Mammary Tumors: New Finding in Dogs

By Ken Tudor, DVM on Aug. 15, 2013

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.