Cancer In Your Dog And Cat And How To Face It

Today And Tomorrow

Ron Hines DVM PhD

  Pain control 

 Pain in cats 

Entyce for dogs or Elura for cats?

 Perhaps some comfort here   

Cancer in our pets is a very broad and complicated subject. Its causes in dogs and cats are no better understood than cancer in humans and potentially successful treatments no different. So, this article only scratched the surface of an immense landscape. I am not a cancer specialist; but like all veterinarians in general practice, I deal with the tumors in dogs and cats on a day-to-day basis. The great majority of these are older pets. Anything credible you read online about the treatment of cancer in humans apply equally well to cancer in your dog and cat.

The first problem is that new breakthrough treatments are often extremely expensive. When they involve the manipulation of the immune system, the medication produced (generally a mAb) must be custom engineered to be accepted by the immune system of your dog or cat. The ones you see on TV and the internet were designed for human immune systems. They would be quickly inactivated by your dog or cat. But I am optimistic that in the near future mAb therapy for pets will be available. In 2017, Zoetis (a spin-off of Pfizer) purchase Nexvet, an Irish company that developed a rapid and economical way to quickly make mAbs designed for humans acceptable to dogs or cats. That should speed development of a host of new therapy options for veterinarians.

CRISPR gene editing is another breakthrough in medicine that has the potential to give veterinarians cancer treatment options that they do not have today.  Read about that here and about the potential of monoclonal antibodies in cancer treatment here &  here.  

I try to furnish pet owners with honest, practical advice during their difficult times. It is the same advice I would want if our roles were reversed. Many of the cancer drug and radiation therapies currently offered by veterinary oncologists, universities and hub veterinary centers undoubtedly extend the lives of pets. But because something can be done does not necessarily mean it should be done. I often question the quality of these relatively short periods that can sometimes be added to your pet’s life as well as the emotional costs of that time to both of you. I have doubts whether many of these therapies are truly performed for the good of the pet. My sense that they are often performed for the peace of mind of an owner who is unable to bear the grief or accept the fact that all life comes to an end. Dogs and cats fear death considerably less than you and I do. Please feel no guilt or shame if you accept that.

How Likely Is It That My Pet Will Develop Cancer?

The likelihood that your dog or cat will develop cancer during its lifetime varies from place to place and breed to breed – but not by that much. In the UK for example, the likelihood that your dog will develop a tumor increases sharply after the age of 6 years. But once your dog reaches the age of 10, that likelihood begins to decrease. In 130,684 dogs that were followed there, about 2,671 cases of cancer appeared per year. Skin tumors led the list, followed by tumors of the digestive system, breasts, immune system, glands and mouth, in that order. Other studies in California dogs and dogs in Ontario ranged from 1134 to 4817 tumors per 100,000 dogs. Cat in the Ontario study had a much lower incidence of cancer than dogs, 748 per 100,000 cats. However, considerably more of the tumors on cats were the much more serious, malignant kind. A problem with that Canadian study was that it did not adjust the incidence for age. (Cats that come to veterinarians tend to be young ones; and those cats are the least likely to have cancers other than those caused by feline leukemia and feline AIDS) You can read those studies here and on breed disposition here.

Why Do Cancers Occur?

Your pet’s cells are forever growing and replacing themselves, and growth gone awry is the basis of most 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 tumorous.

Cell growth is strictly controlled by instructions written into the DNA code of every cell in the body. In tumorous cells an error has occurred in this script, allowing the cells to grow out of control and evade the immune system policemen in charge of preventing it. When these errors are minor, the cells still look and act a lot like normal ones. We call tumors formed from these cells benign. When the errors are major, we call the tumors malignant.

The Characteristics of Cancerous Cells:

Small snippets of tumor tissue are called biopsies. Examination of biopsies allows veterinary pathologists to determine if the growth is benign or malignant, the types of cells involved and what the best treatment might be. 

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 skin of dog’s (usually when they are too chubby). Another type are the cauliflower-shaped papillomas that form within the skin and protrude out as “warts”. Under a microscope, the cells of these tumors look very much like normal tissue. The borders of these tumors are usually regular and well-defined making them easy to remove surgically. Skin tumors on cats tend to be much more worrisome. Some turn out to be the dangerous fibrosarcomas associated with prior over-vaccination.

