What is cancer?
Cancer is a disorder of cell growth (uncontrolled cell division) that results in an abnormal mass of tissue (tumor) without a purpose. The growth exceeds that of normal tissue, is un-coordinated, and persists after the cause has gone. There are many different types of cancers.
A few definitions
- "Neoplasia" is Greek for "new growth". Not all neoplasms are malignant. In fact, many are not life threatening and are referred to as "benign".
- "Malignant" means "life threatening". Malignancy is often shown by tumor names ending in "carcinoma" or "sarcoma".
- "Cancer" is the Latin word for 'crab' and the name describes the way the cancer adheres to adjacent tissues. Another more descriptive name for cancer is "malignant neoplasia". "
- "Tumor" is Latin for a swelling. Tumors may include swellings that are due to non-neoplastic causes.
- "Oncology" is the study of neoplasia.
"In strict medical terminology 'cancer' is used for malignant neoplasia."
Note that in strict medical terminology 'cancer' is used for malignant neoplasia but that in general usage it is often used as a broader term encompassing benign and malignant tumors.
What causes cancer?
Cancer is essentially the result of non-lethal genetic damage to cells (mutations in cellular DNA). Causes of such mutations include radiation, chemicals, hormones and infections. Some damage to cellular DNA is a daily "wear and tear" event, and all mammals have many safeguards to prevent or repair such damage. Nonetheless, such protective mechanisms are not flawless. In some individuals there are even defects in such defences, resulting in a higher than expected occurrence of cancer. Some of those defects in protection are inherited; for example, some purebred dogs have inherited predispositions to develop specific forms of cancer. In other instances, the protective mechanisms are unable to cope with excessive injury.
The mutated DNA upsets the normal regulation of cell growth, and the "altered" cells no longer obey the rules governing coordinated cell activity, instead growing in an uncontrolled and uncoordinated fashion.
Why has my pet developed cancer?
All cells have the potential to develop cancer. It is in many cases a matter of chance and misfortune.
The more divisions a cell undergoes, the more probable is a mutation; therefore, cancer tends to be more common in those cells that divide more frequently and in older animals whose cells have undergone many divisions. Pets are living longer, and because cancer frequency increases with age, we are seeing more cancer cases.
"Cancer tends to be more common in those cells that divide more frequently."
In some cases, an animal has been exposed to carcinogens, factors in the environment that cause or promote cancer. These include sunlight, some chemicals and some infections.
Some animals have a greater tendency (genetic susceptibility) to cancer. Some breeds have far more cancers than others, often of specific types.
A few tumors need hormones to start growing or to enable them to persist. These tumors are classified as "hormone dependent".
Can my animal catch cancer from another animal? Can my animal transmit cancer to others?
For the vast majority of cases the answer to both questions is "No!"
Some viruses and other microorganisms can cause cancer in animals. Animals may become infected with one of these agents from their mother before or at birth, through direct contact with other animals of the same species, or through bites of 'vectors' such as fleas or ticks. Feline leukemia virus, for example, can cause cancers of the blood and lymphoid system in cats. Occasionally, an infected queen will transmit the virus to her kittens before or at birth. However, feline leukemia virus is more commonly spread by close contact with infected cats that shed the virus in saliva, urine and feces. If your cat is infected, it can pass the infection to other cats.
If your pet is infected with a specific transmissible cancer, your veterinarian will advise you of this and you must take steps to prevent your pet from infecting other animals.
How does cancer affect my pet?
The most obvious symptom of most cancers is a lump that continues to enlarge. This lump may ulcerate, bleed or cause other physical effects (pressure, displacement, etc.) on the surrounding tissues.
Benign cancers only enlarge in a local area by uniform, smooth expansion but malignant cancers may invade the surrounding tissues with "tentacles" or irregular projections (i.e. "cancer the crab"). In fact, pathologists use these differences in growth habit to help decide if the cancer is benign or malignant. Malignant neoplasia may also be malignant because it spreads widely through the body. Widespread distribution of a cancer may occur by "direct seeding", which occurs when cancer cells break away from the original tumor and seed in body cavities (such as the pleural cavity of the chest or peritoneal cavity of the abdomen). Malignant cancer may also be spread when cancer cells invade the blood, which then carries tumor cells to distant tissues where the cells lodge and start new tumor masses (metastases). The most common sites for metastatic cancer to develop are the liver, the lungs and the lymph nodes, although other sites can be affected.
