Cat's Age Matters When Spaying to Reduce Cancer Risk

By Joanne Lynn Intile, DVM, MS, DACVIM on Aug. 20, 2014

Mammary cancer is a particularly frightening diagnosis for cat owners. Over 90 percent of feline mammary tumors are malignant, meaning they grow in an invasive fashion and spread to distant sites in the body. This is in contrast to dogs, where only about 50 percent of mammary tumors are malignant.

Tumors tend to affect older, unspayed female cats, but all cats, including males, are at risk.

The age at which a female cat is neutered plays a role in protecting against tumor development, with the greatest benefit seen for kittens spayed before 6 months of age, who have a 91 percent reduction in risk compared to non-spayed cats. Spaying between six months and one-year results in an 86 percent reduction in risk, spaying between 1-2 years leads to an 11 percent reduction in risk, and spaying after age two does not reduce the risk of mammary cancer development at all.

Sometimes owners will detect a mammary mass incidentally while petting their cat. Other times the cat will draw attention to a tumor by showing signs of increased licking or chewing at the affected area. Masses can also be discovered “accidentally” during routine physical exams.

The size of the tumor at the time of diagnosis makes a difference in patient outcome:

Cats with tumors less than 2cm in diameter at the time of removal have a median survival time of 4.5 years.

Cats with tumors greater than 3cm in diameter at the time of removal have a median survival time of 6 months.

Because tumors can go undetected for a long time and the size of the tumor is prognostic, routine physical exams are of absolute necessity for pets. (see Regular Exams Can Save More Than Your Pet's Life) This is especially true for cats known to be neutered later in life, or for those cats adopted as adults with an unknown medical history.

Surgery is the mainstay of treatment for cats with mammary tumors. The current recommended “surgical dose” for cats with no evidence of spread of disease is a procedure called a staged, bilateral radical mastectomy. This entails surgical removal of all of the mammary tissue on one side of the body, followed by removal of the tissue along the opposing side following about a 2-week healing period.

Many owners are anxious when they hear the details of this type of surgery. Though it is an aggressive procedure, what I try to remind them of is that the surgery is less invasive than one that opens up a body cavity, and we are very pro-active about our pain management measures.

It’s always difficult to make this type of a decision for our companions — one where we know we are making a choice because that has the best chance for prolonging their life but also knowing there will be an impact, albeit temporary, on their quality of life.

A few important considerations for submitting feline mammary tumors for biopsy:

It is essential that all of the removed tissue be submitted for histopathology. Most feline mammary tumors are carcinomas or adenocarcinomas, but other histological subtypes do occur.

Submitting all of the tissue also allows us to know if there were additional tumors located in other mammary glands. Often I see a report indicating pre-cancerous tissue was removed in glands adjacent to the one with the tumor.

The biopsy report will also let us know if there are adequate surgical margins on the tissue, or if the chance of regrowth is more significant because cancerous tissue was left behind.

The biopsy should also provide information as to the grade of the tumor. The pathologist should examine specific histological features under the microscope to assign a grade to the tumor (grade 1, 2, or 3).

Each of the factors listed above help veterinary oncologists decide on risk assessment and for need of further therapy beyond surgery.

Based on the information above, I often discuss using chemotherapy after surgery to treat what is known as “microscopic residual disease.” These are tumor cells that may have spread to distant sites in the body prior to removal. The most commonly prescribed chemotherapeutics for feline mammary tumors are doxorubicin, carboplatin, and cyclophosphamide, though many other options exist.

We lack studies that adequately “prove” that treating with chemotherapy after surgery is truly beneficial for cats with mammary tumors. Although one study showed survival in cats receiving chemotherapy after surgery was not improved when compared to cats undergoing surgery alone, the disease-free interval was increased, meaning the patients receiving chemotherapy felt well for a longer time period.

Chemotherapy can also be used to treat cats with tumors that cannot be removed surgically, or for cats with spread of disease. Roughly half of those cats would show some form of response to treatment, and about 1 in 5 will achieve a remission (i.e., a period of time where no tumor would be detectable). Cats that showed a response to therapy have median survival times of about six months compared to less than three months if they do not respond to treatment.

Owners of cats with mammary tumors often ask me what will happen “in the end.” In my experience, there are usually one of two outcomes:

Cats develop large, non-resectable tumors that grow rapidly and become ulcerated and infected and ultimately make them feel sick and have a poor quality of life, or
Cats develop spread of the tumor to their lungs, and show signs of difficulty breathing due to the physical presence of the tumors or due to fluid building up around the lungs secondary to the tumors.

A diagnosis of mammary cancer can be a scary and overwhelming. However, it’s important to arm yourself with all of the facts. Often, the best way to do this is to seek consultation with a veterinary oncologist or a veterinary surgeon prior to any major treatment decisions. The information you gain will be well worth the price of the referral, and could just mean the difference between life and death for your cat.