Skin cancer encompasses a broad category of tumors that includes any uncontrolled growth of cells of the skin or associated structures such as glands, hair follicles and supportive tissues (fat and connective tissue). Metastasis to the skin from tumors elsewhere in the body can occur but these are not considered skin tumors because they did not start in the skin.
The skin is the most common site of cancer in the dog and the second most common site of cancer in the cat. Dogs have up to six times the number of skin tumors as cats; however, skin tumors in dogs are more likely to be benign than those in cats. Skin cancer frequently occurs in dogs that are middle-aged to older (six to 14 years of age), although there are several types of skin cancers that occur in young animals.
Breeds of dogs that have been found to have a higher incidence of skin cancer include
boxers, Scottish terriers, bull mastiffs,
Basset hounds, Weimaraners, Kerry blue terriers and
Norwegian elkhounds.
The cause of most skin cancers is unknown.
Exposure to the sun has been shown to cause a higher incidence of two types of skin cancer: squamous cell carcinoma and hemangioma. Light-colored dogs with thin haircoats that spend time in the sun have a higher risk of developing either squamous cell carcinoma or hemangioma. There may be a genetic basis for the development of certain cancers.
Skin cancers can be divided into four categories, depending upon the cell type that is involved. These categories are epithelial tumors, mesenchymal tumors, round cell tumors and
melanomas.
Epithelial tumors. These tumors involve the skin itself, glands in the skin or the hair follicles.
Mesenchymal tumors. These tumors are derived from cells that surround or support the skin such as fat, connective tissue, blood vessels and nerves.
Round cell tumors. These tumors are named because of their appearance under the microscope and include lymphosarcoma, mast cell tumors, histiocytomas, plasma cell tumors and transmissable venereal tumors.
melanomas. These tumors are derived from melanocytes – cells that provide pigment in the skin.
In dogs, the most common tumors are lipomas, mast cell tumors, sebaceous gland adenomas/hyperplasia and papillomas. Most skin cancers appear as a lump in or underneath the skin or as a sore that does not heal. Animals with skin cancer may experience discomfort or itchiness because of the cancer. This may cause them to chew or scratch at the affected area. With certain types of cancer, there may also be redness to the skin or it may have a flaky appearance.
