The Dog Cancer Survival Guide (40 page)

Some human cancers can be cured with chemotherapy, but not in dogs. Instead, it is used to prolong life and increase quality of life by shrinking tumors and/or suppressing their growth and spread. This emphasis on life quality and extension is why I consider chemotherapy a palliative cancer treatment, not a curative one (although most oncologists wouldn’t think of it this way).

Some guardians wonder why chemotherapy can’t cure dog cancer, and the reason is really very simple: curative doses of chemotherapy typically come with high levels of toxic side effects. These side effects may feel awful to humans, but they can be tolerated because we understand why they must be endured. It is impossible to give dogs the same understanding, and most guardians and vets would find the severe side effects, typical in human treatments, unacceptable for dogs.

The goal of chemotherapy is to put cancer into remission, which is a reduction (not necessarily elimination) of the signs and symptoms of illness. During a remission, you may see your dog’s tumor shrink (visibly or on images) or disappear, which is commonly referred to as a “reduced tumor burden.” Her energy and appetite may return, and she may be in less pain, eating and drinking normally, sleeping regularly, playing, wanting to go for walks and being more engaged with you.

Veterinary oncologists consider chemotherapy a success if tumors stop growing or shrink or if they continue to grow at a much slower rate. The length of time a remission lasts varies widely, depending upon the tumor type, the stage and grade of the cancer, and many other factors. For example, Dr. Ettinger’s preferred lymphoma protocol can extend life by thirteen to fourteen months, while other cancers may allow a dog just a few extra months. Remissions that last two weeks or more are considered stable and called “durable.”

Researchers know chemotherapy must be as strong as possible to slow cancer – but, with stronger treatments, the side effects increase. So, what is an acceptable level of side effects? To induce remission in the majority of dogs, researchers have a set of guidelines; to be effective, any given chemotherapy protocol will not cause death in more than 1% of treated dogs, and hospitalization in no more than 5%. These results must be achieved with as few side effects as possible, most of which will be manageable at home, with over-the-counter medications.

While I understand Dr. Dressler’s perspective, as a chemotherapy specialist, I must respectfully disagree with him. A 5% hospitalization rate seems reasonable when I consider the benefits chemotherapy can offer dogs who are good candidates. A one in twenty risk of hospitalization seems well worth the pay-off for another year with your dog, in the case of the best lymphoma protocol, for example. Chemotherapy does not work for every cancer (please see my section for more information). When it does, you, the dog owner, ultimately have to decide for yourself: what’s your risk tolerance? As a medical oncologist, I am very proactive about managing side effects, and I have rarely hospitalized chemotherapy patients. The majority of them are treated as outpatients, and they are happy dogs. Please read my section of this book for more of my advice and another perspective.

From my perspective, chemotherapy can be a tough sell for certain cancers. Because I treat many other conditions in addition to cancer, a 5% hospitalization rate – one in twenty dogs – seems really high, in comparison to treatments used for other diseases. Many of the guardians I see in private practice feel this way, too – for some, the knowledge that their dogs have a one in one hundred chance of dying as a result of chemotherapy will cause them to walk right out the door. For others, the expense, the concerns about toxicity, the possible side effects, and the realization that there is no sure outcome, make chemotherapy a less attractive option than surgery, for example (at least surgery is guaranteed to get some cancer out of the body). Of course, there are certainly guardians who feel chemotherapy is an attractive option for their dogs. Chemotherapy certainly has its place, and should be used in the appropriate circumstances, but there are many factors involved.

In addition to the inherent risks, relapses or recurrences of the cancer are not only possible, but also, very probable. After a remission, the cancer comes back, and usually stronger. At this point, it will be your choice whether you use a “rescue protocol” to attempt a second remission. Depending upon the type of tumor and how long the first remission lasted, rescue protocols may entail the same drugs administered in the same doses, or a completely different protocol. If your dog achieves a second remission, it may not seem very long before another relapse occurs.

