What is cancer?
A tumor or neoplasm is an abnormal growth of tissue in which cells proliferate more rapidly than in the tissue from which they came. These cells are not subject to the same control mechanisms that keep normal cells in check. Normal cells grown in a tissue culture medium will cease growing when they have touched each other and filled their container; this is called "contact inhibition" and explains why our liver grows to a certain size and doesn't keep growing until we burst. Tumor cells growing in cell culture dishes, however, don't display the same

Figure 1: Cartoon depicting the difficulty controlling cancer.

respect for boundaries. Instead, they grow relentlessly, tumbling over each other and spilling over the edges of their container. When we fully understand the reasons that these cells escape normal control mechanisms, we will begin to be able to devise ways to stop their growth (fig.1).

There is a difference between benign tumor cells and malignant tumor cells. Benign tumors are well-differentiated---in other words, they differ only slightly in appearance and behavior from their tissue of origin. These tumors are slow growing and noninvasive, do not spread throughout the body, and will often have a fibrous tissue capsule around them. All these characteristics make benign tumors easy to remove completely in most cases. Unless allowed to grow to a huge size, they are seldom a threat to life. However, they may cause significant problems if they are located in a vital organ, such as brain or intestine. In fact, the most significant alteration that a benign tumor can make in its host organism is that of spatial encroachment on surrounding normal tissue, leading to obstruction or replacement of the normal tissue by tumor. To identify a benign tumor, we use the suffix "-oma"; for example, a benign tumor of bone (osseous tissue) would be an osteoma, and a benign tumor of fibrous tissue would be a fibroma (fig.2).

Figure 2: Definitions for identifying benign tumors based on the tissue of origin.

The term "adenoma" is used to suggest a benign tumor of a glandular structure or organ, such as a mammary (breast) adenoma or a thyroid adenoma. The term "papilloma" or "polyp" is applied to wart-like projections from epithelial surfaces like skin or intestine. In some instances, though, the terminology used to name cancers can be misleading. For example, the commonly-used term "lymphoma" is used to name a malignant tumor of lymphoid tissue; there is no such thing as a benign lymphoid tumor. A better term for this type of cancer might be "lymphosarcoma," which implies malignancy. Similarly, the term "melanoma" may be used to describe either a benign or malignant proliferation of the pigment cells of the skin, the melanocytes. Usually, the adjective "malignant" is used with cancerous pigmented tumors, as in "malignant melanoma." Another term used to describe the malignant form of the pigmented tumor is "melanosarcoma." The use of the term "melanoma" can be confusing for owners, who sometimes are frightened by the report of a "melanoma" diagnosed in their pet, when it turns out that these pigmented skin tumors are often benign. In dogs, pigmented tumors found in the mouth or on one of the digits of the foot are generally malignant, however.

Figure 3: Oral melanosarcoma.

Malignant tumors often grow rapidly. The cells send spreading fingers into the normal tissue nearby, making it difficult to remove the tumor entirely. Thus, these tumors frequently regrow even after radical surgery. The cells show marked de-differentiation, or anaplasia; in other words, they do not look like the cells from which they originated. Generally, malignant tumors are referred to as cancers (fig.4). Cancers of epithelial or glandular structures are

Figure 4: Definition for identifying malignant tumors based on the tissue of origin.

referred to as "carcinomas," while cancers of other tissues are referred to as "sarcomas." The major danger of a malignant tumor is its ability to metastasize, or spread throughout the body. Vital organs are invaded, and death will ultimately occur. Carcinomas as a rule spread via the lymph vessels (fig.5), although some can "skip" the lymph nodes and go directly to blood vessels (hematogenous metastasis). An example of a carcinoma in the dog that can metastasize either by lymphatics or hematogenously is the mammary carcinoma. On

Figure 5: A dog with enlarged lymph node due to metastasis.

the other hand, sarcomas usually metastasize hematogenously instead of through the lymphatic channels. Organs that are frequent sites of blood-borne metastasis include the liver and lungs, both of which can be rapidly compromised by tumor nodules (fig. 6).

Most cancers are named for the type of cell or the organ in which they begin, i.e., breast cancer, pancreatic cancer, lung cancer, etc.

Figure 6: A radiograph of tumor nodules in lungs.

When cancer spreads, the new tumor nodules in other organs have the same kind of abnormal cells as the original tumor (known as the primary tumor) and therefore keep the same name. For example, if cancer of the lung spreads to the brain, the cancer cells in the brain are lung cancer cells despite the fact that they are now in a different location. The disease is called metastatic lung cancer, not brain cancer.

The medical science that concerns the diagnosis and treatment of cancer is called oncology, and those who practice it are called oncologists. There are medical oncologists, who make the diagnosis, stage the tumor, and prescribe a rational course of treatment, surgical oncologists, who perform the specialized surgical operations to remove the cancer, and radiation oncologists, who administer radiation therapy. Several of these specialists may be involved in the treatment plan prescribed for your pet.