Most dogs with mammary tumors do not exhibit any systemic signs of illness when they are diagnosed. The tumors are confined to the areas along the mammary chain, and are easy to detect with a very simple clinical exam. Dogs do not have abundant breast tissues (unless they are nursing). Therefore, tumors if present, are easily palpable from the surface in the area around the nipple. In contrast to human breast cancer, even small nodules/tumors can be detected this way and do not require mammography. Dogs have 4-5 pairs of mammary glands so a total of 8-10 individual glands. Dogs that are at increased risk of mammary tumors because they have not been spayed or were spayed later in life should be monitored regularly for mammary nodules/tumors. Owners of high-risk dogs should learn how to do this themselves in between the routine annual veterinary visits. Early detection and early treatment are crucial for good outcomes in dogs with mammary tumors.
Mammary tumors are the most common tumors in older intact female dogs, or female dogs that are spayed later in life. Mammary tumors are hormone dependent tumors and removing the ovaries and the most important source of estrogen significantly decreases the risk of developing mammary tumors. Mammary tumors in dogs represent a diverse group of histological subtypes and varied clinical behavior, spanning from benign to highly aggressive tumors. Approximately 50% are benign and therefore not associated with any risk of metastasis, however, if left untreated, some of these benign tumors may transform over time and become malignant. Therefore, surgical removal of all mammary tumors is recommended. Surgery also provides the tissues needed to make an accurate histopathological diagnosis. Dogs often have more than one mammary tumor when they are diagnosed, and all of the individual tumors should be removed and submitted for histopathological evaluation. Carcinomas and various carcinoma subtypes represent the most common malignant group and also here there is significant diversity in biological behavior and outcome. Clinicians use information about tumor type, tumor grade, tumor size and stage when making recommendation whether additional systemic treatment is indicated, or if surgery +/- ovariohysterectomy (spay) is adequate. Dr Sorenmo’s and Dr Durham’s recent publication on bioscoring in canine mammary tumors (see literature list) provides a comprehensive system to assess the risk for metastasis and therefore the need for additional systemic therapy in dogs with mammary tumors.
We believe that a well-informed owner can be a more effective partner in his/her/their dog’s care. Therefore, we have provided a literature list for interested owners. Many of these publications are based in part of data collected through the PennVet Shelter Canine Mammary Tumor Program.
We have established a diverse and comprehensive team of clinicians and basic researchers with expertise spanning from clinicopathological aspects of mammary tumors to molecular biology focusing on both molecular changes within the tumors as well as the tumor micoenvironment. The value for translational research is clear and through our collaborations we have already shown that the dog is indeed a realistic and genuine model for breast cancer research and benefit both women and dogs with this disease.