• Abnormal swellings that persist or continue to grow
  • Sores that do not heal
  • Weight loss
  • Loss of appetite
  • Bleeding or discharge from any body opening
  • Offensive odour
  • Difficulty eating or swallowing
  • Hesitation to exercise or loss of stamina
  • Persistent lameness or stiffness
  • Difficulty breathing, urinating, or defecating 



Mammary tumors are the most common tumors in female dogs who have not been spayed. Mammary tumors can be small, simple nodules or large, aggressive, metastatic growths. With early detection and prompt treatment, even some of the more serious tumors can be successfully treated. Cats also suffer from mammary tumors and they have their own unique set of problems that are discussed in a separate article. 

Which dogs are at risk for developing mammary tumors?

Mammary tumors are more common in unspayed, middle-aged female dogs (those between 5 and 10 years of age), although they can, on rare occasions, be found in dogs as young as 2 years. These tumors are rare in dogs that were spayed under 2 years of age. Occasionally, mammary tumors will develop in male dogs and these are usually very aggressive and have a poor prognosis.

The risk of breast cancer is almost eliminated in dogs that are spayed before their first heat. 
Spaying greatly reduces the chances of a female dog developing this condition. In those females spayed prior to their first heat cycle, breast cancer is very, very rare. The risk of malignant mammary tumors in dogs spayed prior to their first heat is 0.05%. It is 8% for dog spayed after one heat, and 26% in dogs spayed after their second heat.It is believed that the elimination or reduction of certain hormonal factors causes the lowering of incidence of the disease in dogs that have been spayed. These factors would probably be estrogen, progesterone, a similar hormone or possibly a combination of two or more of these.

What are the types of mammary tumors in dogs?

There are multiple types of mammary tumors in dogs. Approximately one-half of all mammary tumors in dogs are benign, and half are malignant. All mammary tumors should be identified through a biopsy and histopathology (microscopic examination of the tissue) to help in the treatment of that particular type of tumor.

The most common benign form of canine mammary tumors is actually a mixture of several different types of cells. For a single tumor to possess more than one kind of cancerous cell is actually rare in many species. This combination cancer in the dog is called a 'benign mixed mammary tumor' and contains glandular and connective tissue. Other benign tumors include complex adenomas, fibroadenomas, duct papillomas, and simple adenomas.

The malignant mammary tumors include:

  • tubular adenocarcinomas

  • papillary adenocarcinomas

  • papillary cystic adenocarcinomas

  • solid carcinomas

  • anaplastic carcinomas

  • osteosarcomas

  • fibrosarcomas

  • malignant mixed tumors.

What are the symptoms of mammary tumors?

Mammary tumors present as a solid mass or as multiple swellings. When tumors do arise in the mammary tissue, they are usually easy to detect by gently palpating the mammary glands. When tumors first appear they will feel like small pieces of pea gravel just under the skin. They are very hard and are difficult to move around under the skin. They can grow rapidly in a short period of time, doubling their size every month or so.

Mammary Cancer in Dogs The dog normally has five mammary glands, each with its own nipple, on both the right and left side of its lower abdomen. Although breast cancer can and does occur in all of the glands, it usually occurs most frequently in the 4th and 5th. In half of the cases, more than one growth is observed. Benign growths are often smooth, small and slow growing. Signs of malignant tumors include rapid growth, irregular shape, firm attachment to the skin or underlying tissue, bleeding, and ulceration. Occasionally tumors that have been small for a long period of time may suddenly grow quickly and aggressively, but this is the exception not the rule.

It is very difficult to determine the type of tumor based on physical inspection. A biopsy or tumor removal and analysis are almost always needed to determine if the tumor is benign or malignant, and to identify what type it is. Tumors, which are more aggressive may metastasize and spread to the surrounding lymph nodes or to the lungs. A chest x-ray and physical inspection of the lymph nodes will often help in confirming this.

Mammary cancer spreads to the rest of the body through the release of individual cancer cells from the various tumors into the lymphatics. The lymphatic system includes special vessels and lymph nodes. There are regional lymph nodes on both the right and left sides of the body under the front and rear legs. They are called the 'axillary' and 'inguinal' lymph nodes, respectively. Mammary glands 1, 2, and 3 drain and spread their tumor cells forward to axillary lymph nodes, while cells from 3, 4, and 5 spread to the inguinal ones. New tumors form at these sites and then release more cells that go to other organs such as the lungs, liver, or kidneys.

