What is Cancer?

So, what is cancer? In this article, cancer expert Gerry Polton discusses some general features of cancers in pets and explains some of the important terms you may hear.

Cancer is an abnormal growth of tissue that :

  1. Continues to grow even if whatever started it is no longer present
  2. Doesn’t have the normal structure of the tissue it arises from and
  3. Usually, but not always, forms a distinct lump (or lumps) of tissue

Types of Cancer

The development of cancer is also known as neoplasia and a lump of cancerous tissue is often referred to as a neoplasm or tumour.

Cancers may be either benign or malignant (see later) and successful cancer management is critically dependent upon the clinician’s ability to understand the nature of the problem with which they are confronted. Not all cases are helped by veterinary intervention.

Cat with a lymphoma on its head. Lymphomas are a type of cancer involving white blood cells.

Cancer typically arises as a lump or a series of lumps. It can occur anywhere on the body. Most pet cancers identified by owners are superficial masses on the skin surface or just beneath. Internal cancers are more frequently found by the veterinary surgeon during investigation of non-specific symptoms shown by the pet, such as weight loss or weakness.

Some cancers will spread to other parts of the body; the medical term for this is metastasis. Cancers that metastasise are described as being malignant. Some cancers remain localised and don’t spread around the body. These are known as benign cancers. Although they are less likely to cause life-threatening disease, benign cancers can still cause harm through progressive enlargement and interference with the function of neighbouring structures.

X ray showing multiple chest metastases in a dog. They appear as round white opacitites overlying the heart (arrows)

Cancer Management in Pets

To recommend the best treatment options for a pet with cancer, the clinician needs to understand the extent of the tumour. We call this the clinical stage. This term describes the degree of local spread of the tumour into surrounding normal tissues, and whether there is evidence of metastasis (spread) to the regional lymph nodes and/or to other organs such as the lungs.

Cancers differ in their behaviour, and whether they are likely to invade local tissues and/or spread around the body. Understanding the behaviour of individual cancers allows the veterinary surgeon to perform appropriate tests to define the clinical stage of the disease.

We also need to understand the cancer’s capacity for malignancy. We call this the tumour grade. Tumour grade is determined by microscopic examination of biopsy specimens by a specialist pathologist. A high grade tumour would be considered much more likely to metastasise (spread around the body) than a low grade tumour. The tumour grade therefore affects the outlook for the patient and decision making on treatment.

Ultimately, in cancer management, we are trying to predict the future. The best outcome for one patient may not be the best for another. In order to understand how to help owners make decisions for their pet, the veterinary surgeon must consider :-

  • The tumour and its stage and grade
  • The patient
  • The owner’s circumstances and wishes
  • The therapy options and outlook for the patient
  • Their own competence to deliver the treatments offered (some of which are very complex)

As in people, pets suffer from a wide range of different cancers. Many of these can be treated successfully without unpleasant side effects for your pet. In the next article we’ll explain the treatment options available, and the implications of each treatment for you and your pet.