• Joan Rest, BVSc, PhD, MRCPath, MRCVS

Canine Mammary Tumors

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What is a mammary tumor?

This is any tumor originating from the mammary gland tissues. In the dog, most tumors of this type are cured by complete surgical removal, but over time a few progress to malignancy and start to spread to other parts of the body (metastasis). A dog may have multiple tumors in different mammary glands.

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 occurs due to non-lethal genetic damage of cells (mutations in the DNA genome). Some animals have a genetic tendency to develop cancer and this risk increases with increasing age. However, sex hormones are the most important single risk factor for mammary tumors in dogs. If the ovaries and uterus are removed (ovariohysterectomy or spaying) at an early age, there is less risk of these tumors developing. Conversely, giving a dog female sex hormones increases the incidence.

"Sex hormones are the most important single risk factor for mammary tumors in dogs."

Cancer induction is a multi-step process and some tumors stop before becoming true cancers. Hyperplasia (excessive growth) and dysplasia (abnormal growth) of the mammary glands are due to hormonal disturbances. Some progress to become freely proliferating entities without function (benign cancers) and a few may become invasive and spreading (malignant cancers).

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 (genetic susceptibility) to cancer. Some breeds have far more cancers than others, often of specific types. The more divisions a cell undergoes, the more probable is a mutation, so cancer is more common in older animals. Mammary tumors need hormones to start growing, and some benign ones will regress if the source of the hormones is removed.

Are these common tumors?

These are very common tumors in female dogs, mainly in middle aged to older animals. They are rare in male dogs.

The incidence of mammary tumors is between eight and twenty six percent when dogs are spayed after the first estrus. Spaying bitches before first oestrus reduces the incidence to 0.5%, so this will affect the probability of your dog developing more mammary tumors.

How will these cancers affect my pet?

The most obvious effect is a lump or multiple lumps in the glands. Some tumors produce a secretion (clear, milky or bloodstained fluid which may be expressed from the teat). Benign tumors rarely ulcerate or bleed but large tumors may have physical effects by pressing on the surrounding tissues or structures, or may lose some of their blood supply causing parts of them to die. Inflammation and secondary bacterial infections are possible, causing pain and general signs of illness.

How are these cancers diagnosed?

Your veterinarian will often suspect the diagnosis of a mammary tumor based on its typical appearance and location. However, accurate diagnosis of the type of mammary tumor relies upon microscopic examination of tissue.

Cytology, the microscopic examination of cell samples, is not an accurate method of diagnosis for mammary tumors. The diagnosis, prediction of behavior (prognosis) and a microscopic assessment of whether the tumor has been completely removed rely on microscopic examination of tissue samples (histopathology). Your veterinarian will send tissue samples to a specialized laboratory for assessment by a veterinary pathologist. 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 completely 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."

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.

Ovariohysterectomy (spaying) early in life markedly reduces the incidence of mammary cancer. Spaying at the time of tumor removal does not affect growths that are already cancerous but can make some mammary hyperplasias disappear. Spaying at the time of tumor removal is unlikely to prevent further tumor development.

"Some medical hormonal treatments used successfully in women may induce cancers in dogs."

Both canine and human mammary tumors are hormone dependent, but the hormones involved are different. Some medical hormonal treatments used successfully in women may induce cancers in dogs rather than curing them.

Can these cancers disappear without treatment?

Ovariohysterectomy does not affect established cancer or prevent recurrence, but removal of the ovaries causes most hyperplastic lesions to regress.

Development of cancer is a multi-step process so it may stop at some stages. These benign cancers have the potential to progress to malignancy, some rapidly and others taking months or years. As they have this potential, early surgical removal is always recommended.

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 a diagnosis, which will indicate the type of tumor and how it is likely to behave. However, there is significant variation between animals in their response to tumors and the probability of further tumor development.

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

Benign non-cancerous growths are either hyperplasias (overgrowth) or dysplasias (abnormal growth). These include growths arising from the epithelium that normally produces milk (lobular or epithelial hyperplasia, occasionally called adenosis), growths that also include the connective tissue between the glands (fibroadenomatous change or fibroepithelial hyperplasia), and growths due to expansion of the ducts which take the milk to the teats (cystic ducts, ductal ectasia or hyperplasia). In women, fibroadenomatous change is a risk factor for development of mammary carcinoma and in cats, most hyperplasias progress to neoplasia. However, the situation in dogs is less clear, and some hyperplasias resolve after spaying without progressing to cancer.

Benign mammary cancers are called adenomas. Some arise only from the milk-producing epithelium (simple adenomas). Others include mammary tissues such as the myoepithelium and connective tissue between the glands. These adenomas have a number of different names and the structure may be complicated. Some of the names used include complex adenoma, fibroadenoma and benign mixed tumor. In general, complex (mixed) tumors tend to have the best outlook (prognosis), while tumors arising from the ducts have the least favourable prognosis. Tumors may also be classified by the type of epithelial growth because this influences the probability of recurrence. The behavior of a few mammary adenoma tumor types is difficult to predict, since some are borderline malignancy and others occur at multiple sites. In general, smaller mammary tumors have a better prognosis, but some tumors are very large but benign.

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 be certain of complete cure once your dog has developed 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. The likelihood of malignant tumors increases with age. There is also a familial incidence for tumors.

"The likelihood of malignant tumors increases with age."

Multiple tumors are common. In a very large and long study, 60% of female dogs had more than one mammary tumor and in a shorter study, approximately 25% of dogs developed more tumors following excision 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 there is no regrowth of a tumor 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

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