Mammary gland tumors are common in the dog, and surgery to remove cancerous mammary glands, called a mastectomy, is a routinely done procedure in veterinary medicine. Male dogs almost never get mammary tumors; as well, female dogs that have been spayed before their first heat cycle rarely acquire breast cancer. Statistics demonstrate that if a female dog is spayed after her first heat cycle but before her second cycle, her potential to develop mammary tumors is slightly greater than the dog that was spayed prior to a heat cycle. If two cycles occur, then the spay procedure, an even higher incidence of breast cancer is demonstrated. And spaying after three heat cycles has no effect on diminishing the potential to develop mammary tumors. In short, the sooner a dog is spayed the less the chances for mammary tumors to develop in the future; but after three or four heat cycles, spaying has almost no effect on protection against tumor development. Keep in mind that spaying any dog at any time (as long as the patient is healthy) may be advisable to prevent a very serious uterine infection called PYOMETRA. Also see images of the SPAY (technically called an Ovariohysterectomy) surgery procedure.
Visual inspection and a physical exam probably will not be helpful in determining whether or not a growth is benign or malignant. A biopsy can be done on these growths to determine the cell types, which are highly variable in canine mammary tumors, and to establish the degree of malignancy. Malignancy means that they have the tendency to spread invasively into surrounding tissues and also to be spread by the lymph system to other parts of the body. Most veterinarians will suggest a chest X-ray prior to any mammary gland surgery to see if there is any evidence of metastasis (new tumors as a result of "seeding" from a distant, primary tumor). If there is evidence of tumor spread to other areas, the decision to do a mastectomy may not be advisable because metastasis of mammary gland tumor to the lungs or other body tissues almost always signals a very poor prognosis for recovery in the dog.
Chemotherapy for mammary gland cancer, and/or radiation treatment, can be done but the main effort of treatment is surgical excision of any suspicious mass.
The case shown below is of an eight year old dog that had been spayed and that had small masses in the right third, fourth and fifth mammary glands for a number of months. All of a sudden a rapidly growing tumor near the right fourth nipple attracted the owner's attention and surgery was performed. There was no evidence of metastasis in this patient and removal of the third, fourth and fifth mammary glands was done. Since there was no evidence of tumors in the first and second gland, those were left intact but careful vigilance will be needed to ensure that if any evidence of tumors show up, further surgery may need to be done.
It is a good idea to routinely (about once a month) check your dog for breast tumors. If anything unusual seems to be present, than have your veterinarian do a thorough physical exam. At this time a discussion regarding the pros and cons of surgery is important. Remember, it is always easier on the patient to have a small mass removed as contrasted with the removal of a large amount of tissue such as in this case.