Before your vet removes skin tumors he/she will clip its fur coat off close to the skin and make the area as free of bacteria as possible. I usually send owners home with a surgical marker to mark the position of even the smallest tumors. The one the client brought the pet in to have removed may be the largest, but it is uncommon for there to be just one papilloma or lipoma on your pet. It is exasperating for pet owners and veterinarians alike to find others a few weeks or months after the initial surgery.

Every veterinarian has techniques they are most comfortable with. I personally do not like to take dogs into deep anesthesia for the removal of these superficial tumors. It is possible to just heavily tranquilize the pet and infiltrate the area surrounding the tumor with a combination of local anesthetic lidocaine  (Novocaine®) and epinephrine (to constrict blood vessels and reduce bleeding in the area) just before and during the procedure. This both numbs the area and stops loss of blood. Then the tumors can be easily removed with a scalpel or electrosurgical unit, the incision sutured, and a drain added if need be. I generally freeze or cauterize off small papillomas. I have never had a tumor of this type regrow. Lipomas or benign fatty tumors are present just under the skin. They are considerably more common in dogs than in cats. I have only seen them on chubby dogs. They are not a threat to your pet’s health. If the dog looses sufficient weight, these tumors will shrink on their own. Because they are invariably encapsulated and have little blood supply, they are quite easy to remove. Lipomas that have grown around nerves or large blood vessels are considerably more difficult to remove (and it may be wiser to leave them alone as long as possible.) But even with these, the outcomes are usually good if sufficient care is taken to preserve the nerves and blood vessels in the area of the lipoma. Your veterinarian can do a simple microscopic test in the office to confirm that the tumor is a simple lipoma.

Tumors that arise from glandular and surface cells are called carcinomas. Tumors that arise from 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 (spread) 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 the life-threatening ones.

Cancers that move often become trapped in the sieve-like structures of the lungs, liver, bone marrow and kidneys. When they do, the symptoms that we see are due to the physical destruction of these organs as the tumor grows and crowds out healthy cells. Not anything toxic the tumor cells produce (see the tests). Metastatic tumors are usually highly vascular. That is, they are rich in blood vessels to supply the nutrients that the fast-growing tumor cells require.

Cancers have 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 your pet was born with which made it particularly susceptible to one form of cancer or another.

Boxers and the giant breeds of dogs are renowned for their predisposition to certain tumors. Other risk factors, such as cancer-causing (oncogenic) chemicals, may be found in the pet’s environment or diet (formaldehyde, chlorine-containing compounds, nitrites, etc.). They are the same products you should avoid to preserve your own health.

Some of these chemicals cause the cell’s 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 tumor-causing viruses have also been found to cause cancer in animals. Cancer often results from the combined effects of genetics, physical and chemical carcinogens. Your pet’s immune system plays an important role in detecting and eliminating new cancers.

Any factor or disease that causes immunosuppression increases the risk of tumors occurring in your pet. Feline immunodeficiency disease and feline leukemia both of which are caused by retrovirus, are conditions leading to a variety of tumors in cats. Hormones that cause various body organs to proliferate excessively can also progress to cancer.

Breast or mammary tumors in dogs are quite common and occur almost exclusively in older unspayed females. This is because of the twice-a-year hormone rises that unspayed female dogs experience, associated with their estrus or heat cycles. Luckily, breast tumors in dogs are much less dangerous than breast tumors in people. The ones that occur in dogs rarely metastasize or threaten your dog’s life.

As in people, the earlier we detect and remove the more dangerous cancers from pets the more successful we are. Skin tumors are rather easy to diagnose – and some of them do need immediate removal. But tumors within the body often only show up as weight loss, low-grade fever, weakness, and lethargy. By the time these cancers are large enough to detect they can be in advanced stages and very difficult to treat successfully.


