C. Guillermo Couto, DVM, Dip. ACVIM
Professor and Head, Clinical Oncology/Hematology Service
Veterinary Teaching Hospital
The Ohio State University
Columbus, OH 43210

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Over the past several decades several treatment modalities have been used in dogs with cancer. However, until two or three decades ago, surgery remained the mainstay of treatment for pets with cancer. Nowadays, non-resectable (i.e.; masses that cannot be taken out surgically) or metastatic (i.e.; tumors that have spread) malignancies can be treated with varied degrees of success using one or more of the modalities described below. Some useful definitions are included in Table 1.

Surgery is used both as a diagnostic (i.e.; biopsy) and treatment tool in dogs with cancer. In order to completely eliminate every single tumor cell, the surgical excision needs to be wide (3 to 5 cm margins around and below the tumor). Radiotherapy consists of delivering radiation to the neoplastic tissues, either by using a beam (e.g.; cobalt therapy) or by placing radioactive implants in and around the mass; it is effective in patients with mast cell tumors, several carcinomas, and a few sarcomas. Chemotherapy consists of delivering drugs to the tumor, either by administering systemically or locally. Even though chemotherapy has a high prevalence of adverse effects in people, most dogs and cats receiving this treatment have excellent quality of life. Immunotherapy (use of biologic response modifiers) consists of manipulating the patient’s immune system specifically or nonspecifically to assist in tumor cell kill; although there are some commercially available immunomodulators, none of them have yet proven to be effective when evaluated in independent studies. Hyperthermia consists of applying heat (usually 42C) to the tumor or the patient by a variety of methods; by itself, it is effective in small superficial tumors, and in combination with radiotherapy it is beneficial in dogs with fibrosarcomas and hemangiopericytomas. Cryosurgery is based on locally freezing the tumor cells; the release of tumor-related antigens may stimulate a local immune response against the tumor. Cryosurgery has been used successfully in dogs and cats with small superficial neoplasms. In phototherapy, the patient receives a photosensitizing drug that is rapidly eliminated by most normal cells, but that is retained by the tumor cells. Upon exposure of the tumor to a laser beam, the excitation of the photosensitizing molecules results in release of energy and cell death. However, this technique is only effective in tumors of less than 0.8 to 1cm in diameter/depth. Thermochemotherapy consists of generating high temperatures in and around the tumor, and delivering chemotherapy locally or systemically; the high local temperature may result in differential uptake of the chemotherapeutic agent/s by the tumor cells.

When evaluating a dog with a malignant tumor, most owners will elect to treat their pets. Although euthanasia still remains a reasonable alternative for cancer treatment in small animals, every effort should be made to investigate other treatment options, particularly since so many dogs with malignant tumors enjoy excellent quality of life for extended periods.

Cancer treatment can be either palliative or curative. The palliative treatment is aimed at alleviating the clinical signs, whereas cure refers to the complete disappearance of the tumor. These two approaches sometimes overlap (i.e.; a treatment initially thought to be palliative may result in cure or vice-versa). Every effort should be made to eradicate every single cancer cell in the body (i.e.; obtain a cure) upon diagnosis. This means immediate action, rather than a “wait-and-see” attitude. With very few exceptions, malignancies do not regress spontaneously. Therefore, by delaying treatment in a patient with highly suspected or confirmed malignancy, we are, at least theoretically, enhancing the probability of the tumor disseminating locally or systemically (and therefore decreasing the probability of obtaining a cure). Surgery, radiotherapy, and hyperthermia are potentially curative treatments; while chemotherapy and immunotherapy are usually palliative.

As discussed in the previous paragraph, once a diagnosis of cancer is made, the first decision the veterinarian must face is whether the tumor on his/her patient is potentially curable. If so, every effort should be made early on to completely eliminate the very last tumor cell in the patient. Most local and some regional tumors can be cured with appropriate treatment (e.g.; aggressive surgery, radiotherapy, or combinations of both). Examples of these tumors include fibrosarcomas, hemangiopericytomas, and small mammary carcinomas, among others.

