Illustration courtesy Dr. Wendy Brooks

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 healthcare agencies, and research is ongoing. Every woman is encouraged to check for breast lumps regularly. But what about pets? It turns out our furry friends (mostly female but male as well) are also at risk for this common form of cancer. The mammary cancer picture is a little different between cats and dogs. The information here concerns the situation of our feline friends. In cats, mammary cancer is the third most common cancer, with the most common victim being a senior female cat around age 10-12 years who either remains unspayed or was spayed well into adulthood.

Dogs are lucky, as only about 50% of mammary tumors are malignant for them. For cats, approximately 90% are malignant with rapid spread to adjacent glands and the nearest set of lymph nodes. Cats generally have four pairs of mammary glands (thoracic, cranial abdominal, caudal abdominal, and inguinal – see the illustration). The most commonly affected glands are the thoracic and inguinal glands. An owner should be accustomed to feeling for even small lumps in these areas. Because veins connect both the right and left sets of glands, it is easy for tumor cells to cross from one side to the other, though usually, the glands on the same side as the original tumor are seeded first.

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At first, the tumor is small and may feel like a pebble or dried pea. The tumor should be removed as soon as possible in order to remove it completely. If left alone, mammary tumors get larger and harder and ultimately burst through the skin, creating a smelly, infected ulcer.

Tumors spread from the mammary glands to local lymph nodes and then on to the chest, brain, bone, and even spleen. Chest radiographs will be needed to assess tumor spread before surgery can proceed. Basic blood tests will also be needed.

Risk Factors for Mammary Cancer

• Not surprisingly, being female is the most significant factor.
• Siamese cats are overrepresented. They also tend to develop mammary tumors at a younger age than other breeds (nine years is average for Siamese and 14 for non-Siamese).
• Exposure to progestin hormones (not used much anymore but previously used for behavioral issues such as urine marking) is a risk factor.

Protective Factors

Early spaying is the single most significant protective factor. Spaying before the age of six months results in a 91% reduction in risk. Spaying before the age of one year results in an 86% reduction in risk. Spaying before the age of two years leads to an 11% reduction in risk. Giving birth to kittens has no affect on mammary cancer risk.

Cats spayed before age six months have a 9% rate of mammary cancer development later in life. Cats spayed between ages six and 12 months have a 14% incidence of mammary cancer development later in life.

Types of Tumors

Feline mammary tumors are generally either carcinomas or adenocarcinomas. Papillary or tubular carcinomas tend to be the least aggressive. Ductular carcinomas seem to kill cats about four times faster. Anaplastic carcinomas are the worst kind, usually dispatching the patient three times faster than that.

The pathologist examining the biopsy tissue uses different cellular criteria of malignancy to grade the tumor as Grade I, Grade II, or Grade III. A grade I tumor generally has a good chance of a one-year survival time, depending on its size and tumor type. Grade II tumors have a 57% chance of survival after a year unless chemotherapy accompanies surgery. Grade III tumors treated with surgery alone had no survivors at one year from the time of diagnosis.

Treatment: Surgery

Photo courtesy of Dr. Wendy Brooks

Surgery is the mainstay of therapy, and the goal is not only to remove as much tumor as possible but also to remove potential areas for the tumor to spread. This is best done with the removal of not only the chain of mammary glands where the tumor is located but the removal of the entire other side as well. This sounds very aggressive, but survival times can be more than doubled with this approach.

There is not usually enough skin to remove both mammary chains on the same surgery so it is common to allow three to five weeks of healing time between the removal of each chain. Removing both chains not only prevents the tumor from crossing to the other side but it prevents development of a new tumor on the second side as well.

If only one side is involved but is deemed too advanced for cure, removing the second mammary chain is likely not worth it.

Aggressive surgery yields a disease-free interval of 1 year in about 50% of cats and 2 years in 32% of cats. If lymph node involvement is already present at the time of surgery or if the tumor recurs after the first surgery, then 51-66% of cats will have a 5.5-month disease-free interval.

If the tumor is in an obviously advanced state at the time of the first surgery, then a less aggressive surgery to palliate the infection may be more appropriate. This will at least make the cat more comfortable by losing the smelly, infected external tumor. Alternatively, radiation therapy can be used to temporarily "dry up" the stinky leaking gland, especially if the cancer poses too great an anesthetic risk for tumor removal.

Treatment: Chemotherapy

To maximize disease-free intervals, surgery is followed by chemotherapy. The most common drugs used are Adriamycin and cyclophosphamide. This combination was tested in a group of cats who had either incomplete surgical removal of their tumor or obvious tumor spread to lymph nodes or beyond at the time of surgery. About 50% of cats showed at least some response though 21% actually achieved remission (i.e. no detectable tumor, at least temporarily). In the 50% of cats that showed at least some response to therapy, median survival time increased to 150 to 180 days as opposed to 75 to 86 days in the 50% that did not respond. In studies where adriamycin was used alone, survival statistics were not improved over surgery alone - meaning the cancer still spread - but the disease-free interval was increased, meaning the patients felt good longer.

Not all veterinarians are comfortable treating mammary gland cancer. Discuss with your veterinarian whether a referral to a specialist would be best for you and your pet.

How Do Cats Die From This Disease? When is Euthanasia the Right Choice?

There are two problems that tend to lead to the cat’s demise. One is simply the infected ulcerated external tumor. The primary tumor smells rotten and bleeds. Secondary tumors arise and also break through the skin. Cats are fastidious creatures and do not like having growths of this type on their bodies, plus owners tend not to like rotten, bleeding flesh in their homes. Cats are commonly euthanized due to the progress of the external tumor.

The other problem is tumors spread to the chest. The lungs are invaded by tumor cells, and ultimately, the cat will not be able to breathe.

Cats with advanced lung involvement at the time the tumor is diagnosed have a median survival time of only one month.

Life quality decisions for cancer patients are difficult as the disease progresses inexorably but slowly, and there are good days mixed with bad days. Quality of life issues center around inherent discomfort from the disease versus enjoyment of other aspects of life (socialization, appetite, etc.). Criteria for a euthanasia decision are largely personal, but some guidelines have been established to assist in making this difficult decision. See our quality of life scale to assist you in determining when the time is right. Remember, your veterinarian is available to discuss life quality issues if you need additional guidance.

Download a poster on mammary gland cancer awareness for pets.