X-rays (radiographs) are often the 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 CAT-scan and MRI imaging equipment to use when simpler techniques fail or when difficult surgery is contemplated. Since smaller veterinary hospitals do not have this equipment, they often make more use of biopsies and exploratory surgery to diagnose and treat cancer. When the abdomen is opened and all the organs inspected, small tumors or those that were not visible on radiographs for one reason or another are sometimes obvious. When they are not, biopsies of the major organs, sent to 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 and threaten your pet’s life.

The most common cancers in dogs are those in or near the skin. Skin cancers make up over half the total number of cancers that occur in dogs they are considerably less common (but more worrisome) in cats :


Among the most common skin cancers that veterinarians encounter are papillomas. These are small, cauliflower-like, viral-produced tumors that proliferate as a dog ages. They are common on the muzzle, trunk and extremities of dogs with graying hair. The vast majority are not malignant and cause no damage beyond being easily nicked or worried into bleeding by the pet as it grooms. When they form on the edge of the eyelid, they do need to be removed as they tend to irritate the cornea. The vast majority are easily removed by your vet with a tranquilizer and the aid of a bit of local anesthetic.


The next most common cancer in dogs are lipomas. They are soft, and often have the consistency of a baggy filled with water. They are almost never life-threatening. They often occur in multiple locations just under a dog’s skin. They are much rarer in cats and ferrets but quite common in parrot-like birds. They will shrink, but not entirely disappear, if you put your pet on a lower calorie diet. Read about that here.

Mast Cell Tumors

The next most common skin cancers in dogs are mastocytomas (or mast cell tumors). These distinctive tumors are oval, firm and slightly raised. Sometimes their center is brown or bluish. Mastocytomas are usually only locally invasive (malignant) and do not readily metastasize to other organs. Cancerous cells project outward from the tumor into what appears to be normal skin. So, when they are removed, a considerably larger diameter than the visible tumor needs to be excised to be sure that all tumorous cells are removed. The biggest problem occurs when mast cell tumors are on the extremities (legs) where insufficient extra skin may be left to close the incision after the tumor is removed. Your dog’s individual genetics play an important part in its susceptibility to mast cell tumors as well. (read here)

Skin tumors on cats are considerably more likely to be malignant than those on dogs. Your vet need to removed them as rapidly as they are discovered and always sent the removed tissue to a veterinary pathologist for evaluation. In an unfortunately high a number, these tumors have already metastasized to other locations before they were discovered.

Fibrosarcomas In Cats

The most common cause of these tumors is thought to be repeated vaccinations with vaccines. You can read about fibrosarcomas in cats here. This is a heartbreaking condition because it often occurs in relatively young cats, and it was usually preventable. It can sometimes be treated successfully with surgery – but the surgery must be very aggressive, removing a large amount of tissue surrounding the tumor – possibly even bone. However, surgery alone is only successful if every single tumor cell is removed or destroyed and this is quite difficult to do. The larger the mass and the longer it has been there, the less likely that surgery will be successful. Up to 70% of these tumors re-grow after surgery. Chances of your pet’s survival increase if the surgery is followed by combinations of radiation therapy and immunotherapy. When you do decide to have your cat vaccinated, insist that the shot be given low on a rear leg or, better yet, in the tail. If a tumor occurs there, that portion of the pet’s body can be sacrificed preserving its life. Read about fibrosarcomas in cats here

Mammary Gland Tumors

Mammary gland tumors have a high incidence in older unspayed dogs. They often begin to develop between six and ten years of age and are encouraged by the hormone, progesterone, associated with estrus, and reproduction. Dogs that have been spayed before their second estrus (heat cycle) are less likely to develop these tumors later in life. The most common form is the fibrous and hard mixed mammary carcinoma. They form most frequently in the posterior breasts nearest the rear legs. The breasts involved often give small amounts of milk or milk-like fluid. Most are well encapsulated and easy to remove. They are usually not highly malignant and most of the time vets remove them them long before they have metastasized. Of special concern are tumors that are ulcerated (bleeding) and which have infiltrated the skin. These might be malignant. Also worrying are mammary tumors that involve the lymph nodes of the groin and those at the base of the foreleg. It is not uncommon to sent sections from the same tumor of this type to two different pathologists and receive differing opinions as to the tumor’s degree of malignancy. This is because determination of malignancy is a subjective process. Spaying females before their second heat cycle (but never as puppies) can help reduce the likelihood of these tumors. 