Dr. Ettinger is in the trenches of chemotherapy, and these rates seem normal to her (although of course she wishes they were better). She takes care to inform guardians of the real odds, and her clients can afford and are willing to take the chance chemotherapy offers them, hoping that their dog will achieve some length of remission. There are many chemotherapy drugs; not all of them are useful for each cancer. There is a lot to understand, which is why Dr. Ettinger, the expert in chemotherapy, has written about these drugs, in addition to other conventional advice. Her section begins on
page 289
, so please make a point of using it for your research.

Unlike radiation and surgery, which treat specific, localized areas, chemotherapy agents can circulate throughout the body, via the bloodstream. For this reason, chemotherapy can be useful when the cancer has become systemic, by spreading into the surrounding tissues (local invasion) or into the lymphatic system, bloodstream, or distant sites in the body (metastasis). For cancer types that have a high risk of future metastasis, chemotherapy may also be used as a preventive measure.

Chemotherapy is often used as a follow up to surgery. If there is micrometastasis – invisible spread – it will sometimes undergo a growth spurt immediately after surgery. Using chemotherapy five to seven days post-surgery can stunt some of that growth and make recurrence less likely. However, using chemotherapy during this time can also affect the body’s ability to heal from the surgery, and for some cancers (osteosarcoma, specifically), waiting for fourteen days may not affect the ultimate outcome. For these reasons, Dr. Ettinger recommends waiting at least ten to fourteen days after surgery to start chemotherapy for most patients.

There are published chemotherapy protocols for most of the common dog cancers, which oncologists use as guidelines, not recipes. For most cancers, there is no one “standard” protocol acknowledged as “the best.” Oncologists make their plans based on their observations of the particular dog, their experience with that particular cancer type, stage, and grade, and their personal preferences.

Because there are so many variables – and so many drugs – I strongly recommend reading Dr. Ettinger’s advice and consulting with an oncologist, if you are considering chemotherapy. If you don’t have a veterinary oncologist in your area, your vet can contact one of the commercial laboratories, which often have oncologists on staff.

The goal of most chemotherapy protocols is to give a series of the highest, most potent dose possible – otherwise known as the
maximum tolerated dose.
The shorter the time is between doses, the more effective the therapy. This intense schedule kills the maximum number of cancer cells. The trick, for oncologists, is to minimize (and manage) the possible side effects, while the chemotherapy has a chance to work.

As powerful as chemotherapy agents are, some cancer cells almost always survive the treatment. Why? It could be because some cancer cells just did not come in contact with an effective dose of the chemotherapy agents. If a tumor is very large, or very dense, the agents might kill the cells on the margins, but not penetrate into the very center, especially if the blood supply to the cancer is inadequate. Sometimes it’s more complicated. Some tumor cells activate a structure in the cell membrane, called a p-glycoprotein pump, to literally pump out certain chemotherapy drugs.

(P-glycoprotein pumps can be present in normal cells, too, but in those cells the pumps are used to remove toxins.)

The p-glycoprotein pump is encoded in the multi-drug resistance (MDR) gene. When it’s turned on, this pump can eliminate specific chemotherapy drugs from the cell. It can eliminate many other drugs, also, even those with completely different chemical structures. This is called multi-drug resistance, and in these cases it is very hard to find a chemotherapy protocol that will be able to shrink or slow the cancer.(This is why dogs with lymphoma with multi-drug resistance have a worse prognosis.)This is an interesting area for researchers, some of whom are developing drugs that might affect multi-drug resistance and help future patients respond to chemotherapy.

While MDR is a real phenomenon for some dogs, others have the opposite problem: they carry a genetic mutation in the MDR gene, which actually makes them more susceptible to chemotherapy and other drugs. While these dogs are more likely to suffer severe side effects from chemotherapy, they also could become critically ill or even die from the use of some common parasite-control products, antibiotics, sedatives and pain medications.