Written by: Dr. Bob Rosenthal, Veterinary Oncologist
Throughout the years, dogs have benefitted from improvements in veterinary care. More are living longer as a consequence of improved nutrition, better vaccination protocols, and evolving standards of care in both general and specialty practices. If there is a downside to living longer, it is that more dogs live to so-called “cancer ages,” late middle age to geriatric. If a dog reaches ten years of age, it has a 50-50 chance of developing cancer. How can a rational treatment plan be developed?
There are pros and cons to every type of therapy, and in veterinary medicine there is still, unfortunately, some uncertainty as part of almost every therapeutic decision. The Morris Animal Foundation, supported by the Orvis Canine Cancer Campaign, supports a wide array of investigations into treatments, looking for ways to decrease that uncertainty. For the pet owner, the place to start is with a basic understanding of what their veterinarian has in her armamentarium. Therapy will always be a balancing act with efficacy and toxicity, two sides of a coin. This dictum applies to the three conventional approaches to treatment – surgery, radiation therapy, and chemotherapy.
Surgery to treat cancer has a very long history. References to surgery for cancer go back to Egyptian documents from over 3,400 years ago. The concept behind surgery is to cut away all lesions. Although surgery is almost always the quickest way to rid the patient of the largest number of tumor cell, it is strictly a local therapy and does not address metastasis, the propensity of malignant cancers to spread throughout the body. The efficacy/toxicity balance concerns how much tissue can be taken away (efficacy) without unduly compromising normal function or the cosmetic appearance. Even in tumors that cannot be cured by surgery, the reduction of a tumor mass to microscopic residual disease can go a very long way to make other approaches better.
The birth of radiation therapy came quickly on the heels of the discovery of X-rays and radioactivity in the 1890s. Since that time, there have been incredible advances in the technology involved, and veterinary medicine has used those technologies well. Radiation therapy is most commonly a local treatment to clean up what malignant cells the surgeon may have been unable to remove. On some occasions, radiation may be the primary mode of treatment, for example with tumors of the nasal passages or in brain cancer. The goal, as always, is to maximize the impact on tumor cells while sparing nearby normal tissues, and this is where very sophisticated treatment planning computer programs are essential.
Chemotherapy also has a long history. The documented use of drugs to treat cancer goes back to Greece about 2,200 years ago, but the modern era was initiated by studies conducted after World War I. Nitrogen mustard, used as a weapon in that war, was the basis of an important class of anti-cancer drugs called alkylating agents. Today many more drugs, acting by different mechanisms and with different toxicities, are available. These are potent agents that must be handled carefully. Drugs treat the whole body, unlike either surgery or radiation therapy. They are, therefore, ideal in attacking microscopic metastatic disease. In a few cancers, medical management would be the primary mode of therapy.
Multi-modality therapy involves the use of two or all three of these approaches. It is likely that the owner of a dog with cancer will be presented with various options for therapy. Difficult decision-making is made easier when owners understand that there are various ways to attack the problem, that every therapy carries some risk and uncertainty, and that each approach plays a different role. Such understanding certainly increases the power of the owner-veterinarian team.