Lymphatic 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, dogs and ferrets. In cats, these tumors can occur under the immunosuppressive effect of the feline immunodeficiency virus and Feline Leukemia virus. Cats are also quite susceptible to a form of lymphoma that invades their intestinal walls (intestinal lymphoma). 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 these cancers appear spontaneously. The tumors appear as sold growths which begin in the lymph nodes or bone marrow or as individual cells freely circulating in the blood, in which case they are called leukemias. Animals as young as four years may develop these tumors. These animals are often presented to veterinarians with painless, enlarged lymph nodes over the whole body; but occasionally, it is a single lymph 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 early 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 when those are the ones affected. This type of tumor in dogs responds well to chemotherapy. It does not respond that well in cats. Many of these tumors are treated by veterinary oncologists using the CHOP protocol. Other “chemo” drugs commonly utilized to treat lymphomas and other cancers are vincristine,   L-asparaginase,    cyclophosphamide,    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, Ears and Tongue

Tumors of the mouth, lips ear 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. White areas exposed to the sun are common places for them to form. A variety of tumors form here. They include squamous cell carcinomas, adenocarcinomas, fibrosarcomas and 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 may have to be force-fed or fed intravenously. As I mentioned earlier, I am not inclined to suggest radical surgical procedure to most clients, but I do make them aware that these procedures exist. There is some evidence that the viruses responsible for papilloma might be involved in the formation of some of these oral tumors in dogs as well. Chemotherapy has not been very rewarding in these cases. Before considering chemotherapy for your pet, ask your veterinarian what the average increased in life expectancy after 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. (read here)

Bone Cancer

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 veterinarians initially because of lameness. X-rays of these tumors are highly distinctive and usually easy to diagnose. Because they often metastasize to the lungs, vets include a chest film of every dog radiograph for this problem. Not all cases are so advanced that tumors in the lungs can be detected. That does not mean they are not there. When the tumor has moved, later in the disease these dogs might 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 thought to be helpful in cases where the tumor is inoperable because of location. Most dogs do quite well with only one rear leg. But I rarely suggest that a front leg be removed from a large dog because of the extent of disability that produces.

Keeping Your Companion Happy

Our pets are blessed by the Creator in that they do not fear the future or dwell on their illnesses. Unlike us, they do not worry about the passage of time or where they will spend eternity. So if they even consider it, they face death with considerable peace. Veterinary medicine has made tremendous strides in delaying the death of pets. I have an article that explains strategies your veterinarian can use to do that. You can read the abstract here. But would your pet want you to do those things? My feelings are that it probably would not. Will you be doing those things for the pet’s happiness or for your happiness? Your pet will remain happy if it has something good to eat, a loving home and the pleasure of your company. Pet’s key off of their owners emotions. So if you are sad, they will be sad – if you are at peace and accepting, they will be at peace too.

Be wary about purchasing products online that claim to cure cancer. They do not work. I have never understood how anyone could be so cruel and cold-hearted as to sell them. That goes for any Google Ads that might sneak onto my website as well. They are placed there by a robot and – as you know-robots treat the honest and the dishonest equally.

Keeping Your Pet Eating

Nutritional support is very important for all cancer patients. This is especially true for cats because minimal cancer-induced stress can make them disinterested in eating and bring on hepatic lipidosis. Cancer cachexia is a form of malnutrition that affects many pets with cancer – especially those in which the disease is widespread in their bodies. Offering flavorful, highly digestible and energy-dense diets can reverse some of the signs of cancer, prolong their lives and increase their happiness. Weight loss is always a bad sign in dogs and cats dealing with cancer. The best diets for pets with cancer are rich in fat and protein and low in carbohydrates. They are also the ones you make yourself. Read about them here. Stimulate them to eat with savory home-cooked aromas, pet them, hug them, encourage them, love them. The two medications that are the most helpful in stimulating appetite are mirtazapine (Remeron®) and Entyce®.

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