Cats Get Breast Cancer Too, and There’s a Lot We Can Learn From It

Understanding aggressive tumors in pets may lead to better treatments for the nastiest forms of the disease in people

The patient, in a rare moment of calm. Victoria Jaggard

Felix seems determined to test the idea that cats have nine lives. I adopted him as a kitten from someone whose outdoor cat got unexpectedly pregnant. When I took him for his first vet visit, he was riddled with parasites, from ear mites to intestinal worms. A medley of kitty drugs eventually cleaned him up with no lasting effects. At age five he burrowed through the screen door on my balcony and took a dive, falling six stories and collapsing a lung. That required X-rays, an overnight stay in an oxygen tank and another round of meds.

Then, in January, I found a lump on his chest, close to his right front paw. Hours of web searches and an initial vet visit both came to the same conclusion: my male cat potentially had breast cancer.

Cat cancer is something I was already painfully familiar with. My other cat Sally had developed a lump in her cheek three years ago at age 16, and I spent a lot of time taking her for test after test before I finally got the grim diagnosis. She had oral squamous cell carcinoma, and it was basically inoperable. This particular cancer is fairly common in cats but notoriously aggressive, with a 1-year survival rate of less than 10 percent. In the end, all the ultrasounds, oncologist visits and desperate attempts to feed her via syringe didn't help, and she died within a few months.

With that nightmare experience still fresh in my mind, Felix's lump became an obsession. This time, I was going to fight for the earliest possible diagnosis and treatment. My morbid curiosity also kicked into high gear, especially as I saw so many quizzical looks when I said, "… and they think it might be breast cancer." What, exactly, was happening to my cat?

It turns out that, beyond surgery options, the study of mammary cancer in cats suffers from a dearth of coordinated clinical research. But a coalition of vets and doctors will soon be gathering in Washington, D.C., to help build the case that better understanding of canine and feline tumors could be a huge benefit to dealing with the disease not just in pets, but maybe also in people.


It's no medical surprise that cats can get mammary cancer. Cats of both genders have eight mammary glands, with four along either side of their tummies. Even if you find only a single lump, when a biopsy comes back cancer, the usual recommended action is to remove the entire chain on that side. According to Felix’s vet, the four glands are connected to lymphatic vessels that can transport cancer cells through the body, so doing a radical mastectomy is the best way to be sure you cut out the problem. Some vets even advise removing the chains on both sides, just to be safe. 

Because of the lymphatic connection, vets will often check whether the nearby lymph nodes show any abnormalities, and some will go ahead and remove those too during a mastectomy. Our vet also suggested we do a lung X-ray before any kind of surgery, because that's a common spot cancer will spread from the mammary chain. Once it's in the lungs, things get dire, and some vets will say you should consider cancelling surgery and moving instead to kitty hospice care. If the cat is cleared for a surgical procedure, all that's left is to wait and hope.

Cats Get Breast Cancer Too, and There's a Lot We Can Learn From It
Illustration by Shaylyn Esposito

"Surgery is usually all we do to provide treatment for a primary tumor," says veterinary oncologist Karin Sorenmo at the University of Pennsylvania. "In women, we offer breast-sparing surgery, because that is important for women psychologically." That leaves some breast tissue in place but requires the patient to go through follow-up doses of radiation or chemotherapy to beat back any lingering cancer cells and reduce the odds of recurrence. "Cats and dogs are different that way—they don't have self-image issues if we do a big surgery," she says. Giving a cat radiation therapy also means putting it back under anesthesia, which carries its own risks. "It's better to get it all out."

The disease is most common in older breeding females. "The risk for developing breast cancer overall is dependent on exposure to hormones," says Sorenmo. "There's a seven-fold increase in risk in cats that have not been spayed, and spaying has to occur at a very early age if you're going to have the best benefit." Sorenmo says she has seen mammary cancers in male cats too, more often if they have been taking hormone therapies like progesterone-based drugs for behavioral problems such as spraying or aggression.

If Felix had a tumor, he would simply be unlucky. He was spayed as a young cat and has had no behavioral problems (or at least ones serious enough to require medication—he is a cat, after all). One vet told me we could start with antibiotics and then see how the lump evolved; if it was a cyst or some type of infection, it might go away on its own. But while this type of cancer is extremely rare in males, in general feline mammary tumors are malignant 86 percent of the time. In other words, if Felix’s lump was a tumor, it was most likely a really bad one.


The aggressive nature of mammary cancer in cats is part of what intrigues Sorenmo the most, and one of the reasons she and other experts think finding out more about the feline version could be a boon to humans. According to the National Cancer Institute, the number of new human breast cancer cases has been stable for the past 10 years, but the number of deaths has actually been on the decline, going down by 1.9 percent on average each year from 2002 to 2011. Thanks to early detection efforts, doctors are finding more breast tumors while they are still localized and the cancer has not spread into other regions of the body. Surgery and drug options are improving too, and today 98.5 percent of people who are diagnosed with localized breast cancer are still alive at least five years later. But the situation can be much worse for people who are in more advanced stages or who have particularly nasty forms of the disease.

