Dear Dr. Dog: My dogs are aging and I find myself worrying that some dread disease like cancer is on the horizon. I’d like to know more about how cancer is treated in dogs, as opposed to humans. Can you help?
Approximately one in four dogs will be diagnosed with cancer, and there are few cancers that we can prevent. Therefore, our efforts as veterinary oncologists are focused on treating cancer, with the additional goal of improving the quality of life for our patients. We strive to provide treatment options that do not compromise what our patients love to do — whether it’s going for walks, playing ball, or just looking out the window. In other words, our treatments are intended to maximize therapy while minimizing side effects.
Let’s look at some of the treatment options available to our canine companions diagnosed with cancer:
Surgery is often our first line of defense in canine cancer treatment. The first surgery offers the best chance of a cure if the entire tumor can be removed with wide surgical margins. In addition to tumor removal, surgery is used for diagnostic purposes, relief of pain and clinical signs, reduction of tumor size (called debulking), and enhancement of other forms of treatment.
Removal of a tumor is comparable to pulling a weed in the garden. If you pull the weed and leave the roots behind, the weed will grow back. If we leave cancer cells behind, the tumor will grow back — and it can grow back more aggressively. Therefore, treatment must involve removal of the entire tumor or follow-up with another treatment modality such as chemotherapy and/or radiation.
Chemotherapy targets rapidly dividing cells and is indicated for tumors that metastasize (spread) readily. Chemotherapy can be used to:
- Slow the spread of cancer;
- Shrink or slow progression of the tumor;
- Prevent the local recurrence of tumors after incomplete surgical removal;
- Reduce the size of some tumor types prior to treatment with surgery or radiation; and
- Sensitize the tissues to radiation treatment, prior to the radiation.
It may ease your mind to know that veterinary oncology is not as aggressive with chemotherapy, so dogs and cats generally do not experience the same side effects as humans. Much lower doses of chemotherapy are used than in human oncology, which means very few of our patients become ill or require hospitalization during or after treatment. As mentioned earlier, we do not allow our treatments to compromise the patient’s quality of life.
Some tumor types can be cured by surgical removal but may be in locations not amenable to complete surgical removal. For example, most soft tissue sarcomas can be cured with wide surgical resection. However, when these tumors are on a leg, for instance, wide surgical resection is impossible without amputation. In such cases, radiation treatment is used as follow-up therapy to control the disease that has been left behind.
Radiation treatment is also used as a primary treatment for tumors that cannot be surgically removed, such as nasal or brain tumors. It is sometimes used for palliation of pain or to slow the growth of certain tumor types.
A new use for radiation therapy is in dogs with lymphoma. Historically, these patients have been treated with chemotherapy alone, but half-body radiation is now offered in addition to chemotherapy. Studies are ongoing, but there is some indication that this may extend survival times.
New Treatment Options
Metronomic (Antiangiogenic) Chemotherapy. This continuous, low-dose, oral chemotherapy, administered at home on a daily basis, targets the blood vessels supplying the tumor rather than the tumor itself. One main advantage with this form of chemotherapy is that it can be given at home, and the patient only needs periodic re-checks and bloodwork at the veterinarian’s office. A few studies have shown that this form of therapy can slow progression of cancer.
Melanoma Vaccine. Malignant melanoma is a very aggressive cancer and is the most common oral tumor in dogs. Malignant melanoma is usually treated with surgery and/or radiation for local disease. However, another treatment modality is needed to address the aggressive metastatic behavior of this cancer. Injectable chemotherapy has been minimally effective in targeting metastasis, but the recent development of the melanoma vaccine, a form of immunotherapy, has been promising. The vaccine contains human DNA that encodes for tyrosinase, which is required to synthesize melanin. By using human DNA, the dog’s immune system can recognize this protein as foreign and mount an immune response against it. Melanoma vaccine has resulted in increased median survival times when combined with aggressive surgical excision and/or radiation treatment, and it eliminates the use of injectable chemotherapy. The vaccine is typically given every two weeks for a total of four treatments and is boostered every six months thereafter.
Of course, many dogs live long lives without ever developing cancer. Fortunately, there are a number of effective treatment options available to address the disease when it does appear, and many more are on the horizon. The best news of all is that we are usually able to treat canine cancer while maintaining a good quality of life for the patient.
Danielle O’Brien, DVM is an intern at San Francisco Veterinary Specialists. Next August, she will begin a three-year medical oncology residency at UC Davis. Aarti Sabhlok, DVM, Board-qualified (Oncology) is part of the SFVS Veterinary Oncology Group. For more information on SFVS and its oncology services, visit www.sfvs.net.