GENERAL CONSIDERATIONS

Over the past several decades, a variety of therapeutic modalities have been used in dogs and cats with cancer (Box 76-1). However, until two or three decades ago, surgery remained the mainstay of cancer treatment for pets. Today, nonresectable or metastatic malignancies can be treated with varied degrees of success, using some of the modalities listed in Box 76-1.

When evaluating a cat or a dog with malignancy, the clinician should bear in mind that in most cases owners elect to treat their pets, if given the option. Although euthanasia still remains a reasonable choice in some small animals with cancer, every effort should be made to investigate treatment options.

Depending on the tumor type, biologic behavior, and clinical stage, a clinician may recommend one or more of the treatments listed in Box 76-1. However, in addition to tumor-related factors, many other factors influence the selection of the optimal treatment for a pet with cancer. These include patient-related, owner-related, and treatment-related factors.

PATIENT-RELATED FACTORS

It is important to remember that the best treatment for a particular tumor does not necessarily constitute the best treatment for a particular patient or the best treatment from the owner’s perspective. The most important patient-related factor to be considered is the animal’s general health and activity or performance status (Table 76-1). For example, a cat or dog with markedly diminished activity and severe constitutional signs (i.e., poor performance status) may not be a good candidate for aggressive chemotherapy or the repeated anesthetic episodes required for external beam radiotherapy. Age by itself is not a factor that should be considered when discussing cancer therapy with the owner (i.e., “age is not a disease”). For example, a 14-year-old dog in excellent health is a better candidate for chemotherapy or radiotherapy than a 9-year-old dog with chronic kidney disease or decompensated congestive heart failure. Patient-related factors should be addressed before instituting specific cancer treatment (e.g., correct the azotemia, improve the nutritional status with enteral feeding).

TABLE 76-1 Modified Karnovsky’s Performance Scheme for Dogs and Cats

Modified from International Histological Classification of Tumors of Domestic Animals, Bull World Health Organ 53:145, 1976.

OWNER-RELATED FACTORS

Owner-related factors play an important role in determining the treatment to be implemented in small animals with cancer. Every clinician is aware of the impact of the owner-pet bond. This bond is so important that it often dictates the treatment approach used in a given patient. For example, owners may be so apprehensive about having their dog with lymphoma receive chemotherapy that they refuse such treatment; thus the optimal treatment cannot be used in this patient.

In my experience, pet owners should be made a part of the medical team. If they are assigned tasks to perform at home, such as measuring the tumors to monitor the response to treatment, taking their pet’s temperature daily, and monitoring their pet’s performance status, they assume responsibility for the fate of their pet and are therefore quite cooperative. The clinician should always be available to answer concerned pet owners’ questions and guide them through difficult times. The clinician should discuss all potential treatment options with the owner, emphasizing the pros and cons of each (e.g., beneficial effects and potential for adverse effects of treatment A versus B versus C versus no treatment). The clinician should also clearly explain what will (or should) happen during the pet’s treatment, including a thorough description of the potential adverse effects by presenting different case scenarios (i.e., best-case scenario versus worst-case scenario). By observing these easy steps, the clinician usually cultivates realistic expectations on the part of the owner and ensures that the interaction with the owner is smooth and uneventful. As discussed in later paragraphs, the option of euthanasia may also be addressed at this time, either as an immediate option or as an eventual option if treatments fail.

Another very important owner-related factor is finances. In general, the treatment of a cat or dog with disseminated or metastatic malignancy is relatively expensive, as judged by the average clinician. However, it is the owner who should determine whether this treatment is indeed too costly. It is relatively common for an owner to spend $3,000 to $10,000 to treat a dog or cat with surgery, radiotherapy, or chemotherapy. In other words, all treatment options should be described and offered to the pet owner, regardless of their cost. Occasionally, owners spend what most people consider to be exorbitant amounts of money to treat their pet with cancer or other diseases.

TREATMENT-RELATED FACTORS

Several important treatment-related factors must be considered when planning cancer therapy. First, the specific indication should be considered. Surgery, radiotherapy, and hyperthermia are treatments aimed at eradicating a locally invasive tumor with a low metastatic potential (and potentially curing the patient), although they can be used palliatively in dogs or cats with extensive (bulky) disease or in those with metastatic disease. On the other hand, chemotherapy usually does not constitute a curative treatment, although palliation of advanced disease can easily be accomplished for several tumor types. Immunotherapy (the use of biologic response modifiers) also constitutes an adjuvant or palliative approach (i.e., tumors are not cured by immunotherapy alone). Recently, targeted molecular therapy aims at blocking specific pathways present in neoplastic but not in normal cells. In general, it is best to use an aggressive treatment when the tumor is first detected (because this is when the chances of eradicating every single tumor cell are the highest) rather than to wait until the tumor is in an advanced stage—that is, to “treat big when the disease is small.” Removing “only” 99% of the tumor cells will not lead to a cure.

