It is important to remember that the best treatment for a particular tumor does not necessarily constitute the best treatment for a particuliar 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. 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 for 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 that a 9-year-old dog with chronic renal failure or decompensated congestive heart failure. Patient-related factors should be addressed before one institutes specific cancer treatment (e.g., correct the azetomia, improve the nutritional status with enteral feeding).

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The effects of anticancer drugs in dogs and cats on a neoplastic cell population follow first-order kinetic principles (i.e., the number of cells killed by a drug or drug combination is directly proportional to one variable-the dose used). These drugs kill a constant proportion of cells, rather than a constant number of cells. Therefore the efficacy of a drug or drug combination depends on the number of cells in a given tumor (e.g., a drug combination that kills 99% of a cell in a tumor containing 100,000,000 cells leaves 1,000,000 viable cells).

Different types of anticancer drugs kill tumor cells by different mechanisms. Drugs that kill only dividing tumor cells (i.e., that do not kill cells in the G0 phase) by acting on several phases of the cycle are termed cell cycle phase-nonspecific drugs. Alkylating agents belong to this group. Drugs that selectively kill tumor cells during a given phase of the cell cycle are termed cell cycle phase-specific drugs.

Most antimetabolites and plant alkaloids are phase-specific drugs. Finally, drugs that kill neoplastic cells regardless of their cycle status (i.e., they kill both dividing and resting cells) are termed cell cycle-nonspecific drugs. These latter drugs are extremely myelosuppressive (e.g., nitrosoureas) and are infrequently used in veterinary medicine.

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Chemotherapy in dogs and cats is primarily indicated for animals with systemic (e.g., lymphoma, leukemias) or metastatic neoplasms, although it can also be used for the management of nonresectable, chemoresponsive neoplasms that have historically proved refractory to radiotherapy or hyperthermia (primary chemotherapy). It can also be used as an adjuvant treatment after partial surgical debulking of a neoplasm (e.g., partial excision of an undifferentiated sarcoma) and is indicated for the control of micrometastatic disease after the surgical excision of a primary neoplasm (e.g., cisplatin, carboplatin or doxorubicin therapy after limb amputation in dogs with osteosarcoma; VAC after splenectomy for dogs with hemangiosarcoma).

Chemotherapy can also be administered intracavitarily in dogs and cats with malignant effusions or neoplastic involvement in the cavity/area in question (e.g., intrathecally administered cytosine arabinoside in dogs and cats with pleural carcinomatosis).
As a general rule, chemotherapy should not be used as a substitute for surgery, radiotherapy, or hyperthermia; nor should it be used in animals with severe underlying multiple-organ dysfunction because this increase the risk of systemic toxicity.