain samples of the suspected abnormal cells so that a diagnosis can be made. Blood samples are a routine part of the investigation of any suspected cancer patient – partly to detect any adverse effects of the cancer, and partly to detect the presence of any other disease.
With some cancers, occasionally more sophisticated techniques may be required to either make (or confirm) the diagnosis, or to plan the most appropriate treatment. Computed axial tomography (so-called ’CAT’ or ’CT’ scans) or magnetic resonance imaging (MRI scans) are becoming more widely available for pets and can be very valuable, especially, for example, in the diagnosis of brain tumours, and in assessing the extent of tumour invasion.