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Find information on animal health topics, written for the veterinary professional.

* This is the Veterinary Version. *

Cancer Treatment

By Manuals Staff,

Your veterinarian has several options for treating pets with cancer. There are 3 common treatment options for animal cancers: surgery, chemotherapy, and radiation therapy (also called radiotherapy). Each of these options can be used alone or in combination with other treatments. The specific treatment program your veterinarian will recommend will depend on the specifics of your pet’s condition. In selecting the treatment, your veterinarian will consider the type of cancer, how quickly it grows and spreads (the stage or grade of the cancer), and the location of the cancer.

The goal of any cancer treatment program is to completely and permanently eradicate the cancer. Sadly, a complete cure is often not possible. For pets whose cancer cannot be cured, the veterinary team can provide treatment to minimize the animal’s pain and discomfort and enhance its quality of life.

Oncology is the medical specialty that deals with all aspects of cancer in both humans and animals. Veterinary oncologists are veterinarians who study cancer in animals, including prevention, development, diagnosis, and treatment. A veterinary oncologist may be a part of the medical team treating your pet or the treatment may be conducted by your regular veterinarian and veterinary clinic staff.

Surgery

Surgery is considered the cornerstone for treating most cancers in animals. It is one of the oldest forms of cancer treatment and frequently the most effective one. Today, surgery may be combined with radiation therapy and/or chemotherapy, depending on the characteristics of the case.

When cancer surgery is performed, the main goal is usually to remove all the cancerous cells in the animal’s body. Sometimes, if the cancer is detected early (before it grows too large or spreads to other parts of the body), surgery can completely cure the animal. Other goals of cancer surgery can include removing an unsightly tumor to improve the animal’s appearance or comfort or removal of a tumor that is interfering with the animal’s normal body functions. These goals can improve the quality of life for the animal.

Surgery is most successful when the cancer involves a tumor that has not spread beyond its original location. Unfortunately, however, not all tumors can be surgically removed. Some are in inaccessible sites. And, there are times when the costs to the animal might outweigh the benefits. For example, removing a large tumor might require removing a vital organ or may cause a pet to lose a vital body function. If the cancer is in more than one location or has spread (metastasized), then surgery is not as likely to be an effective treatment.

A biopsy is a surgical procedure in which a piece of a tumor is removed for study and analysis by a pathologist. The pathologist’s report will provide important information such as the type of cancer and its characteristics. Combined with information about the size and location of the cancer, your veterinarian can develop the best treatment program for your pet.

Even if surgery cannot be used to completely remove a tumor, it can be used to remove part of a tumor, a process known as debulking. There are several possible benefits to debulking. First, partially removing a tumor can reduce the signs of cancer and make the animal more comfortable through improved mobility or reduced pain. Debulking can also be used to improve the effectiveness of radiation therapy or chemotherapy. These cancer treatments have the greatest chance for success when fewer cancer cells are present.

Surgery may also be prescribed by your veterinarian to manage or reduce the side effects of other treatments. During radiation therapy, for example, normal, healthy tissue may be damaged (see Cancer Treatment : Radiation Therapy). Removal of the damaged tissue will encourage more rapid healing. In other cases, the surgical placement of a feeding tube may be necessary when either the cancer or its treatment makes normal eating physically impossible or very painful.

Pain management is an important part of surgical treatment (see Introduction to Pain Management). After the surgery, medication is usually provided to reduce pain and make the animal more comfortable.

Radiation Therapy

One of the most common treatments for cancer in both humans and animals is radiation therapy. This treatment is sometimes also called x-ray therapy, radioisotope irradiation, or cobalt therapy.

Cancer cells divide more frequently than normal cells. Cancer cells also have a weakness; they do not recover from radiation damage as quickly or completely as normal cells. Radiation works as a treatment for cancer because it kills cells that divide rapidly or, in other cases, because it damages the cancer cells so severely that they cannot divide and grow (see Radiation Therapy). Radiation therapists work to deliver just enough radiation to the cancer cells to destroy or injure them and prevent them from reproducing.

Radiation therapy is often used in addition to treatment with surgery or chemotherapy or both. The therapy or combination of therapies prescribed for a particular animal will be selected by your veterinarian based on which options offer the best chance of controlling or eradicating your pet’s cancer. For brain tumors, nasal tumors, and other tumors in the head and neck, radiation therapy may be the treatment of choice. For cancers of the spine or pelvis, it may be the only practical treatment option.