In some cases, the highest success rates are obtained by combining two or more treatment modalities. For example, the combination of surgery and chemotherapy (with or without immunotherapy) has resulted in a significant prolongation of disease-free survival in dogs with osteosarcoma of the appendicular skeleton and in dogs with splenic hemangiosarcoma. Similarly, the combination of surgery and radiotherapy has resulted in a prolongation of disease-free survival in dogs and cats with spindle cell sarcomas.

The complications and adverse effects of different treatments also constitute treatment-related factors to be considered when planning therapy. Complications of chemotherapy are addressed in Chapter 78. As discussed later, the animal’s quality of life should be maintained (or improved) during cancer treatment. At our clinic, this is the priority in a cat or a dog with cancer receiving treatment. Our motto is “The patient should feel better with the treatment than with the disease.”

Cancer treatment can be either palliative or curative. Given the current paucity of information regarding specific tumor types and treatments, these two approaches sometimes overlap (i.e., a treatment initially thought to be palliative may result in cure, or vice versa). As discussed earlier, every effort should be made to eradicate every single cancer cell in the body (i.e., obtain a cure) shortly after diagnosis. This means taking immediate action rather than adopting a wait-and-see attitude. With very few exceptions, malignancies do not regress spontaneously. In other words, by delaying treatment in an animal with confirmed malignancy, the clinician is only increasing the probability that the tumor will disseminate locally or systemically, thereby decreasing the likelihood of a cure. As discussed earlier, surgery and radiotherapy are potentially curative treatments, whereas chemotherapy and immunotherapy are usually palliative.

If a cure cannot be obtained, the two main goals of treatment are to induce remission while achieving a good quality of life. The term remission refers to shrinkage of the tumor. When objectively evaluating the effects of therapy, the clinician should measure the tumor or tumors and assess the response using the criteria given in Box 76-2. The quality-of-life issue is quite important in small animal oncology (see preceding paragraphs). In a quality-of-life survey of owners whose pets had undergone chemotherapy for nonresectable or metastatic malignancy conducted in our clinic, more than 80% responded that the quality of life of their pets was maintained or improved during treatment. If a good quality of life cannot be maintained (i.e., the animal’s performance status deteriorates), the treatment being used should be modified or discontinued. We are currently conducting a prospective study evaluating quality of life and pain before, during, and after therapy in dogs and cats with cancer.

Palliative treatments are quite acceptable for small animals with cancer and to their owners. For example, even though chemotherapy rarely achieves a cure for most tumors, veterinarians can provide a cat or dog (and its owner) with a prolonged, good-quality survival. Although these patients ultimately die of tumor-related causes, the owners are usually pleased to have a pet that is asymptomatic for a long time. Another common example that is frequently forgotten is palliative surgery; for example, in dogs or cats with ulcerated mammary carcinomas and small pulmonary metastases, euthanasia was once recommended. However, it is now known that performing a mastectomy or lumpectomy (even if the owners decline chemotherapy) is likely to result in several months of good-quality survival, until the metastatic lesions finally cause respiratory compromise. In another example, dogs with apocrine gland adenocarcinoma of the anal sacs and metastatic sublumbar lymphadenopathy benefit from surgical resection of the primary tumor and/or metastatic nodes, even if adjuvant chemotherapy will not be considered. Removal of the primary mass improves clinical signs of straining in these patients; because the colon and rectum are compressed ventrally by the enlarged lymph nodes and laterally or dorsally by the primary mass, removal of one of the lesions easily alleviates clinical signs. Sublumbar (or iliac) lymphadenectomy and chemotherapy in dogs with metastatic apocrine gland adenocarcinoma of the anal sacs in our clinic result in survival times of 1 to 3 years.

Needless to say, the clinician should also address the presence of paraneoplastic syndromes even if specific antineoplastic therapy is not contemplated. For example, treatment of hypercalcemia of malignancy with bisphosphonates causes remarkable improvement in the quality of life of affected dogs. We have used either etidronate (Didronel, Procter and Gamble Pharmaceuticals, Cincinnati, Ohio, at a dosage of 10 to 20 mg/kg, administered orally q12h) or pamidronate (Aredia, Novartis Pharmaceuticals, East Hannover, N.J., at a dosage of 1 to 2 mg/kg, administered intravenously q6-8 weeks) in dogs with tumor-associated hypercalcemia in which the neoplastic disease could not be surgically removed or that had failed chemotherapy. In most dogs serum calcium concentrations were maintained within normal limits, and no appreciable toxicity was detected.

Finally, most cats and dogs with cancer are treated using a team approach. This team includes the pet, the owner, the medical oncologist, the oncologic nurse, the surgical oncologist, the radiotherapist, the clinical pathologist, and the pathologist. A smooth interaction among the members of the team results in marked benefits for the pet and its owner.