Mammary Tumors in Dogs - Malignant

Mammary Tumors in Dogs - Malignant

These notes are provided to help you understand the diagnosis or possible diagnosis of cancer in your pet. For general information on cancer in pets ask for our handout "What is Cancer". Your veterinarian may suggest certain tests to help confirm or eliminate diagnosis, and to help assess treatment options and likely outcomes. Because individual situations and responses vary, and because cancers often behave unpredictably, science can only give us a guide. However, information and understanding about tumors and their treatment in animals is improving all the time.

We understand that this can be a very worrying time. If you have any questions please do not hesitate to call your veterinarian.

What is this tumor?


This is a tumor originating from the mammary gland tissues that has been classified as malignant (invasive and capable of spreading). Although some of these cancers are cured by surgical removal, others will recur locally and some spread to other parts of the body (metastasis). A dog that has a tumor of this type often has or will develop further tumors in other mammary glands.

"A dog that has a tumor of this type often has or will develop further tumors in other mammary glands."

Malignant cancerous growths may arise from the epithelium that normally produces milk (simple mammary carcinoma or adenocarcinoma and secretory carcinoma). Others arise from the ducts (ductal or ductular carcinoma). Some involve other tissues such as the myoepithelium and other connective tissue between the glands. These have a number of different names includingcarcinosarcomacarcinoma or sarcoma in benign tumor, complex adenocarcinoma andmalignant mixed tumor. Tumors are also classified by the type of epithelial growth because this influences the probability of recurrence. Tumors that only contain myoepithelium or connective tissue are called sarcomas.

Any mammary lesion may become inflamed (mastitis) and a few lumps are solely due to inflammation.

What do we know about the cause?

The reason why a particular pet may develop this, or any cancer, is not straightforward. Cancer is often the culmination of a series of circumstances that come together for the unfortunate individual.

Cancer is the result of non-lethal genetic damage to cells (mutations in the DNA genome). Some animals have a genetic tendency to develop cancer and the risk increases with increasing age. However, sex hormones are the most important single factor promoting development of mammary tumors in dogs. If the ovaries and uterus are removed by ovariohysterectomy (spay) at an early age, there is markedly less risk of these tumors developing.

Cancer induction is a multi-step process and some tumors remain benign (non-spreading). Others progress rapidly to malignancy and some are malignant from the start.

In some species of animals, viruses are important factors in inducing mammary cancer but, as far as we are aware, this is not true in dogs.

Why has my dog developed this cancer?

Some animals have a greater tendency (inherited genetic susceptibility) to cancer. Some breeds have far more cancers than others do, and certain breeds tend to get certain types of cancer. The more divisions a cell undergoes, the more probable is a mutation to occur, so cancer is more common in older animals.

"Once the tumors become malignant, hormones no longer influence their growth."

Mammary tumors need hormones to start growing. However once the tumors become malignant, hormones no longer influence their growth.

Are these common tumors?

Mammary tumors are common in female dogs, mainly in middle aged to older animals. They are rare in male dogs. In the dog, malignant (spreading, life-threatening) tumors of mammary glands are less common than benign ones. Fortunately, the most malignant types (ductal/ductular carcinomas and sarcomas) are the least common of all.

How will the cancer affect my pet?

The most obvious effect is a lump or multiple lumps in the mammary glands. Some tumors produce secretion (clear, milky or blood-stained fluid which may be expressed from the teat). Ulceration may occur. These tumors are often firmly attached to the surrounding structures with poorly demarcated edges. Inflammation and secondary infection are possible, causing pain and general signs of illness. Large tumors may lose some of their blood supply and ulcerate, or their size may cause physical effects on the surrounding structures.

Weight loss due to loss of body fat and muscle may occur in the later stages of malignant cancer. The immune system is often damaged which allows cancers to develop and infections to persist. A few tumors induce disease signs that are not readily explained by local or wider spread of the cancer but, in contrast to similar tumors in women, this is rare with canine mammary tumors. If the cancer metastasizes to the lungs, there may be shortness of breath and difficulty breathing. A few cancers spread through the skin, producing inflammation.

How are these cancers diagnosed? 

Clinically, your veterinarian will suspect the diagnosis of one of these tumors based on the typical appearance and results of a physical examination. However, accurate diagnosis of the type of mammary cancer (and therefore how it will behave) relies upon microscopic examination of tissue.

Cytology, the microscopic examination of cell samples, is not an accurate method of diagnosis for mammary tumors. Accurate diagnosis, prediction of behavior (prognosis) and a microscopic assessment of whether the tumor has been completely removed rely on a histopathology examination (microscopic examination of specially prepared and stained tissue sections). Your veterinarian will submit the samples to a specialized laboratory, where a veterinary pathologist will reach a diagnosis and give a prognosis based on the histopathology findings. Whenever a mammary tumor is submitted for assessment, the sample always needs to include the margins, or edges of the lump, so that the pathologist can determine whether the cancer has been fully removed.

