Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066




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We have all heard of breast cancer in women. With approximately one woman in eight or nine falling victim to this form of cancer, there are awareness campaigns from numerous health care agencies and research continues. What many pet owners do not know is that the incidence of mammary tumor development in the dog is higher yet with one in four unspayed female dogs affected. This is a huge incidence yet awareness among owners of female dogs is lacking.



A female puppy spayed before her first heat cycle can expect never to develop a mammary tumor of any kind. The incidence of tumor development in this group is nearly zero.

If she is allowed to experience one heat cycle before spaying, the incidence rises to 7% (still quite low).

If she is allowed to experience more than one heat cycle, the risk is driven up to one in four.

  • Since most female dogs come into heat the first time before age one year and breeding an immature female dog is not recommended, this means one must generally choose between a litter of puppies or mammary cancer prevention.
  • Because mammary tumors are promoted by female hormones, spaying at any age is helpful in tumor prevention. Just because a female dog is in the high risk group, that does not mean it is too late to reap benefit from spaying.

Canine Spay Surgery
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canine mammary glands
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If your dog is unspayed, was known to have had puppies, or was spayed in adulthood, she fits into the high risk group for mammary cancer development. It is important to be somewhat familiar with the normal mammary anatomy of the female dog. There are five sets of mammary glands as shown though the average female dog has only nine. (It is not unusual for asymmetry of mammary glands to be found). The normal glands should be soft and pliant, especially towards the rear legs. There should be no firm lumps. If a lump is detected, see your veterinarian at once regarding possible removal. Most tumors occur in the glands nearest the rear legs.

Mammary tumors are most commonly diagnosed in dogs ages 9 - 11 years.
You should begin conscientiously checking your female dog for growths starting at age 6 years.
This includes both unspayed female dogs and female dogs spayed as adults.


The good news, if there is some, is that approximately 50% of the tumors formed by female dogs are benign. Since one cannot tell by looking at a tumor which it is, the tumor must be removed or a part of the tumor must be sampled for biopsy. The laboratory can determine whether the tumor is benign or malignant based on the cells and their architecture within the tissue. Alternatively, a “needle aspirate” can be performed where a syringe is used to withdraw some cells from the growth and the lab can determine whether the tumor is benign or malignant with enough accuracy to determine how aggressive the surgical approach should be. Needle aspirate may be a helpful pre-operative procedure in many cases but it should be understood that biopsy is ultimately what is necessary to determine the extent of disease. There is enough ambiguity in needle aspirates that they are frequently skipped.

Mammary Tumor Fine Needle Aspirate

Fine needle aspirate of a mammary carcinoma
(courtesy of the University of Georgia)



Staging the tumor before planning removal is ideal and this means checking to see if the tumor has spread to the lungs. If the tumor has already spread, removing the original ("primary") tumor in the mammary gland is likely not going to be helpful, unless the tumor is eroded or open and needs removal for quality of life reasons. Further, cancer in the chest is an important anesthetic risk factor and its presence should be ruled out prior to surgery. If there is cancer in the chest, therapy must be directed towards the whole dog rather than towards the localized tumor.

To see the most detail in the lungs, most oncologists recommend three views of the chest (profile with left side down, profile with right side down, and either top to bottom or bottom to top). These views allow maximum inflation of all parts of the lung so that even small tumors can be seen. After surgery for a malignant mammary tumor, similar radiographs are periodically recommended to screen for evidence of tumor spread in case some cells had managed to escape surgical removal. In general, if no spread has been detected after two years, the tumor can be considered permanently cured.

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Approximately 50% of malignant mammary tumors in the dog have receptors for either estrogen or progesterone. This means that the presence of these female hormones promotes the growth of these tumors. Benign tumors also have female hormone receptors and can also be stimulated by hormonal cycling of the female dog. This means that spaying is important even if a tumor has already developed; in one study, female dogs spayed at the time of their tumor removal (or in the two years prior to the tumor removal) lived 45% longer than those who remained unspayed.



The following are common classes of mammary tumors that might be found on a biopsy:


A benign glandular tumor for which no treatment is necessary.


What is mixed is the type of cell that makes up the tumor: the epithelial cells that line the glandular tissue and the mesenchymal cells that make up the non-glandular portion. (“Mixed” does not refer to a mix of benign and malignant cells.) The mixed tumor can be either benign or malignant and the biopsy will indicate this.


Adenocarcinomas can be “tubular” or “papillary” depending on that gland cells the tumor arises from. Adenocarcinomas behave malignantly but how aggressively malignant they are depends not on whether they are tubular or papillary but on other cellular characteristics described by the pathologist (such as how quickly the cells appear to be dividing and how closely they resemble normal gland cells). When the oncologist reads the description he or she will be able to determine how aggressively to combat the tumor.


A highly malignant tumor that generates tremendous inflammation locally with ulceration, pus, and discomfort. This type of tumor tends to spread early in its course and is virtually impossible to treat. Because many less malignant and more common mammary tumors also become ulcerated, this particularly malignant tumor may not be diagnosed before surgery and frequently is unable to heal from the very surgery needed to identify it. Fortunately, this especially tragic tumor type accounts for less than 5% of mammary tumors.

In general: approximately 50% of malignant mammary tumors will have already spread by the time of surgery.

This, of course, means that the other 50% are locally confined and surgery is curative.



The type of tumor is obviously very important in determining the prognosis; further, spaying at the time for tumor removal or prior is also an important factor in determining prognosis. Other factors include:

  • The size of the tumor. Tumors with diameters larger than 1.5 inches have a worse prognosis than smaller tumors.
  • Evidence of spread to the lymphatic system (such as the presence of tumor cells in a local lymph node or visible tumor cells with in lymphatic vessels on the biopsy) carries a worse prognosis.
  • Deeper tumors or tumor adherence to deeper tissue structures carries a worse prognosis.
  • An ulcerated tumor surface carries a worse prognosis.
  • A history of especially rapid growth carries a worse prognosis.

The biopsy sample will not only identify the tumor type, it will also indicated whether or not the tumor
was completely removed (so called “clean” or “dirty” margins).

If the tumor was not completely removed, one may wish to consider a second surgery to remove more tissue.



Both radiation therapy, chemotherapy and anti-estrogen therapy have been used for incompletely removed tumors. Sometimes it is most appropriate to monitor for recurrence with periodic chest radiographs.

Specialized care is often required for cancer patients. To find a cancer specialist, please visit:

Page last updated: 5/21/2017