What is the treatment?

Surgical Removal: Upon finding any mass within the breast of a dog, surgical removal is recommended unless the patient is very old. If a surgery is done early in the course of this disease, the cancer can be totally eliminated in over 50% of the cases having a malignant form of cancer. The area excised depends on the judgment and preference of the practitioner. Some will only remove the mass itself. Others, taking into consideration how the cancer spreads, will remove the mass and the rest of the mammary tissue and lymph nodes that drain with the gland. For example, if a growth were detected in the number 2 gland on the left side, we would therefore remove glands, 1, 2, and 3 and the axillary lymph node on that side. If it were found in the number 4 gland on the right side, then glands 3, 4, 5, and the inguinal lymph node on that side would be completely removed. With some tumor types, especially sarcomas, complete removal is very difficult and many of these cases will have tumor regrowth at the site of the previously removed tumor.

Owners may confuse a surgical removal of a mammary gland in the dog with a radical mastectomy in humans, with all of the associated problems. In humans, this type of surgery would affect the underlying muscle tissue which complicates the recovery. In the dog, however, all of the breast tissue and the related lymphatics are outside of the muscle layer, so we only need to cut through the skin and the mammary tissue. This makes the surgery much easier and recovery much faster. A radical mastectomy in a dog means all the breasts, the skin covering them, and the four lymph nodes are all removed at the same time. Although this is truly major surgery, suture removal usually occurs in 10 to 14 days with normal activity resuming at that point.

Many veterinarians will spay a dog having a mastectomy (unless she is very old). The value of this in decreasing the recurrence of tumors is still controversial.

Chemotherapy and Radiation Therapy: Chemotherapy has not been a very successful nor widely used treatment for mammary tumors in dogs. However, with the constantly changing and improving drugs available, a veterinary oncologist should be consulted to find out if there is an effective drug available for your dog's particular type of mammary cancer. The effectiveness of radiation therapy has not been thoroughly researched. Some anti-hormonal drug regimens are being tested in dogs. At this point in time, surgical removal of the tumors is the treatment of choice.

How can I prevent mammary cancer in my dog?

There are few cancers that are as easily prevented as mammary cancer in dogs. There is a direct and well-documented link between the early spaying of female dogs and the reduction in the incidence in mammary cancer. Dogs spayed before coming into their first heat have an extremely small chance of ever developing mammary cancer. Dogs spayed after their first heat but before 2.5 years are at more risk, but less risk than that of dogs who were never spayed, or spayed later in life. We all know the huge benefits of spaying females at an early age, but every day, veterinarians still deal with this easily preventable disease. Early spaying is still one of the best things pet owners can do to improve the health and ensure a long life for their dogs.


Mammary cancer is a very common cancer and can often be successfully treated, if caught early. If all non-breeding dogs and cats were spayed before their first heat this disease could be almost completely eliminated. If you find a growth or lump in the mammary tissue of your dog, you should inform your veterinarian immediately and not take a "wait and see" attitude.

Race Foster, DVM
Holly Nash, DVM, MS
Drs. Foster & Smith, Inc. 
Web Site


We've 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 higher yet with one in four unspayed female dogs 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% (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 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 doesn't mean it is too late to reap benefit from spaying.

Early 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 ten 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.

Benign vs. Malignant

The good news, if there is some, is that approximately 50% of the tumors formed by female dogs are benign. Since one cannot tell which it is by looking at a tumor, the tumor or part of it must be removed and 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, in which a syringe is used to withdraw some cells from the growth and the laboratory 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.

Hormone Receptors

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 mammary tumor removal or two years prior lived 45% longer than those who remained unspayed.

Types Of Tumors

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

  • Fibroadenoma: 
    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: 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.

  • Adenocarcinoma: 
    Adenocarcinomas can be tubular or papillary, depending on the 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.

  • Inflammatory Carcinoma:
    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 difficult to treat. 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.

What Else Determines Prognosis?

The type of tumor is obviously important in determining the prognosis; further, spaying at the time of 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.

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.

By Wendy C. Brooks, DVM, DipABVP 
Educational Director, VeterinaryPartner.com
Web Site 

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