How To Care For A Pet With Cancer

Oh, the “Big C”. It goes without saying that cancer is the last thing you, as a pet owner would like to think about. Although this is a scary and depressing topic, it is a real issue that we must face in the veterinary world as well as the human world. Although it’s not fun to talk about, it is necessary to understand the nature of the disease, the signs and symptoms, and finally the treatment.

*What causes cancer? Cancer is essentially the result of non-lethal genetic damage to cells (mutations in the DNA genome). Causes of such mutations include radiation, chemicals, hormones and infections. Some damage to cellular DNA is a daily “wear and tear” event but all mammals have many safeguards to prevent or repair such damage. Nonetheless, such protective mechanisms are not flawless. In some individuals there are even defects in such defenses, resulting in a higher than expected prevalence of cancer. Some of those defects in protection can be inherited and in purebred dogs this can result in inherited predispositions to cancer development. In other instances it is excessive injury. The mutated DNA upsets the normal regulation of cell growth allowing uncontrolled and careless growth of the “altered” cells that no longer obey the rules governing coordinated cell activity.

*How is cancer diagnosed? Cancer is often suspected from clinical signs (a lump, loss of appetite and energy, loss of weight for example). X-rays may be useful in detecting internal tumors, including metastases. Blood tests can help indicate some tumors. In order to identify most tumor types, it is necessary to obtain a sample of the tumor itself. Surgical invasion may be needed such as needle aspiration, punch biopsy, full excision or exploratory surgery. The simplest approach in many cases is the aspiration (suction removal) of tumor cells with a syringe and needle. It does not require general anesthesia or surgery. Microscopic examination of the cells obtained is called cytology. A more reliable test however, requires surgical removal of a tissue sample under anesthesia. The preparation and microscopic examination of stained sections from the removed tissue is called histopathology. This is done at a specialized laboratory where the slides are examined by a veterinary pathologist. The histopathology report typically includes words that indicate whether a tumor is ‘benign’ (non-spreading, local growth) or ‘malignant’ (capable of spreading to other body sites). Malignancy is often shown by tumor names ending in “carcinoma” or “sarcoma”. These, together with the origin or type of tumor, the grade (degree of resemblance to normal cells) and stage (how far it has spread) indicate how the cancer is likely to behave. The veterinary pathologist usually adds a prognosis (what will probably happen). This may include a prediction about the probability of local recurrence or metastasis (distant spread).

*Can cancer disappear without treatment? Cancer rarely disappears without treatment but as development is a multi-step process, it may stop at any stage. The body’s immune system can kill cancer cells using mechanisms that specifically target tumor cells that are recognized as “foreign”. These mechanisms include immune system cells such as cytotoxic lymphocytes and macrophages and antibody production. Not all tumors are recognized as foreign and even when they are, the immune system is rarely 100% effective in eliminating the cancer. Rarely, loss of blood supply to a cancer, by pressure on its own supply for example, will result in tumor cell death but the dead tissue will probably need surgical removal.

*How does cancer affect my pet? The most obvious effect of most cancers is an enlarging lump, but most lumps are not malignant. This lump may ulcerate, bleed or have other physical effects (pressure, displacement, etc...) on the surrounding tissues. Benign cancers only enlarge locally by smooth expansion but malignant cancers may invade regionally with tentacles (i.e “cancer the crab”). In fact, we use these differences in growth habit to help decide if the cancer is benign or malignant spreading widely through the body. Widespread distribution of a cancer occurs when cancer cells break away from the original tumor and seed in body cavities such as the pleural cavities in the chest or peritoneal cavity of the abdomen) and when cancer cells invade the blood. The cells may then be carried to distant tissues where the cells lodge and start new tumor masses (metastases).

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