In healthy human breast tissue, the cells have receptors that relay messages from the hormones progesterone and estrogen, which help the cells grow and function. About 40 percent of the time, breast cancer cells have these hormone receptors too, which is actually a good thing, because it means they usually respond to hormone-based treatments that can direct the cancerous cells to slow down or even stop growing. Sometimes, though, breast cancer is double negative, meaning it lacks these receptors. Triple-negative breast cancer is missing both hormone receptors and the receptor for a protein called HER2, another target of drug therapies. These cancers are tougher to treat and quick to spread.

"When cats develop mammary cancer, it is much more malignant, similar to double- or triple-negative cancers in women," says Rodney Page at the Flint Animal Cancer Center at Colorado State University. If the tumor is small and hasn't spread to the lymphatic system or the lungs, surgery is often very successful, he says. "Beyond surgery, chemotherapy has been tried the most, and there are some cancer chemo-therapeutics for cats that have been studied. But we don't have large clinical studies that show they are successful. The situation in cats is going to require some new thinking. It's an opportunity to identify new strategies."

For a lot of human cancer studies today, researchers induce tumors in animals such as mice to develop new drugs and to figure out the environmental and genetic underpinnings. But Sorenmo and Page, among others, think that looking to feline or canine cancer might offer a unique advantage to basic research.

"Cancer is cancer, whether it appears in a golden retriever or a human," says Page. "Pets live in the same households as their owners and are exposed to the same volatile organic compounds or whatever else the exposure looks like." That means pets that develop the disease are ideal subjects for teasing out the long-term triggers in people too, and new therapies developed to prevent or treat cancer in companion animals could be similarly useful for humans.

"Dogs and cats live such shorter periods of time, and many of their biological processes happen so much faster, so we can get answers to some questions much quicker," says Sorenmo. Because cats and dogs have multiple mammary glands in a chain, it's even possible for tumors of various stages to appear together, offering a chance to simultaneously see how a tumor develops and grows.

In June, Page will be speaking at a workshop put together by the Institute of Medicine of the National Academies, which will bring together human and veterinary oncologists to assess the status of research and figure out how they can better collaborate. Right now about 20 academic centers in the U.S., including Colorado State, conduct clinical trials for cancers in pets and examine how their findings can relate back to people, under the umbrella of the National Cancer Institute's Comparative Oncology consortium. For instance, Page and his colleagues are about to wrap up a nationwide lifetime study of cancer in 3,000 golden retrievers, a dog breed that is at especially high risk for various types of the disease.

"This isn’t a new philosophy; certainly this type of comparative research has been going on for decades," David Vail, a veterinarian at the University of Wisconsin-Madison, told the News in Health NIH newsletter last May. "But, it’s probably been just in the last 10 years that clinical trials involving pets have become well-organized."

The trick now is to put the latest trials to good use in human cancer efforts in both academia and industry. "We conduct clinical studies with the same consent and rigor that occur in people. We also worry about pain management and how to help control nausea, vomiting and diarrhea," says Page. "But there is an issue of awareness—a large portion of the population doesn't necessarily think there is a connection. Plus there's the funding issue of how to convince the NIH or corporate drug manufacturers that these are valuable investments to accelerate the pace of finding cures."

Sorenmo agrees: "It all falls into this concept that there are many species, but the diseases we have at the molecular level are very similar, and the flow of information should go both ways," she says.


As with people, dogs and cats have the best chance of survival if cancer is caught early. This can be especially problematic for cats, which are in the habit of masking pain and other ill effects as a survival tactic. As much as I beat myself up about Sally’s death, she took her sweet time letting me know she had a tumor—she acted normally until her lump affected her eating, and by then there wasn't much either surgery or drugs could do. I only noticed Felix's lump because the 13-pound fluff ball likes to be carried around the house like a prince in a palanquin, and my hand accidentally landed on just the right spot.

Page recommends a more proactive approach, like doing regular physical exams for various cancer types—"any vet can show you how"—and getting into the habit of recording changes in the animal's skin, from dark spots to scabs to lumps. Sorenmo adds that you should make sure to rub your cat's belly and gently squeeze the mammary glands, even if it means getting some indignant swipes in return. "Cats sometimes have their own opinion about what they will allow you to do, but it can make a big difference," she says.

Despite my eagerness to get Felix on a treatment path as soon as possible, I opted for a biopsy first, just to be sure. A radical mastectomy would have involved cutting him open from armpit to back leg, while a biopsy would just be a tiny incision near the nipple to remove the mass for lab tests. I was somewhat comforted by the fact that the lump was loose and unchanging, and that his risk was so low.

Happily, Felix was just fine. I almost collapsed from relief when I got the call saying his lump was a benign cyst, and it was small enough that they had gotten the whole thing out during the biopsy. The worst he had to endure was a small scar, a few loopy days on pain meds and a week in the cone of shame. This is totally normal, says Page. Older animals get lumps and bumps, and in many cases it's nothing serious. But it's still worth going through the effort to find out, he says: "Sometimes it's not so benign." And maybe in the near future, your vet visit will be helping to save the lives of people as well as pets. 

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