Cancer is one of the leading causes of death in companion animals. It has been estimated that almost half of the animals that live 10 years or longer will die of cancer. The incidence of cancer appears to be increasing in the canine population at a steady rate. Explanations for this increase include a larger population of geriatric pets (advances in health care mean that fewer dogs die of other diseases at a younger age), improved recognition and diagnosis of cancer, increased willingness on the part of both veterinarians and pet owners to treat cancer, as well as both genetic and environmental changes.
One of the questions most commonly asked by pet owners is "What caused my pet's cancer?" The standard answer to that question is "We don't know". The cause of most cancers diagnosed in companion animals (as well as humans) is often poorly understood. We have been successful in finding either a cause or a risk factor in only a few cancers. The small numbers of veterinary patients, incomplete or unknown medical histories and lack of follow-up often hinder our efforts.
The symptoms of cancer in animals are usually the same as in people. Early detection of cancer is just as important in our patients. Successful treatment hinges our ability to "catch it
early". Some of the most common signs of cancer (early warning signs) are listed below.
- An abnormal swelling that persists or continues to grow.
- Sores that do not heal
- Unexplained weight loss
- Loss of appetite
- Bleeding or discharge from any body opening such as the nose or mouth
- Bad odor, especially from the mouth
- Difficulty eating or swallowing
- Reluctance to exercise or loss of stamina
- Persistent lameness or stiffness
- Difficulty breathing, urinating or defecating
- Change in behavior
The terms used to describe cancer can be confusing. The same word can mean different things to different people. The following section is a glossary that attempts to clarify some of the more commonly used terms.
- Cancer: "uncontrolled growth of abnormal cells"; synonym:
- Tumor: literally, it means "a swelling"; an alternative
definition is "a new growth of tissue in which the multiplication of cells is uncontrolled and progressive"; synonym: neoplasm
- Oncology/Oncologist: oncology is the field that deals with the
diagnosis and treatment of cancer while an oncologist is a doctor that specializes in this field (veterinarians can become board-certified in either medical oncology or radiation oncology by completing a residency and passing several certifying exams)
- Malignant: "having the properties of anaplasia, invasion and
metastasis" (these are all characteristics of a tumor that make it capable of causing the death of a patient); a malignant tumor is one which is no longer resembles the cells it was derived from, is
invasive at the site where it starts and has the ability to metastasize (or spread) to other organs
- Benign: opposite of malignant; benign tumors are unlikely to
spread, unlikely to cause the death of a patient and have a favorable outcome (however, there are a few exceptions where benign tumors behave more like malignant ones)
- Primary site: this refers to the site where the tumor started;
"the original tumor"
- Metastasis: "the transfer of disease from one organ or part to
another not directly connected to it"; metastasis is the process by which the tumor spreads from one location to another; the most common sites of metastasis are the lungs or lymph nodes; the most common routes of metastasis are the bloodstream and the lymphatics
- Metastatic lesion: the term used for the site of spread
- Biologic behavior: this refers to how a tumor is likely to behave (for example, how likely it is to metastasize, where are the common sites of metastasis and how invasive is it at the site where it starts)
- Staging: this term refers to the process of determining how advanced a cancer is and if it has spread; we "stage" an animal through the use of tests such as x-rays, ultrasound, blood work, lymph nodes aspirates and bone marrow aspirates; this is helpful
because it allows us to determine the best treatment options and to predict the outcome of treatment;
- Aspiration: "the removal of fluid or gases from a cavity by
suction"; when we "aspirate" something, we place a needle (which is attached to a syringe) into a tissue and draw back on the syringe; cells are thus dislodged from the tissue into the needle and syringe; we can then spread these cells onto a slide so that they can be stained and examined under the microscope
- Cytology: this refers to the microscopic examination of cells
that have been removed from the body (either by aspiration or by other techniques)
- Biopsy: "the removal and examination, usually microscopic, of
tissue from the living body, performed to establish a precise diagnosis"; when we "biopsy" something, it means that we remove a piece of tissue to look at under the microscope (we also use the term to refer to the sample we have obtained); a biopsy is crucial in making a diagnosis of cancer
- Carcinoma: "a malignant growth made of epithelial cells tending
to infiltrate the surrounding tissues and give rise to metastasis";
epithelial cells "line" body surfaces
- Adenoma: "a benign epithelial tumor"
- Sarcoma: "a tumor made up of a substance like the embryonic
connective tissue"; sarcomas are malignant tumors of connective tissue origin (such as cartilage, bone and muscle)
- Anaplastic/Undifferentiated: "a loss of differentiation of cells and of their orientation to one another"; tumors that are anaplastic are usually considered very malignant and aggressive
- Margins: this is a term used to refer to the edges of the
surgical specimen; "clean margins" means that no tumor cells are visible at the edges (sometimes tumors come back despite clean margins); "dirty margins" means that tumor cells are visible at the
edges (therefore, tumor cells have been left behind)
- Grading: this term refers to the evaluation of microscopic
features of a tumor that allows the pathologist to assign the tumor "a grade"; the grade of a tumor is important because it allows us to predict how aggressive a tumor will be
- Prognosis: "a forecast as to the probable outcome of an attack of disease"; this refers to how we think a patient will do
- Protocol: we usually use this term to refer to the specific chemotherapy plan that is used (for example, the "Weekly Sequential protocol" is what we commonly use for treating lymphosarcoma); it can also refer to the overall treatment plan
- Cycle: chemotherapy drugs are often given in the same order on the same schedule repeatedly; the term "cycle" refers to the basic plan that gets repeated over and over again; the cycle is different for each chemotherapy protocol