The histopathology report typically includes words that indicate whether a tumor is 'benign' (non-spreading, local growth) or 'malignant' (capable of spreading to other body sites). These, together with the origin or type of tumor, the grade (degree of resemblance to normal cells or 'differentiation') and stage (how large it is and extent of spread) indicate how the cancer is likely to behave.

What types of treatment are available?

The most common treatment is surgical removal of the cancerous tissue. Sometimes this is just the lump and sometimes the whole gland and draining lymph node are removed. If there are several tumors, your veterinarian may perform a "radical mastectomy", in which multiple glands and their associated lymph nodes will be removed.

"However spaying at the time of tumor removal does not usually affect growths that are already malignant because these are already beyond hormone control."

Ovariohysterectomy (spaying) early in life markedly reduces the incidence of mammary cancer. However spaying at the time of tumor removal does not usually affect growths that are already malignant because these are already beyond hormone control. The exception is the secretory carcinoma. This is usually a tumor of young female dogs and is dependent on the female hormone estrogen. Ovariohysterectomy therefore removes the stimulus to further growth.

Decisions on additional treatment include consideration of the quality of life such treatment will give an animal. Additional therapy is not suitable for all types of cancer and often has significant side effects. Some types of adjunctive therapy are only available at specialist centers. Current recommendations are therefore surgery as the primary treatment and, possibly, additional therapy for aggressive tumors with lymphatic invasion and poor surgical margins (but not secondary tumors). Some medical hormonal treatments used successfully in women induce cancers in dogs rather than curing them. Chemotherapy has not been proven to be successful in dogs with mammary cancer. New research is focussing on using certain enzymes to destroy tissue, gene therapy and inhibitors of new tumor blood vessel formation (angioneogenesis). Immunotherapy is not helpful because mammary tumors are poor producers of antigens.

Can these cancers disappear without treatment?

Generally, no. After malignant mammary cancer has developed, an ovariohysterectomy does not cause it to disappear, nor does it prevent recurrence.

Very occasionally, spontaneous loss of blood supply to the cancer can make it die but the dead tissue will still need surgical removal. The body's immune system is not effective in causing this type of tumor to regress.

How can I nurse my pet?


Preventing your pet from rubbing, scratching, licking or biting the tumor will reduce itching, inflammation, ulceration, infection and bleeding. Any ulcerated area needs to be kept clean.

After surgery, you will need to keep the operation site clean and your pet should not be allowed to interfere with the site. Report any loss of sutures or significant swelling or bleeding to your veterinarian. If you require additional advice on post-surgical care, please ask.

How will I know how this cancer will behave?

The histopathology report will give your veterinarian the diagnosis that will indicate the tumor type and how it is likely to behave. However, there is significant variation between individual animals in their response to mammary tumors and the probability of further cancer development. The behavior of some tumor types is difficult to predict and other tumors can be multiple. Returning for regular post-surgical examinations is therefore important.

"Overall, 35% of malignant tumors metastasize and 70% are cured surgically."

Smaller tumors usually have a better outlook (prognosis). The staging (lymph node or distant metastasis) also affects the prognosis. The type of cancer is also important. Malignant tumors with a connective or myoepithelial component (mixed tumors, carcinosarcomas and sarcomas) tend to be more malignant than epithelial tumors but epithelial tumors arising from the ducts (ductal or ductular carcinoma) are also highly malignant. In one study, less than one in five tumors were of this type but they caused two thirds of fatalities. Overall, 35% of malignant tumors metastasize and 70% are cured surgically. Most secondary tumors develop in the lungs of dogs and, in contrast to women, rarely metastasize to bones.

When will I know if the cancer is permanently cured?

'Cured' has to be a guarded term in dealing with any cancer.

It is very difficult to promise complete cure once your dog has developed malignant mammary tumors but the following general guidelines may help. Mammary tumors are age dependent so an older bitch will have a higher probability of recurrence and the proportion of malignant tumors increases with age. There is also a familial incidence for tumors. German Shepherd dogs tend to have a worse prognosis than other breeds.

Multiple tumors are also common. In one study, 60% of female dogs had more than one mammary tumor and in a shorter study, approximately 25% of dogs developed more tumors following removal of one. Tumor growth in different glands is usually multifocal (meaning the tumors develop in many sites), rather than from spread or seeding of tumor cells from a single initial site.  It is therefore advisable to have your dog rechecked at regular intervals to ensure that there is no tumor regrowth and no new ones have appeared.

Are there any risks to my family or other pets?

No, these are not infectious tumors and are not transmitted from pet to pet or from pets to people.

This client information sheet is based on material written by: Joan Rest, BVSc, PhD, MRCPath, MRCVS
© Copyright 2009 Lifelearn Inc. Used and/or modified with permission under license.