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A New Way to Treat the Most Common Tumor in Female Dogs

canine mammary tumor

Story at-a-glance -

  • Mammary cancer is the most common tumor in female dogs, and researchers have now developed a simple “bio-scoring” method to help veterinarians and pet parents make better treatment decisions
  • The root cause of mammary cancer is multifactorial; one cause is toxically high estrogen levels, which can occur even in spayed females and male dogs as the result of exposure to xenoestrogens (estrogen mimicking compounds) in the environment
  • It’s important to take steps to reduce your dog’s exposure to xenoestrogens

Researchers at the University of Pennsylvania have developed a new method of staging canine mammary tumors that will help both veterinarians and pet parents better understand an affected dog’s prognosis as well as treatment options. This new "bio-scoring" system, created by a team of veterinary scientists led by Penn’s School of Veterinary Medicine, was recently published in the journal Veterinary and Comparative Oncology.1

Mammary Tumors Present Unique Treatment and Outcome Challenges

Mammary gland cancer is the most common tumor found in female dogs, and is similar to breast cancer in humans, in that it’s multifactorial (has multiple causes) and fueled by the hormone estrogen. This type of cancer is most commonly seen in intact female dogs, for example, strays and dogs used as breeders in puppy mills. Among these populations, 1 in 4 may have mammary tumors.2

Despite the prevalence of canine mammary cancer, determining the best course of treatment and likely outcome isn’t often obvious. According to Karin Sorenmo, a veterinary oncologist at Penn's Vet and lead study author, the "golden rule" in treating these patients has depended on the tumor stage (stage I — localized and small, up to stage IV — metastasis to distant organs).

"That's fine for some tumors that tend to behave in a certain way," Sorenmo told Penn Today, "but with mammary tumors it can be such a diverse disease."3

This makes deciding which dogs will do best with just surgery to remove the tumor, which might benefit from chemotherapy, and which are candidates for hormonal treatment, challenging.

"In testing this new approach," said Sorenmo, "we found it helps identify the dogs that develop metastasis better than other methods. I'm very excited about it."

New Bio-Scoring System Can Help Determine the Best Treatment Options for Mammary Cancer Patients

Sorenmo developed the new scoring method using data previously gathered in her Penn Vet Shelter Canine Mammary Tumor Program, an innovative program that helps homeless dogs with mammary cancer receive both life-saving treatment and a foster or permanent home.

The researchers evaluated data on 96 dogs from that program, plus 31 additional dogs treated in a similar fashion at the Norwegian University of Life Sciences. The records used for the evaluation included information on the dogs’ cancer stage and grade, treatments received, and outcome — specifically, how long it took for the cancer to metastasize.

Sorenmo and her colleagues learned that dogs with advanced stage cancer but a low-grade tumor (one that isn’t fast-growing) can have a better outcome than dogs with a lower stage but more advanced grade. They also discovered that the histology of a mammary tumor — the molecular markers that characterize cancer cells — also influences how the tumor behaves and responds to treatment.

In order to pull all the data together, the researchers borrowed the concept of a scoring system used to categorize human breast cancers. They ultimately decided to look at three pieces of information to attempt to more accurately determine a dog's prognosis: the stage, grade, and histological type of the tumor.

"We found that if you had a grade 1 tumor, no matter how large it gets, the vast majority of the time it doesn't cause any trouble," Sorenmo says.

Since many of the dogs in the study were abandoned or in shelters and therefore had to wait for long periods to receive care, their tumors were unusually large. This allowed the researchers to identify this size-independent effect. They also found that even when the lymph nodes were involved in grade 1 tumor patients, the finding was insignificant — the cancer didn’t metastasize in those dogs.

The researchers were able to use the overall bio-score to determine which dogs were at lower risk of developing metastasis and might do fine with surgery alone. Dogs with a higher bio-score, on the other hand, might benefit from more aggressive treatment options, including chemotherapy.

Preventing Mammary Cancer Is as Important as Treating It

The work the Penn Vet team is doing to treat dogs with mammary cancer is invaluable. However, in my experience there’s something very important missing from this equation: why did these dogs develop mammary tumors in the first place? We can cut cancers out of the body, but we can't address why the body allowed the disease to occur unless we identify why the immune system failed and allowed cancer cells to proliferate.

For dogs with mammary tumors, it’s critical to evaluate the chemical load (including unidentified sources of xenoestrogens) in their environment. This type of cancer is linked to estrogen levels, which can be toxically high not only in intact females, but also in spayed females and neutered males.

