Cancer Treatment Options for Pets

Saturday, September 01, 2007

Michelle Cohen, DVM, Diplomate ACVIM (Oncology), Diplomate ACVR

Garden State Veterinary Specialists


Cancer treatment in pets is becoming more advanced and parallels treatment available for people with cancer. Many of the same chemotherapy drugs or radiation equipment that is used in human medicine is also used to treat animals. With the advances that are being made in veterinary oncology, the diagnosis of cancer in an animal is no longer a “death sentence” and many animals are experiencing long remission and are able to enjoy a good quality of life for an extended amount of time. The side effects of cancer treatment in animals are typically minimal, as our goal is quality of life.

There are many types of modalities available to us for treating cancer in animals. The mainstays of treatment in veterinary oncology are surgery, radiation, chemotherapy, and more recently, immunotherapy. The type of treatment recommended depends on the type of cancer the animal has and the overall condition of the patient. Sometimes, only one treatment is necessary, and in other cases, a multimodality approach is necessary.

Surgery is a very important part of cancer treatment for many types of cancers. Surgery is recommended when a patient has a localized tumor than can be removed. Surgery is our best chance at local control if a patient has a tumor that can be removed. Sometimes, additional therapy is recommended following surgery, depending on the tumor type, or the completeness of excision. For example, if a dog has a osteosarcoma, which is the most common type of bone cancer in dogs, surgery is recommended to remove the leg, however, because this type of cancer has a very high metastatic rate (spread to other locations), chemotherapy is recommended. Or, if a tumor is removed but the biopsy reports that tumor cells extend to the margins of the specimen, this means that the entire tumor was not removed, and has a good chance of recurring. In this instance, radiation therapy would be advised. Surgery is recommended in cases where the entire tumor can be removed or debulked and where the after-effects will not impinge on the function of the animal or quality of life. Surgery is typically not recommended when the patient already has metastatic disease or when there are other debilitating medical conditions present. In most cases, recovery from surgery can take a few days to several weeks, depending on the extent of the procedure.

Radiation therapy is the use of high energy rays of particles or waves to destroy cancer cells in a localized area. There are 2 main types of radiation therapy – teletherapy or external beam radiation is the use of radiation generated by a machine which the patient is placed under, while brachytherapy is the use of a radioactive source placed into or on the patient. Teletherapy is most commonly used in veterinary radiation oncology. Radiation therapy is a local therapy, like surgery, and has specific indications. The main indications for radiation are 1)post-operatively 2)pre-operatively 3)intra-operatively and 4)primary therapy. Post-operative radiation is one of the most common reasons radiation is used in veterinary oncology. Radiation is often a very effect treatment for patients with tumors that were not completely excised. With this therapy, many times we are able to get a very long term control of the primary tumor, depending on the type. Pre-operatively radiation is used in instances where the tumor is too large to be removed and radiation may help shrink the tumor in order to make surgery possible. Intra-operatively radiation is not commonly used, but is sometimes useful for abdominal tumors. The abdomen is surgery opened and the tumor or tumor bed is exposed to the radiation source while vital organs are moved and shield from the radiation. Radiation as a primary therapy is also used in certain instances where the tumor is effectively controlled by radiation therapy alone and where surgery doesn’t have additional benefit, such as with nasal tumors, or when the tumor is located in a place where surgery is not an option – such with deeply located brain tumors.
There are 2 main ways in which external beam radiation cancer be used – either definitive therapy or palliative therapy. Definitive therapy is an intensive treatment protocol where long term control is anticipated and side effects are expected to be well tolerated. Typically this consists of a low dose treatment given daily 5 days a week for 15-19 treatments. Palliative therapy is a less intensive treatment protocol where long term prognosis is guarded. The goal of palliative therapy is to provide relief of pain or improve function and make the patient feel better for the time he/she has left. The type of treatment that is recommended depends on the type of tumor and the overall health of the patient.
When a patient requires radiation therapy, we will often perform a CT scan to determine the extent of the tumor and to assist with treatment planning purposes. At Garden State Veterinary Specialists, we are able to perform 3D conformal therapy, with the use our treatment planning computer. This computer allows us to utilize the CT scan images and reconstruct a 3D image so that we can visualize the tumor in all dimensions and create a treatment plan that will allow us to maximize dose delivered to the tumor and minimize dose to the surrounding normal tissues. We are then able to take this plan to the linear accelerator and deliver a course of treatment that will allow us to treat the tumor as effectively as possible while decrease the potential side effects.
Side effects of radiation are typically well tolerated by animals. The only side effects that we see are side effects in the area being treated. Side effects that we see depend greatly on the location of the tumor. Sometimes the skin may develop radiation dermatitis, or irritation of the skin, ranging anywhere from mild redness to ulceration of the skin. The hair in the radiation side will typically fall out but will usually grow back after a few months. Radiation over the mouth region can cause mucositis, which can range from a temporary drying of the mucous glands to oral ulceration. Typically, all of these will heal within 2-4 weeks after stopping radiation. If the patient is uncomfortable, we will prescribe medications to help minimize the side effects. Permanent side effects are uncommon but can include a change in fur color in the irradiated site, and a small chance of cataracts if the eyes are in the field.

Chemotherapy may be warranted in certain circumstances. Chemotherapy is recommended as a primary therapy for cancers such as lymphoma, where the tumor is sensitive to chemotherapy. It is recommended for cancers in conjunction with radiation or surgery when the tumor has a high metastatic rate. Chemotherapy is well tolerated by most patients. Approximately 85% of patients on chemotherapy will not have any side effects or minor side effects. Approximately 15% may experience mild gastrointestinal side effects, 3-5% can experience side effects requiring hospitalization, and 1-3% can experience life threatening side effects.

Immunotherapy is a rapidly evolving part of cancer treatment. This involves using agents to stimulate the body’s immune system to attack the cancer. For example, we now have a melanoma vaccine, which is licensed for use in dogs with oral melanomas, and has shown significant improvement in survival times when used in combination with standard treatments.

If your pet has been diagnosed with cancer, your veterinarian can discuss treatment options with you and help your family decide how best to handle your pet’s medical condition.

This article is for informational purposes only and should not be used as a substitute for the professional advice of your veterinarian.

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