Cancer In Your Dog or Cat

Ron Hines DVM PhD 4/21/06

Cancer is a tremendously broad and complicated subject. To keep this article manageable, I have only scratched the surface of the subject. I am not a cancer specialist, but I deal with tumors of my client’s older pets on a regular basis. I try to furnish them with sound, practical advice during these difficult times. Many cutting edge cancer therapies offered by veterinary oncologists and hub veterinary centers undoubtedly do extend the lives of pets. But I question the quality of these short periods added to the pet’s life and the emotional cost involved. I also have doubts weather these therapies are truly performed for the good of the pet. Instead, they are often done for the peace of mind of owners unwilling to accept the fact that life must come to an end – often before we want it to.

Your pet’s cells are forever growing and replacing themselves, and growth gone awry is the basis of all cancer. Normal cell growth and replacement fills a bodily need. When cells grow for any reason other than the good of the body we call them cancerous or a tumorous. Cell growth is strictly controlled by instructions written in DNA code in every cell. In tumorous cells an error has occurred in this script allowing the cells to grow out of control. When these errors are minor, the cells still look and act a lot like normal. We call tumors formed from these cells benign. When the errors are major we call the tumors malignant.

Small snippets of tumor tissue are called biopsies. Examination of biopsies allows us to determine if the growth is benign or malignant. A fibrous capsule often covers benign tumors and relatively few of the tumor cells are actively growing. Some common benign tumors of dogs are the lipomas or fatty tumors that form under the dog’s skin and the cauliflower-like papillomas that form within the skin. Under the microscope, the cells of these tumors look very much like normal tissue. The borders of these tumors are usually regular making them easy to remove surgically. Before I remove skin tumors from a dog I shear its coat off close to the skin. I send the owners home with a surgical marker to mark the position of even the smallest tumor. Just before surgery, I infiltrate the area of the tumor with a combination of Novocain and epinephrine. This numbs the area and stops the loss of blood. Then I excise the tumors with a scalpel and suture the incision. I freeze or cauterize off small papillomas. I have never had a tumor of this type regrow. Lipomas or fatty tumors are present just under the skin. They are only found on fat dogs. If the dog looses sufficient weight, these tumors shrink or disappear on their own. Because they are invariably encapsulated they are quite easy to remove. I have never had one regrow. Lipomas that have grown around nerves or large blood vessels are more difficult to remove. The outcomes are also good if sufficient care is taken to preserve the nerves and blood vessels.

Tumors that arise from glandular cells are called carcinomas. Tumors that arise from the skin, muscle, bone and fibrous connective tissues of the body are called sarcomas. When cancers are found in their original location they are called primary tumors. When they have moved to a new location in the body they are called metastatic tumors. Only malignant tumors have the capacity to move to new locations. Because of this and their invasiveness, they are life threatening. Cancers that move often become trapped in the sieve-like structure of the lungs, liver, bone marrow and kidneys. When they do the symptoms that we see are do to the physical destruction of these organs more than to the tumor cells themselves. Metastatic tumors are usually highly vascular. That is, they are rich in blood vesicles to supply the nutrients that fast-growing tumor cells require.

Cancer has many causes or risk factors. Agents that increase the likelihood of cancer are called carcinogens. Some of the risk factors are written within the genetic code you pet was born with which make it particularly susceptible to one form of cancer or another. Boxers and the giant breeds of dogs are renown for their predisposition to tumors. Other risk factors, such as cancer causing or oncogenic chemicals, may be found in the pet’s environment or diet (formaldehyde, chlorine-containing compounds, nitrates, etc.). Some of these chemicals cause the cells genetic code (DNA) to mutate and so are called mutagens. Physical agents (radiation, asbestos, etc.) can also cause cancer through chronic irritation and inflammation. Certain viruses have also been found to cause cancer in animals. Often cancer results from the combined effects of genetics, physical and chemical carcinogens. The immune system plays an important role in detecting and eliminating new cancers. Any factor that causes immunosuppression increases the incidence of tumors. Feline immunodeficiency disease (feline AIDS) and feline leukemia both of which are caused by retrovirus are conditions leading to a variety of tumors in cats. Hormones that cause body organs to proliferate can also cause cancer. Breast or mammary tumors in dogs are quite common and occur only in older unsprayed females. This is because of the biyearly thormone rises in unsprayed female dogs associated with their estrus or heat cycles.

As in people, the earlier we detect and remove cancers from pets the more successful we are. Skin tumors are rather easy to diagnose. But tumors within the body often only show as weight loss, low-grade fever, weakness, and lethargy. By the time these cancers are large enough to detect they are in advanced stages and difficult to treat.

X-rays are my first choice in diagnosing internal tumors in dogs and cats. Many tumors are bulky and distort the shapes of the organs they reside in making them readily apparent on x-rays. Many can also be seen using ultrasound equipment. Large veterinary facilities and universities have more sophisticated CT and MRI imaging equipment. Since smaller veterinary hospitals do not have this equipment, we make more use of biopsies and exploratory surgery to diagnose cancer. When the abdomen is opened and all the organs inspected, tumors that were not visible on radiographs are often obvious. When they are not, biopsies of the major organs examined by a pathologist often discover the tumor. Pathology reports also reveal the aggressiveness of tumors and the likelihood that they have already moved or will move.