If a patient has a disseminated (e.g.; lymphoma) or a metastatic tumor (e.g.; osteosarcoma, hemangiosarcoma), the likelihood of achieving a cure is extremely low (< 10%). However, in most of these patients palliative treatments result in considerably long disease-free survival times (e.g.; 12-18 months for dogs with lymphoma or osteosarcoma treated with chemotherapy or surgery/chemotherapy, respectively). Certain tumors, such as transmissible venereal tumors, are likely to be cured with chemotherapy.

If the clinician judges that the patient cannot be cured, a decision has to be made as to whether to use palliative treatment or not. This decision should be made primarily considering the quality of life of the patient with and without treatment. At this point in time, euthanasia should be discussed by the owner and veterinarian as a potential alternative.

Depending upon the tumor type, biologic behavior, and clinical staging, a clinician will recommend one or more of the treatment options previously discussed. However, it should be kept in mind that in addition to the tumor-related factors (e.g.; tumor type, degree of dissemination, response to specific treatments), there are a multitude of other factors which influence the selection of the optimal treatment approach to a cancer patient. These include patient-related, owner-related, and treatment-related factors.

It is important to remember that the best treatment for a particular tumor type does not necessarily constitute the best treatment for a particular patient or owner. The most important patient-relatedfactor to be considered is her/his general health and activity or performance status. For example, a dog with markedly diminished activity and severe clinical signs (i.e.; poor performance status) may not constitute a good candidate for aggressive chemotherapy for the repeated anesthetics needed for external beam radiotherapy. Patient-related factors should be addressed prior to instituting specific cancer treatment (e.g.; correct kidney or liver problems; improve the nutritional status with enteral hyperalimentation, etc.).

Owner-related factors play an important role in small animal oncology. Every clinician is aware of the significance of the owner-pet bond. This bond is so important, that it oftentimes dictates the treatment approach to be used in a given pet. For example, an owner may be so apprehensive about chemotherapy that he/she refuses to treat his/her beloved pet with lymphoma using such a modality; hence, the optimal treatment cannot be used.

It is my experience that owners should be made part of the medical team that treats their pet. If the owners are assigned tasks to perform at home, such as measuring the tumor/s mass/es to monitor response to treatment, take their pet’s temperature daily, and monitor their performance status, they feel responsible for the fate of their pet, and are therefore quite involved. On the other hand, the clinician should always be available to answer the concerned pet owner’s questions and guide them through difficult times.

All potential treatment options should be initially discussed with the owner, emphasizing the pros and cons of different modalities (e.g.; beneficial effects and potential for adverse effects of treatment A versus B versus C versus no treatment); the owner should have a clear understanding of what will (or should) happen during their pet’s treatment (including a thorough discussion of the potential complications). By following these easy steps, the owner’s expectations are usually realistic, and the interaction with the clinician smooth and uneventful. As discussed below, I usually address the option of euthanasia at this time (either as an immediate option or as a potential if treatments fail).

Even though the clinician should attempt to institute treatment as soon as possible, he/she should emphasize to the client that, in general, an immediate decision is not necessary. It is my belief that pressuring the client to make a decision immediately is counterproductive. First, there are very few clinical situations in which a decision needs to be made immediately (e.g. a dog with a ruptured hemangiosarcoma of the spleen that requires immediate surgical intervention); in most instances, the client can wait a few hours, or even days before deciding whether to treat their pet or not. Second, the decision of whether to treat their pet or not is quite difficult, and the owners have to consider scientific facts, as well as finances (see next paragraph), time and emotional commitment, etc. Hence, an approach that I find quite useful is to tell the clients that they do not need to make an immediate decision, since I know that they need some time to think it over. However, I emphasize the fact that they will need to make that decision within the next two or three days.

