According to Dog Owner's Home Veterinary Handbook, there are 10 major types of canine skin tumors. Not all of these types are cancerous, but all skin tumors on a dog need to be carefully watched in order to avoid complications. Contact a veterinarian immediately if a tumor is more than 1 inch in length, if it bursts open or grows rapidly.
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Breed-Specific Skin Cancer
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It's not clear why, but some dog breeds are more prone to getting skin tumors than others. Poodles are susceptible to wartlike papillomas, which are usually benign. Cocker spaniels are prone to melanomas (potentially deadly tumors of pigment cells on dark areas of the skin), basal cell tumors (firm, round and mostly benign tumors on the head and neck) and sebaceous adenomas (harmless tumors of the oil glands found on the skin). Scottish and Boston terriers are prone to melanomas. The rare, and often fatal, cancerous histiocytosis is seen most in flat-coated retrievers and Bernese mountain dogs. Histiocytic sarcoma can appear as lesions in the skin, lymph nodes, spleen, lung, and bone marrow.
Weight
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Being overweight can cause some dogs to develop lipomas, benign oblong tumors made of fat cells. Added weight can also contribute to other types of tumors, but the connection between weight and tumor growth is not as clear as in the case of lipomas. According to Holy Hash, D.V.M., overweight dogs may be more susceptible to other types of canine skin tumors. Also, added body fat produces more body oils and folds of fat that can encourage the growth of tumors or cysts of the oil glands.
Age
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The majority of dogs that develop skin tumors are older than 5 years of age, so it is thought that age plays a factor in skin tumor development, as it does in humans. It could be that older dogs become more easily overweight than younger dogs because they are not as active. Basal cell tumors, mast cell tumors (hairless lumps that easily split open and are found in the lower body) and papillomas are most often seen in older animals.
Sunlight
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Overexposure to sunlight is thought to be the main cause of squamous cell carcinomas in dogs. These tumors tend to appear on white or light colored parts of the body. They easily break open and can get infected. These hard, flat and gray skin tumors can metastasize (spread to other parts of the body), so they must be removed as quickly as possible. People with white or light-colored dogs need to limit their time in bright sunlight. Noses and paws can be protected with human sunscreen.
Perianal Tumors
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One type of canine skin cancer, a perianal tumor, seems to be caused by testosterone. It appears only around the rectums of unneutered male dogs. Although perianal tumors are not often cancerous, there is a chance they can become malignant. Neutering a dog is often the only logical course of treatment.
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