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Mammary Tumors in Dogs
Revised: May 17, 2023
Published: December 23, 2002

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 dogs is even higher than it is for humans: one unspayed dog in four is affected. This is a huge incidence, yet awareness among owners of female dogs is lacking.

Protection from Spaying

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 percent (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 and breeding an immature female dog is not recommended, this means you 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 preventing tumors. Just because a female dog is in the high-risk group doesn't mean it is too late to reap benefits from spaying.
    Tumors in both cranial abdominal mammary glands. It's best to detect and remove tumors before they get this large. Photo by MarVistaVet

Tumor Detection

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 nine to 11 years. You should begin conscientiously checking your female dog for growths starting at age six years. This includes both unspayed female dogs and female dogs spayed as adults.

Fine Needle Aspirate of a Mammary Carcinoma
Photo courtesy of the University of Georgia.

Preparing for Surgery: Radiography

Staging the tumor before planning removal is ideal and this means checking to see if it 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 it should be ruled out before 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.

A female dog has mammary glands in these locations though a given individual may not have them all. Graphic by MarVistaVet

Preparing for Surgery: Tissue Evaluation

There are many factors to consider when planning to remove a mammary tumor. For example, not all tumors in the mammary area are mammary tumors. Some are mast cell tumors or other types of skin tumors. Furthermore, approximately 50 percent of mammary tumors are benign in dogs and removal may not need to be aggressive in this situation. It would be nice to know going into surgery what the tumor type is and whether or not it is malignant. To accomplish this, tissue must be evaluated. An aspiration of cells will yield enough information to determine if the tumor is mammary and may indicate if it is benign or malignant. Unfortunately, there is frequently some ambiguity in the latter part of this evaluation making the evaluation of malignancy challenging. An actual biopsy sample is more accurate.

Frequently, the surgeon will opt to attempt an excisional biopsy first. This not only provides accurate identification of the tumor; it potentially completely removes the tumor as well, allowing for a cure in one surgical procedure and spares the expense of potentially less accurate pre-surgical tissue. Obviously, it is not always possible to remove the entire tumor on the first surgery in this way, but often it is.

Hormone Receptors

Approximately 50 percent of malignant mammary tumors in dogs have receptors for either estrogen or progesterone. Having these female hormones promotes the growth of these tumors. Benign tumors also have female hormone receptors and can also be stimulated by the 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 percent longer than those who remained unspayed.

Types of Tumors

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.

Mixed Mammary Tumor

What is mixed is the type of cell that makes up the tumor. (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, though by far, most mixed tumors are benign.


Adenocarcinomas can be tubular or papillary, depending on the gland cells from which the tumor arises. 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). After reading the description, the oncologist will be able to determine how aggressively to combat the tumor.

Inflammatory Carcinoma

The inflammatory carcinoma is 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 means that the other 50% are locally confined and surgery is curative.

What Else Determines Prognosis?

The type of tumor is obviously important in determining the prognosis; further, spaying at the time of tumor removal or before 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 tumor cells in a local lymph node or visible tumor cells within 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 indicate whether or not the tumor was completely removed (so-called clean or dirty margins). If the tumor was not completely removed, you may wish to consider a second surgery to remove more tissue.

Further Therapy?

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. Oncology is an area that not all veterinarians are comfortable performing. Discuss with your veterinarian whether referral to a specialist would be best for you and your pet.

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