Another owner-related factor that is quite important is finances. In general, the treatment of a cat or dog with disseminated or metastic malignancy is “expensive”, as judged by the average clinician. It is common for a client to spend between $1,000 and $3,000 to treat a tumor-bearing dog with surgery, radiotherapy, and/or chemotherapy. However, it is the owners who should determine if this treatment is indeed “expensive”. In other words, all treatment options should be presented to the client, regardless of their cost. Quite frequently owners will spend what most people consider to be exorbitant amounts of money to treat their beloved dog.

Several treatment-related factors are also important. First, the specific indication of different treatment modalities should be considered. Surgery, radiotherapy, and hyperthermia are treatment modalities aimed at eradicating (and potentially curing) a locally invasive tumor with low metastatic potential, although they can be used palliatively in patients with extensive disease or in those with metastases. On the other hand, chemotherapy does not usually constitute a curative approach. Immunotherapy also constitutes an adjuvant or palliative approach (i.e.; tumors are rarely cured by immunotherapy alone). In general, it is best to use an aggressive treatment when the tumor is first detected (since this is when the chances of eradicating every single tumor cell are the highest) rather than wait until the tumor is in an advanced stage.

Nowadays, 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 significant prolongation of survival with excellent quality of life in dogs with osteosarcoma of the appendicular skeleton, and in dogs with splenic hemangiosarcoma. Similarly, the combination of radiotherapy and hyperthermia has resulted in prolongation of survival or cure in dogs with fibrosarcomas or hemangiopericytomas.

The complications and adverse effects of different treatment modalities also constitute treatment-related factors to be considered when planning therapy. Table 2 lists potential complications from different treatment modalities. As discussed below, quality of life should be maintained (or improved) during cancer treatment. In our clinic, quality of life is the first priority when treating a dog with cancer.

In small animal oncology, the two main goals of palliative cancer treatment are induction of remission and good quality of life. The term remission refers to a decrease in the tumor mass (i.e.; cytoreduction). When objectively evaluating effects of therapy, the tumor/s should be measured, and the response assessed. If a good quality of life cannot be maintained, the treatment should be modified or discontinued.

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. Smooth interaction among the members of the team results in marked benefits for the affected pet and his/her owner/s. In brief, even though cancer still remains a devastating disease, multiple treatment options that result in cures or in marked prolongation of disease-free survival with excellent quality of life are available for dog owners.

TABLES

Table 1: Definition of commonly used terms.

• Cancer: malignant neoplasm. Malignant behavior determined by the tumor’s ability to infiltrate surrounding tissues, reoccur after surgery or radiotherapy, and/or disseminate to distant sites (ie; metastases). Synonyms: malignancy, malignant neoplasm.
• Soft tissue tumors: those affecting solid organs. Generically used to refer to carcinoma and sarcomas. Does not include tumors of the blood or blood-forming organs, such as lymphoma.
• Carcinomas: malignant neoplasms of epithelial (i.e.; lining tissue) origin. When affecting glandular epithelium, the prefix adeno- is used Most carcinomas are treated with surgery plus or minus radio- and/or chemotherapy; radiotherapy is effective in some.
• Sarcomas: malignant neoplasms of mesenchymal (i.e.; supportive tissue) origin. Include collagen-producing neoplasms (i.e.; fibrosarcomas), bone-producing neoplasma (i.e.; osteosarcomas), etc. Most sarcomas are treated with surgery plus or minus radio- and/or chemotherapy.
• Lymphomas: malignant tumors of lymphocytes (a type of white blood cell) affecting solid organs (e.g.; lymph nodes, liver, spleen, etc.). Synonyms: malignant lymphoma, lymphosarcoma, LSA. Dogs with lymphoma are treated primarily with chemotherapy.

Table 2: Potential complication and adverse effects of cancer treatments.

Modified from: Couto CG: Principles of cancer treatment. In Nelson RW and Couto CG: Essentials of Small Animal Internal Medicine. St. Louis, Mosby-Yearbook, 1992, p. 838-841