Great strides have been made in recent years in radiation therapy. There has been a dramatic improvement in the sophistication of radiation therapy equipment and methods and a parallel rise in its success in eradicating cancer. Pet owners have also increased their requests for this treatment for their pets. However, radiation therapy is not a cure-all for cancer. Not all cancers are easily killed by radiation. Some cancers are highly resistant to radiation therapy and cancers of these types cannot be treated effectively with radiation. Thus, whether or not radiation therapy will be prescribed will depend, to a great extent, on the type of cancer to be treated.

Often, radiation therapy is used to either help make chemotherapy more effective or to decrease the size of a tumor in order to make surgical removal possible or more likely to succeed. Thus, radiation therapy is frequently used as a part of a combination treatment program.

Radiation therapy is not administered in a single “zap.” It is delivered in a series of doses over an extended period. By administering the radiation in this way, the killing effect on the cancer cells is maximized while the toxic effects on healthy cells are minimized. This schedule allows healthy cells to repair themselves after radiation exposure. The exact dose and the schedule for delivery will be carefully set based on the type of cancer being treated, how advanced the cancer is, the animal’s response to radiation therapy, and the goal of the treatment. For example, if the treatment goal is to reduce the size of a tumor prior to surgery, the treatment dose and schedule will be different than if the goal is to completely eradicate a tumor. Overall, a radiation therapy program will typically involve 5 doses per week for a period of 4 to 6 weeks.

A cancer cure is not the only possible goal for radiation therapy. In some cases, the radiation therapy goal is to provide some relief from the impact of a tumor or from the spread of cancer to other parts of the body. These steps may allow the animal to feel better even if its life is not lengthened by the treatment.

Great accuracy is required to target the radiation to destroy cancer cells while protecting healthy cells. However, even with great care and accuracy, radiation can damage normal cells close to the cancer. The cells most likely to be damaged are those that normally divide rapidly. These include the lining of the mouth, esophagus, and intestines; hair follicles; bone marrow; and the skin. Radiation can also damage the ovaries or testes.

There are some recognized adverse effects from radiation therapy. The extent and severity of these effects will depend on the size of the area being treated, the dose administered, and the location being radiated. When the radiation site is near sensitive tissues, the effects are likely to be more severe and prolonged. For example, treatment for tumors on the head or neck often causes damage to the overlying skin. Treatment of head tumors may cause inflammation or irritation of the lining of the mouth. For animals with this condition, a feeding tube may be recommended to reduce the discomfort of eating with a sore mouth. Dry eye is another side effect associated with radiation to the head. It is caused by a decrease in tear production due to the impact of radiation on the eyelids. This can sometimes be a permanent condition. Eye drops and other medications are available to help prevent sores from developing and relieve eye irritation. Radiation to any portion of the digestive tract may cause irritation resulting in nausea, lack of appetite, or diarrhea. For these animals, a change in diet may help control the signs.

Chemotherapy

Certain drugs destroy cancer cells. This type of treatment is called chemotherapy. It can be used to manage and treat several types of cancer. When it is used, the most common treatment goal is to shrink, stop the growth of, or destroy the cancer without longterm negative effects on the quality of life for the animal. Veterinarians will prescribe chemotherapy based on the type of cancer to be treated, the stage of the cancer, the overall condition of the animal to be treated, and any financial constraints that may be present.

In an ideal situation, a chemotherapy drug would kill cancer cells in an animal’s body without harming normal healthy cells. Few such drugs have been found. Today, the drugs selected for chemotherapy have been designed to be more damaging to cancer cells than to normal cells. They specifically target cells that divide and grow rapidly. Normal cells will be affected to some extent by chemotherapy drugs; sometimes the drugs can have adverse effects (See Drugs Used to Treat Cancers and Tumors).

Chemotherapy drugs are delivered either through the mouth or by injection. If injection is used, it can be into a vein (intravenous), muscle (intramuscular), or under the skin (subcutaneous). The delivery method will be selected with the comfort and quality of life for the pet in mind balanced against the goal of effective delivery of the drugs.

Some cancers do not respond to chemotherapy. How a cancer responds to a particular drug will depend on the type, size, rate of growth and spread, and location of the cancer. These factors are some of the most important ones in the selection of chemotherapy drugs, their combination, and their dosage. As is the case with other cancer treatments, chemotherapy is most effective when the tumor is small, is at an early stage in development, and has not spread to other parts of the body. When these conditions exist, most cancer cells divide quickly and the chemotherapy drugs are able to kill a larger number of the cancer cells.

Chemotherapy alone usually cannot cure cancer in pets. It is used most often to control cancer and its spread. Thus, chemotherapy is often used to treat cancers that affect the whole body, such as cancer of the lymphatic system (lymphoma). In other cases, chemotherapy is used to fight the remaining cancer cells when a tumor cannot be completely removed with surgery. Chemotherapy is also used to fight types of cancer that spread around the body early in their development.

