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.

Mammary Gland Surgery In The Dog
This view is of the patient just prior to preoperative preparation of the surgical area for surgery.  The large mass between the fourth and fifth nipple is evident.	The surgeon, employing sterile technique, begins the incision near the masses in the glands, being careful not to incise any tumor tissue.	Hemostasis (controlling bleeding) can be challenging in these surgeries due to the mass of tissue to be removed as well as the vigorous blood supply to the area.
One side of the three mammary glands is dissected down to the abdominal wall; then the opposite side is dissected.	Care is employed to remove all the mammary gland tissue but also to leave enough normal tissue that closing the incision will not create excessive tension along the surgical closure.	Blood vessels are tied off and the row of three mammary glands is lifted away from the patient.  Some veterinarians will remove all five glands even if no evidence of tumor in other glands is present.
A major artery and vein to and from the area is the Caudal Superficial Epigastric vessels.  Lymph channels drain from the last three mammary glands through this small channel through the abdominal wall.	After removal of the mammary glands (along with the tumor tissue) the incision is closed in layers from deep to superficial.  The goal is to comfortably close the incision with no tension being placed on the skin.	In this case the skin is closed via two simple continuous sutures to decrease surgical and anesthetic time.  The patient is placed on pain control medication for a few days and sutures are removed in 12 days.