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 Breast Cancer Isn't Just a Human Disease

VETSTREET: BY DR. ANN HOHENHAUS

 

According to the American Cancer Society, breast cancer is the leading cause of cancer death in women.

In 2011, nearly a quarter million cases of invasive breast cancer were diagnosed in the United States — and close to 40,000 deaths occurred.

Breast cancer is an equally important disease in dogs and cats: It is the most common tumor found in female dogs and the third most prevalent tumor detected in cats. Even wild cats can develop the disease — in a study that looked at captive lions, leopards and tigers, 11 out of 26 of these animals had breast tumors.

 

How Cats and Dogs Compare to Humans
Reproductive status impacts the development of breast cancer in both humans and animals. For example, women who've had their first child late in life have a greater risk of developing the disease.

Spaying dogs before their first heat cycle decreases the risk of breast cancer. And while the same data isn't available for felines, one of my patients, Nellie, is a typical case of a cat suffering from breast cancer — she was spayed as a mature cat after she found a forever home, and she may have mothered a litter of kittens while homeless.

For dogs, cats and humans, the mainstay of treatment for breast cancer is surgical excision of the tumor. The tissue is then submitted for a biopsy to determine if further therapy is necessary. Nellie's surgery was performed by her neighborhood veterinarian, and she came to me because a “bad” biopsy indicated that she needed chemotherapy.

Normal breast tissue has estrogen and progesterone receptors that, when stimulated by the body’s production of hormones during pregnancy, ready the breast tissue for lactation. In women, tumor tissue is tested for the presence of these receptors; tumors that come back positive for estrogen and progesterone receptors are treated with drugs like tamoxifen and raloxifene, which block the receptors, inhibiting tumor growth.

Currently, veterinary oncologists do not routinely screen their patients’ biopsies for hormone receptors, but based on scientific research, the majority of breast tumors in dogs and cats lack hormone receptors.

What Your Vet Will Do: Diagnosis and Treatment Options
Other than a physical examination, there are no routine breast cancer screening tests for dogs and cats. A recent study has suggested that mammography and ultrasounds for dogs produce similar results to humans, but the acceptance of mammography as a routine screening test is yet to come.

Finding a breast tumor in a pet leads to a cascade of tests and evaluations. In Nellie's situation, since these tumors can spread to the lungs, she received a chest X-ray to determine if this was the case. Nellie’s veterinarian also ran routine blood tests to determine if contraindications to general anesthesia existed, since surgery was recommended.

Veterinary oncologists will often suggest radical mastectomy, which involves removing the chain of breast tissue along the chest and abdominal walls. The procedure differs from the human version in that the underlying muscle isn't removed.

Nellie underwent a radical mastectomy and recovered at home for two weeks before she came back to see me so we could discuss further treatment options. For dogs who only have small tumors, a lumpectomy may be all the surgery that is needed.

Most of my patients, like Nellie, are spayed. But if your dog or cat has a breast tumor and is not spayed, your veterinarian will likely recommend that this surgery be performed at the same time as the mastectomy.

The veterinary oncology group at the Ryan Veterinary Hospital of the University of Pennsylvania, led by Dr. Karin Sorenmo, has recently shown that spaying at the time of tumor removal decreases the risk that additional breast tumors will form. In an unpublished study presented at the recent World Veterinary Cancer Congress, Dr. Veronica Kristiansen noted that spaying also decreased the occurrence of new tumors in dogs with benign breast tumors.

Many dogs do not require treatment beyond surgery, but treating feline breast tumors remains a challenge because they are highly malignant and intrinsically resistant to drug therapy.

The Current Prognosis Picture for Pets
Nearly all feline breast tumors behave malignantly, but approximately half of all dogs with breast tumors receive good news from their oncologists because about 50 percent of canine breast tumors are benign.

The high frequency of benign tumors in dogs means that surgery alone can cure many canines with breast cancer. In cats, surgery followed by chemotherapy with a drug call doxorubicin resulted in a disease-free span of 225 days. The average lifespan after surgery and chemotherapy was 448 days. Only about 1/3 of cats lived two years.

Nellie responded to treatment exactly as expected and lived for about a year and a half before ultimately succumbing to her tumor.

Cutting-Edge Cat and Dog Research
Human breast tissue that's sent out for a biopsy is tested for a gene known to affect both the prognosis and treatment of breast cancer. The HER-2/neu gene contains genetic information that leads to the synthesis of a cellular membrane receptor that's found to be at increased levels in 25 to 30 percent of human breast cancers. Herceptin® is a monoclonal antibody designed to block the HER-2/neu receptor, as well as block the growth of breast cancer, when the tumor tests positive for HER-2/neu.

A study performed at my hospital, The Animal Medical Center, showed HER-2/neu was commonly elevated in cats with breast cancer. Information like this will help to determine if drugs like Herceptin® will be useful in the treatment of dog and cat patients.

In women, the spread of breast cancer cells to the lymph nodes indicates a less favorable prognosis. Recently, Polish researchers confirmed that the same is true in dogs and found that the presence of tumor cell clusters in the lymph nodes greater than 2 millimeters in diameter translated to a shorter survival rate.

Meanwhile, French researchers at the Nantes-Atlantic National College of Veterinary Medicine presented new information on breast cancer tumor markers at the World Veterinary Cancer Congress. A team of veterinarians used breast tumors from dogs and cats to investigate estrogen and progesterone receptors and the HER-2/neu gene.

In human breast cancer, tumors lacking all three markers carry a poor prognosis. The French team found that these triple negative tumors in both dogs and cats can serve as models for the study of new treatments to help everyone suffering from this terrible disease.

How You Can Help Your Pet
Support early spay programs. And make sure that your puppy or kitten is spayed before the first heat cycle.

Examine your pet. Every pet loves a tummy rub, and this is the perfect opportunity to find a breast tumor while it's still small and very treatable.

Visit your veterinarian regularly. During a complete physical examination, your vet will carefully palpate for small tumors in the breasts.

Support breast cancer research in pets and people. Consider a Think Pink leash for your pet or donate to organizations that fund research for pet diseases, including the Morris Animal Foundation and Winn Feline Foundation.

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