The most common cancers in dogs are skin cancers. Skin cancers make up over half the total number of cancers that occur in dogs. The most common skin cancers that I encounter are papillomas. These are small cauliflower-like viral tumors that proliferate as a dog ages. They are common on the mussel, trunk and extremities of dogs with graying hair. The vast majority are not malignant and cause no damage beyond being nicked or worried into bleeding by the dog as it grooms.. The next most common cancer in dogs are lipomas. They often occur multiply just under a dog’s skin. They are very rare in cats and ferrets. The next most common skin cancer in dogs is the mastocytoma (or mast cell tumor). These distinctive tumors are oval, firm and slightly raised. Sometimes their center is brown or bluish. Mastocytomas are only locally invasive (malignant) and do not metastasize to other organs. Cancerous cells project outward from the tumor into what appears to be normal skin. So when I excise them, I remove three times the diameter of the visible tumor to be sure that all tumorous cells are removed. The biggest problem occurs with mast cell tumors on the extremities in that insufficient skin may be left to close the excision wound.
Skin tumors in cats are much more to be malignant than those of dogs. I remove them as rapidly as I discover them and always send tissue from the tumor to a veterinary pathologist for evaluation. In too high a number, these tumors have already metastasized to other locations before they were removed.
Mammary gland tumors have a high incidence in older unsprayed dogs. They begin to develop between six and ten years of age and are caused by the hormone progesterone associated with estrus, and reproduction. The most common form is the fibrous and hard mixed mammary carcinoma. They form most frequently in the posterior breasts nearest the rear legs. Often the breasts involved give milk or milk-like fluid. Most are well encapsulated and easy to remove. They are usually not highly malignant and most of the time I get to them before they have metastasized. Of special concern to me are tumors that are ulcerated and which have infiltrated the skin. These are often malignant. I also worry about these tumors when they involve the lymph nodes of the groin and base of the foreleg. I have sent sections from the same tumor of this type to two different pathologists and gotten differing opinions as to their degree of malignancy. This is because determination of malignancy is a subjective process. Spaying females before their third or fourth heat cycle can prevent these tumors.
Lymphatic tumors are tumors of certain white blood cells called lymphocytes. They are classified as lymphosarcomas, lymphomas and lymphoid leukemias. These tumors are quite common in cats, ferrets and dogs. In cats, these tumors occur under the immunosuppressive effect of the feline AIDS and Feline Leukemia virus. They are the second and third most common tumors in ferrets and dogs respectively occurring most commonly in Golden and Labrador retrievers and Doberman pinchers. In ferrets and dogs this cancer appear spontaneously. The tumors appear as sold growths called lymphomas which begin in the lymph nodes or bone marrow or as individual cells freely circulating in the blood called leukemia. Animals as young as four years may develop these tumors. These animals are often presented to me with painless, enlarged lymph nodes over the whole body but occasionally it is a single node that is enlarged. Some of the dogs have an increased number of abnormally large lymphocytes in their blood stream but most do not. At this stage the pets do not appear to be ill. Other animals, particularly cats, develop this form of cancer in the walls of their intestines, which leads to diarrhea and weight loss. A biopsy of one of the enlarged superficial lymph nodes confirms the diagnosis. This type of tumor in dogs responds well to chemotherapy. It does not respond well in cats. The drugs commonly used to treat lymphomas are vincristine, L-asparaginase, cyclophosphomide, doxorubicin, and prednisolone. With this treatment, three-quarters of the dogs caught early in the disease live an additional six month or more. Without treatment their average future life span is about four months.

Tumors of the mouth, lips and tongue are relatively common in dogs and cats. These tumors often bleed by the time they are noticed. A large percentage of them are highly malignant – especially in cats. A variety of tumors form here. They include squamous cell carcinomas, adenocarcinomas, fibrosarcomas and malignant melanomas. Dogs that develop these tumors are generally six to ten years of age. A big problem in dealing with these tumors is that they often surround important structures of the mouth and are therefore next to impossible to remove in their entirety. But most of these tumors can be surgically removed (debulked) and the dog or cat then treated with radiation. This procedure is particularly stressful to cats that often have to be force-fed or fed intravenously. I am not inclined to suggest this procedure to most clients but I do make them aware that these procedures exist. There is some evidence that the viruses responsible for papilloma are involved in the formation of some of these oral tumors in dogs. Chemotherapy has not been very rewarding in these cases. Before considering chemotherapy for your pet, ask what the average increased life expectancy the procedure might be. Specialists tend to be highly optimistic about the benefits of their specialty so take their numbers with a grain of salt and really try to pin them down.

Bone cancers or osteosarcomas occur frequently in large and giant breeds of dogs. They tend to form at the growth plates near the ends of the long bones of the legs. These dogs are often brought to me because of lameness. X-rays of these tumors are highly distinctive and easy to diagnose. Because they often metastasize to the lungs, I include a chest film of every dog I radiograph for this problem. Not all cases are so advanced that tumors in the lungs can be detected. Late in the disease these dogs may have a cough. Neutered male dogs have a higher risk of this disease, as do dogs with a previous injury to the leg involved. The best treatment for these tumors when they occur on a leg is the removal of that leg. A chemotherapeutic drug called Cisplatin along with radiation treatment is helpful in cases where the tumor is inoperable because of location. The drug, Feldene (pyroxicam) is a good analgesic in these dogs. This drug may also have some mild chemotherapeutic effects.

Nutritional support is very important for all cancer patients. This is especially true for cats because minimal cancer-induced stress stop them from eating. Cancer cachexia is a form of protein malnutrition that affects many cancer patients - especially those where the disease is widespread. Offering flavorful, highly digestible and energy-dense diets can reverse some of the signs of cancer. The best diets for cancer patients are rich in fat and protein and low in carbohydrates.

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