Mammary Gland Tumors

Mammary gland (breast) tumors are the most common tumor found in dogs, and are three to seven times more likely to occur in an intact female than in a spayed female. This suggests that female steroids, such as estrogens produced by female reproductive organs, are involved in the development of these growths. Hence, spaying female dogs is a preventative measure for this type of tumor.

Although not all mammary tumors are cancerous, 50% of them are malignant. These cancers most often develop in the two most caudal pairs of the mammary glands. Half of the malignant mammary tumors are curable by surgery if detected early, without metastasis. The other 50% of cases either recur locally, or have already metastasized by the time of diagnosis, so surgical treatment is too little, too late. The prognosis for dogs with metastatic mammary cancer is poor.

Several factors determine the tumor's degree of malignancy, and the patient's prognosis. Larger tumors, evidence of metastases on radiographs, and histological evidence of metastasis (the invasion of blood vessels or lymphatics) worsen the prognosis. Metastasis or local recurrence of the cancer after surgery are the most common causes of death. Clinical signs that the tumor may have metastasized include dyspnea (shortness of breath), cough, lymphedema, lameness, hindquarter weakness, and emaciation.

Techniques for diagnosing mammary gland tumors include palpation of the mass, cytology of the cells contained within the mass, and radiographs to search for possible metastases. The most common locations for metastases are regional (local) lymph nodes and the lungs.

Treatment of Mammary Gland Tumors

The standard treatment for canine mammary tumors is surgical excision of the tumor. Radiation is rarely used at the tumor site because the mass can usually be completely removed by surgery. Chemotherapy is recommended if there is histological or radiographic evidence of metastasis. The average survival time of dogs with metastatic mammary cancer is two years after surgery, according to a study in the Journal of Small Animal Practice.

Antiangiogenic therapy may be useful in treating mammary cancer, and there are numerous scientific studies demonstrating that angiogenesis inhibitor drugs can slow or regress mammary tumor growth. The timing of intervention appears to be important, with treatment of early disease being more successful than treatment applied when the tumor burden is extensive.

Malignant mammary tumors in canines are similar to breast cancer in human patients. In human breast cancer, the degree of angiogenesis that has taken place within the cancer directly correlates to the likelihood for metastases. A similar finding has been shown in dogs. Breast, and mammary, tumors are thus regarded as highly angiogenic cancers, and are good candidates for antiangiogenic therapy.

A number of angiogenesis inhibitor drugs are in clinical testing to treat human breast cancer. Two drugs used to treat breast cancer - tamoxifen (Nolvadex) and trastuzumab (Herceptin) - possess angiogenesis inhibitory activity, and this likely contributes to their efficacy in human patients. In addition to its effects of estrogen receptors, tamoxifen prevents blood vessel cells from dividing. The Angiogenesis Foundation is studying tamoxifen as part of multi-targeting regimen for canine cancers. Trastuzumab is a monoclonal antibody directed against the HER2-neu gene associated with breast cancer. This gene helps tumors to produce the angiogenic growth factor VEGF (vascular endothelial growth factor). In animal studies, trastuzumab can inhibit angiogenesis and tumor growth.