Desexing dogs at a very young age does not guarantee they won't end up with mammary cancer, despite what we've been led to believe. As another recent Penn Vet study states:

"These results highlight the dual effect of estrogen in cancer: Estrogen acts as a pro-carcinogen in Estrogen Receptor (ER) positive mammary tumors, but a may have a protective effect in ER negative tumors, potentially via non-receptor mechanisms. The latter is supported by the decreased risk for non-mammary tumors in dogs with high serum estrogen, and explains the increased incidence of certain non-mammary tumors in in dogs spayed at an early age."4

Even animals no longer producing their own estrogen after being spayed or neutered can be exposed to overwhelming amounts of estrogen mimicking chemicals in the environment. I have seen many patients over the years with wildly unbalanced endocrine systems, including male dogs with estrogen levels higher than what is normal for intact females.

Treating Estrogen Toxicity in Mammary Cancer Patients

When I find mammary tumors in a dog, I immediately measure the sex hormone levels. If estrogen is elevated, after removing the tumors I institute a protocol including DIM (diindolymethane) and high-lignan flax hulls, which may help to naturally reduce estrogen levels.5

DIM and flax hulls (not flaxseeds or flaxseed oil) have been shown to promote beneficial estrogen metabolism in both males and females. The body's ability to effectively metabolize estrogen is an important component in the prevention of certain cancers, in particular breast cancer.6

Dietary adjustments, including the elimination of all estrogenic foods (e.g., soy and yams) and highly processed foods created via the extrusion process (kibble) is important because the manufacture of kibble creates carcinogenic byproducts. Feeding a fresh, ketogenic, high-fat, no-carb (starch-free), nutritionally balanced and species-appropriate diet is also part of the protocol, along with beneficial immune-support supplements.

Thankfully there are a growing number of integrative veterinary oncologists popping up around the globe who can work with your veterinary surgeon and local integrative veterinarian to customize a treatment plan to achieve maximum benefits with the fewest possible side effects.

Sources of Xenoestrogens

In my opinion, exposure to xenoestrogens — chemicals that mimic the hormone estrogen — plays a significant role in elevated estrogen levels in dogs. Examples of xenoestrogens include:

Atrazine (weed killer)

Heptachlor (restricted insecticide)

4-Methylbenzylidene camphor (4-MBC) (found in sunscreen lotions)

Lindane, hexachlorocyclohexane (restricted insecticide)

Butylated hydroxyanisole (BHA) (food preservative)

Methoxychlor (banned insecticide in U.S.)

Bisphenol A (used to make plastics)

Nonylphenol and derivatives (laboratory detergents; pesticides)

Dichlorodiphenyldichloroethylene (DDT)

Pentachlorophenol (restricted biocide in U.S.; wood preservative)

Dieldrin (banned insecticide)

Polychlorinated biphenyls (PCBs)

DDT (banned insecticide in the U.S. but not other countries)

Parabens (lotions)

Endosulfan (banned insecticide in U.S.)

Phthalates (used to make plastics)

Erythrosine (FD&C Red No. 3)

DEHP (found in PVC)

Ethinylestradiol (oral contraceptive)

Propyl gallate (used to preserve oils and fats)

The problem with this list is that these chemicals often aren’t plainly labeled as such in many products found around the house. For instance, plug-ins, car fresheners, scented candles, room sprays and gel air fresheners are loaded with chemicals on this list, but manufacturers aren’t required to list them on product labels.

Endocrine disrupters, which damage your dog’s hormonal axis, including estrogen balance, are also found on many fabrics treated with flame-retardant chemicals (dog beds, carpets, couches, draperies).

Another endocrine disruptor is BPA, which is found in the lining of canned dog food containers and plastic food and water bowls, not to mention cleaning supplies that instruct you to call poison control if ingested. Always remember that any product used in your house has the potential to end up inside your pet.

It’s also important to keep in mind that pesticides and chemicals banned in the U.S. still show up on and in products imported from other countries.

12 Ways to Reduce Your Pet’s Exposure to Xenoestrogens

To reduce your dog's exposure to these estrogen-mimicking compounds, and thereby potentially lower the risk of mammary cancer:

  1. Use stainless steel, glass, or ceramic food and water bowls
  2. Avoid plastic storage containers for pet food or water
  3. Don’t microwave pet food in plastic containers
  4. Don’t use non-stick cookware if you cook food for your pet
  5. Avoid using cling wrap that contains DEHA
  6. Avoid pet foods containing soy, the preservatives BHA and BHT, and the food dye FD&C Red No. 3
  7. Use natural pest control around your home and yard
  8. Use alternatives to chemical flea/tick repellents
  9. Use all-natural, non-toxic cleaning supplies inside your home
  10. Buy organic dog beds
  11. Remove fluoride and chlorine from drinking water
  12. Don’t buy canned food unless it’s labeled BPA-free

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