Of all domestic pets, the prevalence of mammary cancer in dogs far outweighs any other species. Female dogs are three times more likely to develop a tumor of the mammary gland then a human female. The incidence of malignancy is exceptionally high in canine mammary tumors. Close to 50 percent of the tumors diagnosed will be malignant cancers, according to the Merck Veterinary Manual.
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Factors
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The normal age of the onset for carcinogenic mammary tumors is between 10 to 11 years of age, according to the Ohio State University College of Veterinary Medicine. A strong indicator of the future development of a mammary malignancy would be if the dog was fed a high-fat diet prior to 1 year of age and if the dog has not been spayed. Overweight dogs also appear to be at an increased risk.
Spread
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Large malignant mammary tumors contain a high risk of spreading to other regions of the body. In dogs the first area of spread is normally the surrounding lymph nodes. Currently, 5 to 10 percent of mammary tumors will proceed to skeletal metastases. Early detection is paramount in successful treatment. Regularly checking the dog's mammary glands for hard lumps or any other abnormality can lessen the risk of spreading if the cancer is caught early.
Hormones
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There are appears to be a direct relationship between hormones and mammary tumors in dogs. The more heat cycles that a dog sustains before being spayed increases its risk. Both mammary tissue and benign tumors resulting in the breast area produce a combination of estrogen and progesterone receptors. However, it appears that less than 50 percent of cancerous mammary tumors produce the hormones, which indicates that at some point during the transition from being a benign tumor to a malignant tumor the tumor becomes less dependent upon hormones, according to the University of Georgia College of Veterinary Medicine.
Breeds and Sex
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A few breeds appear to have a genetic predisposition to developing mammary cancer. The poodle, spaniels and dachshunds have a higher incidence of malignant tumors. Less than 1 percent of all diagnosed malignancies will occur in male dogs. When the rare event does take place the male dog normally suffers from a hormonal imbalance and has an estrogen-secreting testicular tumor as well as a mammary tumor.
Treatment
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Currently, the standard treatment for malignant mammary tumors is complete resection of the mass and removal of any lymph nodes that show involvement. Chemotherapy after surgery is being used by many veterinarians, but the efficiency remains unknown. The accepted drugs are carboplatin, doxorubicin and gemcitabine. Prognosis weighs on the factors of tumor size, metastases and tumor type.
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