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The Daily Vet by petMD

The Daily Vet is a blog featuring veterinarians from all walks of life. Every week they will tackle entertaining, interesting, and sometimes difficult topics in the world of animal medicine – all in the hopes that their unique insights and personal experiences will help you to understand your pets.

Mammary Tumors: New Finding in Dogs

Sexually intact female dogs more commonly have mammary tumors than other tumor types. Fortunately, studies indicate that sixty percent of those tumors are benign. Ninety-nine percent of tumors less than .5 inches in size are benign and fifty percent of tumors larger than 1.5 inches are also benign. Reducing ovarian hormone levels by early spaying has been a long standing veterinary strategy for the prevention of mammary tumors.

With recent studies indicating that early spaying may actually increase the risk of joint disorders and other tumor and cancer types, veterinarians and pet owners are beginning to question the wisdom of early sexual alteration. Such a change in strategy may increase the incidence of benign mammary tumors. A study in the latest Journal of Veterinary Internal Medicine examined the effect of spaying at the time of benign tumor removal.

Why Worry About Mammary Tumors in Dogs?

Benign mammary tumors in dogs have a wide range of irregularity called “atypical mammary hyperplasia.” It is known in human females that greater levels of atypical changes in mammary cells and the amount of exposure to ovarian hormones increases the risk for the later development of malignant breast cancer. It is speculated that the same may be true in dogs. Twenty-five percent of dogs that have undergone surgery to remove benign mammary tumors experience recurrence of more tumors, many of which are malignant with significant risk of metastasis to other parts of the body and early death.

Studies have also shown that the hyperplastic mammary tissue in dogs has large numbers of estrogen and progesterone receptors that suggests a hormonal influence on mammary cell behavior. This study sought to show that spaying and the removal of sex hormones at the time of tumor removal might influence future occurrence of mammary tumors.

The Spayed vs. Non Spayed Group Study

Eighty-four dogs with diagnosed benign mammary tumors were enrolled in this well designed study. Forty-two of the dogs were spayed at the time of tumor removal. None of the dogs had normal mammary tissue removed.

After surgery the dogs were tracked for more than seven years to gather statistical information. The researchers found that after this time, sixty-three percent of the spayed group were tumor free and thirty-sex percent of the non-spayed were tumor free. Statistically this is a reduced risk factor of forty-seven percent for the spayed group.

Disappointing Findings

The difference in death due to mammary tumors was not significant between the groups. The spayed group actually experienced a greater incidence of malignant tumor recurrence than the non-spayed group. Tumor recurrence was not restricted to the same side as the initial tumor.

Forty-two percent of dogs with a single tumor on one side had tumor regrowth on the opposite mammary chain. This suggests that prophylactic (preventative) mastectomy requires the removal of all mammary tissue at the time of tumor removal and spaying. Needless to say, such an approach increases surgical and anesthetic risks and complications and is difficult to justify when only thirty-six percent of the spayed group had tumor recurrence.

Mammary Tumors in Cats

Mammary tumors in cats act differently. It is estimated that eighty-five percent of mammary tumors in cats are malignant. Spaying cats reduces the risk of mammary tumors to .6 percent, a negligible risk.

When to Spay Dogs?

Dogs spayed before their first heat cycle have only a .5 percent risk, or virtually no risk of developing mammary cancer. The risk increases to eight percent when spayed after the second heat. By 2.5 years of age spaying offers no decreased risk benefit. This argues for early spaying since mammary tumors are very common.

However, the new studies suggesting that the absence of ovarian hormones may predispose dogs to a greater risk of joint disease and other cancer types is at odds with early neutering. It is going to take time and much more research to reveal the better course of action. In the meantime, discuss the pros and cons with your veterinarian and keep abreast of the latest research.

Dr. Ken Tudor

Image: Sinseeho / Shutterstock

Comments  7

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  • Scary Thought
    08/15/2013 04:25pm

    Reading that article made me dread that I neutered myt male at just under 5 months. He is six now and already has joint problems. I presume the lack of hormones affects the males the same way. It is appaling that all these proceedures are done , albeit necessary to control the population, yet they shorten our beloved dogs lives. It made me very sad and I question neutering our six month old Boston now...

  • 08/15/2013 04:48pm

    Medicine and human medicine are always evolving as we learn new information. When I was young it was customary to routinely take children's' tonsils out. With research we found that was a bad idea. Like in human medicine, especially, heart disease, research discovers associations. Associations are not causation. For instance we can't say having 2 glasses of wine a day prevents heart disease. We only know that people who drink 2 glasses of wine a day have a lower incidence of heart disease. Is it the wine, a chemical in the wine (the reds), is it the decrease in stress because of the wine or is their something else about these people's lifestyle. We simply don't know yet. It is the same with this and other studies, they are preliminary. We can't say for certain that early neutering caused arthritis in your dog. But you are right to use this information to question neutering your other dog. Do more research and discuss it with your vet. At any point in time we never have all the answers, we just try to make the best decision we can based on what we know. I hope this helps ease your guilt.
    Dr. Ken

  • 08/24/2013 06:29am

    It is not necessary to remove testicles and ovaries for population control. Females can have a hysterectomy and leave the ovaries intact and male dogs can have a vasectomy. Also responsible owners who don't let their pets roam the neighborhood wouldn't have to worry about population control anyway, pregnancy is not an airborne disease. Containment is quite effective.

  • 08/24/2013 01:35pm

    Thank you for that response. We will consider asking for a vasectomy for our Boston when we neuter him later this fall. We have decided to let him develop a while longer hormonally to hopefully prevent joint problems later. He is so hyper though, that we hoped neutering will calm him down. It always has had that effect on our dogs when they were "fixed." It also made them less eager to "wander." However, if the testicles are not removed, I would think that won't happen with the hormones still being produced. Am I correct?

  • 08/28/2013 04:49am

    you are correct, if you don't remove the testicles, and just do a vasectomy the dog cannot reproduce, but still has his hormones. That's why in human males they do a vasectomy for sterilization purposes. Your Boston pup may be hyper at this point, but this is just normal part of adolescence and young adulthood. You can channel that energy by training him to have good manners and by giving him plenty of exercise. It also helps to feed a good quality food with adequate amounts of animal proteins and less carbs. The extra energy that is now a little bit of an inconvenience, comes in handy when your little one gets older or is a senior, then that little extra energy and muscle mass makes a big difference. Just think about all the ads they have on TV about testosterone replacement for men and do some research of the symptoms of low testosterone in humans.

  • Mammary Tumors
    08/15/2013 10:05pm

    "It is known in human females that greater levels of atypical changes in mammary cells and the amount of exposure to ovarian hormones increases the risk for the later development of malignant breast cancer."

    We hear about prophylactic mastectomies for some women. I wonder if there will come a time for prophylactic hysterectomies.

    It's really interesting that the statistics don't hold true for kitties for both partially preventing mammary tumors as well as the incidence of cancerous tumors. Any thoughts why that's true for kitties?

  • 08/16/2013 12:01am

    We perform ovariohysterectomies so the ovaries and hormones are removed. That would not be the case with women where only the uterus is removed. Because estrogen is important for women in other respects (osteoporosis, early menopause, etc.), I doubt that removing ovaries is on the table in human medicine. We have no idea why the higher incidence and vengeful nature of mammary tumors is different in cats.
    Dr. Ken

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