Weight loss due to loss of body fat and muscle is common in malignant cancer and unexplained weight loss can be an important sign of malignancy.
A few tumors induce clinical signs that are not readily explained by local or distant spread of the tumors. These are known as paraneoplastic syndromes. Some syndromes are due to abnormal hormone production by the cancer. (Hormones are internal secretions that pass into the blood and stimulate other organs to action.) Examples of paraneoplastic signs and symptoms include loss of hair, increase or decrease in blood glucose levels, and increased blood calcium levels.
How is cancer diagnosed?
Your veterinarian may suspect that your pet has cancer based on certain clinical signs (a lump, loss of appetite and energy, loss of weight). X-rays may be useful in detecting internal tumors, including metastases. Blood tests can help indicate some tumors. In order to identify most tumor types, it is necessary to obtain a sample of the tumor. Depending upon the type of tumor suspected, your veterinarian may obtain this sample through a fine needle aspiration, a punch biopsy, a tissue biopsy or a full excision biopsy. In some cases, an exploratory surgery or ultrasound guidance may be needed.
"In order to identify most tumor types, it is necessary to obtain a sample of the tumor."
The simplest approach in many cases is the aspiration (suction removal) of tumor cells with a syringe and needle. It does not require general anesthesia or surgery. Microscopic examination of the cells obtained is called cytology. A few tumors can be accurately diagnosed with cytology.
However, in most cases a biopsy sample of the tissue must be examined in order to reach an accurate and reliable diagnosis. Your veterinarian will send the sample to a specialized laboratory for examination by a veterinary pathologist. The preparation and microscopic examination of tissue is called histopathology.
The histopathology report typically includes words that indicate whether a tumor is 'benign' (non-spreading, local growth) or 'malignant' (capable of spreading to other body sites). Malignancy is often shown by tumor names ending in "carcinoma" or "sarcoma". These, together with the origin or type of tumor, the grade (degree of resemblance to normal cells) and stage (how far it has spread) indicate how the cancer is likely to behave.
"The veterinary pathologist usually adds a prognosis."
The veterinary pathologist usually adds a prognosis (what will probably happen). This may include a prediction about the probability of local recurrence or metastasis (distant spread).
Can cancer disappear without treatment?
Cancer rarely disappears without treatment but as development is a multi-step process, it may stop at any stage. The body's immune system can kill cancer cells using mechanisms that specifically target tumor cells that are recognized as "foreign". These mechanisms include immune system cells such as cytotoxic lymphocytes and macrophages and lymphocytes that are responsible for antibody production. Not all tumors are recognized as foreign and even when they are, the immune system is rarely 100% effective in eliminating the cancer. In rare situations, loss of blood supply to a cancer, by pressure on its own supply for example, will result in tumor cell death but the dead tissue will probably need surgical removal.
What types of treatment are available?
The most common and often most effective treatment is surgical removal of the lump. For lumps that are too big or too numerous to be removed or that are in inaccessible locations, other treatments can be considered. These include drugs (chemotherapy), immunotherapy (specific or non-specific stimulation of the immune system), and radiation. Some of these treatments are only available at specialist centers. New approaches such as gene-based therapies are under development. Chemotherapy or radiation are not suitable for all types of cancer and often have significant side effects. Chemotherapy drugs target differences between the cancer cells and normal cells, but there is a fine margin and inevitably some normal cells are also destroyed.
"There are many issues to be considered in the decisions on cancer treatment."
There are many issues to be considered in the decisions on cancer treatment and your veterinarian will discuss these with you.
How do I know if the cancer is permanently cured?
In many cases, the diagnosis and prognosis indicate there can be a high likelihood of complete cure. Sadly, there are some cases where the diagnosis and prognosis indicate that surgical removal will only give transient relief and the cancer will recur or spread. There are a few tumors whose behavior can be difficult to predict.
As in humans, our understanding of cancer in dogs and cats is increasing. Survival rates are improving and many animals are alive and well as "cancer survivors".