When cancer has been diagnosed in an animal, the clinician must try to give the pet owner a prognosis. The owner will want to know how long the animal has to live and whether treatment is likely to cure him. In human oncology, survival statistics for each stage of cancer are available and are updated frequently as new treatments become available; the National Cancer Institute and the National Institutes of Health are agencies funded by the federal government to accumulate this data for people. Because funding for the study of cancer in animals is so limited, we do not have this data for many tumors in animals. So how can the veterinary oncologist give the anxious owner the best estimate of a prognosis? First, the patient with cancer will frequently be middle-aged or older, and other diseases may be present besides cancer. The pet may have heart disease, kidney disease, or liver disease. Therefore, one of the first objectives in deciding whether to treat an animal with a diagnosed cancer will be to identify all the problems. If an animal has decompensated kidney or heart disease, it is possible that it may not live through a major surgical procedure (fig.1).
Figure 1: A critically ill patient in the intensive care unit.
Even small problems can become major once treatment has begun. For example, a cat may be infected with the feline immunodeficiency virus, and may be showing no clinical signs; when chemotherapy further depresses his immune system, however, severe bacterial or fungal infections may become a problem. A dog may have a subclinical bacterial infection of the kidney (pyelonephritis) with no apparent signs; when cancer treatment lowers his neutrophil (white blood cells that protect against infection) count, the infection may become life-threatening. So before initiating treatment, we take care of the problems we can cure. We also assess the patient's other problems in light of how they may affect his lifespan and the proposed cancer treatment.Second, we must know the behavior of the particular tumor that the animal has. Obviously, histopathology should be done on all excised tumors. Once the diagnosis has been made, either by cytology or histopathology, the behavior of the particular tumor can be obtained from current literature or textbooks. These sources will discuss the pattern of spread of the particular tumor. Some tumors spread quickly to other organs, while others spread very slowly or not at all. Some spread via the lymphatics, tiny microscopic channels that drain to lymph nodes and can carry cellular material from node to node. Other tumors spread hematogenously, (see matastasis) or via the blood stream; tumor cells enter capillaries and can quickly be taken to distant locations. It is also possible for tumors to spread both hematogenously and by lymphatics at the same time.
Tumors may manufacture hormones or other substances that circulate through the blood and produce effects at other locations throughout the body. A common tumor of the skin of dogs and cats, mast cell sarcoma, contains histamine within the cancer cells. When excessive amounts of histamine are released from the tumor, the animal may show itching or even have an anaphylactic reaction. Some tumors make a hormone that elevates the animal's blood calcium, leading to weakness of muscles and kidney failure (fig.2) Figure 2: Lymphoma patient with enlarged mandibular lymph nodes and concurrent elevated blood calcium level.
. If a particular tumor is known to make one of these substances, its presence can be checked for as part of the workup.Finally, if the tumor is malignant, the extent of disease must be determined; this is called staging. In most sarcomas, thoracic radiographs are necessary since the lung is a frequent site of blood-borne metastasis. An abdominal ultrasound scan may be performed to check internal organs for evidence of tumor spread. In carcinomas, lymph nodes, lungs, and other internal organs should be evaluated closely, since these sites may already be involved with metastatic cancer at the time of presentation to the veterinarian. Some tumors have an uncharacteristic pattern of metastasis; for example, mast cell sarcomas do not generally show blood-borne metastasis until the cancer is widely disseminated. Early in the course of disease, metastasis of mast cell tumors occurs via regional lymph nodes. In tumors of the oral and nasal cavity, skull radiographs are necessary, since prognosis is not as good if there is bone destruction and invasion (fig.3) Figure 3: A radiograph showing marked bone destruction due to a malignancy in the lower jaw of a dog.
. If a lymph node is nearby, it should be aspirated or biopsied in an attempt to determine whether it is involved with tumor. Even if no evidence of cancer is found in the staging workup, it must be emphasized that only small, microscopic numbers of tumor cells may be present in nodes or organs, and these may be missed by any or all of the methods outlined here.Thus, to summarize, the steps involved in the clinical evaluation of an animal with a tumor are as follows:
1.Diagnose the tumor.
2.Become familiar with the particular tumor's behavior.
3.Determine the stage of disease; is there lymph node, bone, lung, or internal organ involvement?
4.Determine the other medical problems the animal has which might make it impractical or dangerous to treat the cancer.
Such a logical approach to the workup of the pet with a tumor will give the best chance that appropriate, informed decisions will be made about future treatment.