Many of the chemotherapy drugs used to control cancer in people are used for the same purpose in pets. However, animals require dosages that are adjusted for their size and body type. In most cases, a combination of drugs will be used. Your veterinarian will evaluate the individual cancer and the particular needs of your pet when selecting the drug combination, dosage, and administration schedule. Quality of life issues, medical and nutritional support concerns, and pain control are other considerations that the prescribing veterinarian must evaluate when selecting a chemotherapy program. In all cases, your veterinarian must weigh the expected benefits of the drugs with possible adverse effects to select the most appropriate treatment for your pet. The veterinarian will carefully monitor your pet’s physical and behavioral response to the treatment and adjust the dosage to maximize the effect on the cancer while reducing the side effects.

While improvements have been made in chemotherapy for humans—many of the well-known side effects such as nausea, vomiting, hair loss, and fatigue have been reduced in recent years—people still regard chemotherapy as a distinctly unpleasant experience. Animals generally appear able to tolerate chemotherapy better than people, but treatment with some chemotherapy drugs may lead to vomiting or a lack of interest in food. This side effect can be treated with anti-nausea medicine. Intravenous fluids can be used to control such side effects as vomiting, diarrhea, and dehydration. Some chemotherapy drugs may cause a reduction in the number of red blood cells (anemia), white blood cells (leukopenia), or the cells that clot blood (platelets). The loss of white blood cells is probably the most significant of these effects because white blood cell loss lowers your pet’s ability to fight off infections. Your veterinarian will monitor your pet’s condition by taking blood samples. If the white blood cell count becomes too low, antibiotics may be prescribed to prevent infections. For animals with a low platelet count, there is an increased risk of bleeding.

Hair loss is a common side effect of chemotherapy among people. This is less common in pets, though it varies among breeds.

Dogs and cats receiving chemotherapy usually have good to excellent quality of life throughout the treatment program. Side effects, if any, are usually mild. The risk of life-threatening adverse effects is estimated at less than 5% for most types of chemotherapy. The most risky side effects can often be anticipated and either controlled or prevented entirely. If your pet will be undergoing chemotherapy, you should discuss the treatment program with your veterinarian in advance. You need to come to a mutual understanding about what can be expected for your pet and the level of risk that can be accepted.

Chemotherapy may be stopped before the end of the scheduled treatment program if the cancer being treated is not affected by the drugs or starts regrowing following a period of remission. A prescribed chemotherapy program may also be stopped when the animal has received the maximum acceptable total dose for a particular drug or if there are unacceptable adverse effects.

Combination Therapy

The term combination therapy refers to the use of 2 or more treatment options in the fight against cancer. Today, combination therapy is the most frequently used approach to treat cancer in pets. It offers the best opportunity to cure the cancer while maintaining the best possible quality of life for the animal.

Combination chemotherapy offers many advantages over single drug treatment programs. For example, when multiple chemotherapy drugs are used, and each one uses a different mechanism to kill cancer cells, it is less likely that the cancer will become drug resistant. This improves the chances that the treatment will be successful. Also, a combination of drugs can target different cancerous sites, increasing the likelihood of controlling any spread of the cancer. When using drugs with different side effects in combination, the probability is high that any side effects will be no worse than with a single drug given separately. These benefits combine to make a combination therapy program the best choice in many cases.

There is no single best treatment for all cancers. For some cancers, the best approach is one that combines surgery, radiation therapy, and chemotherapy. Tumors and other cancers that are confined to a localized area are often best treated with surgery or radiation therapy. Chemotherapy has the advantage of treating cancer cells that have spread from their original location. In other cases, radiation or chemotherapy is used to shrink a tumor to a size that makes surgical removal possible or more likely to succeed. Radiation or chemotherapy may be used following surgery to kill any cancer cells that may remain.

The stage of cancer development is a factor in selecting the treatment, whether a single treatment mode or a combination of treatment methods. For animals with advanced cancers that cannot be treated with surgery or radiation therapy, combination chemotherapy can be used to reduce the signs of the disease and prolong life.

Prospects for a Cure

During the past century, researchers have made enormous strides toward finding a cure for cancer. But we are not there yet. There is no single and complete cure for cancer in either humans or animals. However, much has been learned about managing and treating this ancient disease. Veterinarians have been successful in using surgery, chemotherapy, and radiation therapy to cure many animal cancers. Meanwhile, research is continuing and the prospects for better cancer treatments are strong.

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* This is the Veterinary Version. *