Shelter dogs provide an ideal population for studying mammary tumors, because only 10 percent of animals received into shelters have been spayed or neutered. The incidence of mammary tumors in unspayed female dogs is at least four times greater than in spayed dogs. A female dog spayed before she comes into her first heat cycle has only a .5 percent chance of developing a mammary tumor. This is why most veterinarians recommend that female dogs be spayed at a young age if they are not going to be bred.

Few of the dogs referred to Penn’s Matthew Ryan Veterinary Hospital for treatment, for example, are unspayed, says Michael Moyer V’90, Rosenthal Director of Shelter Animal Medicine and adjunct associate professor of shelter medicine. “Ninety-five to 99 percent of the female dogs we see have been spayed, which virtually eliminates their risk of developing mammary tumors.”

Not so for shelter dogs. Most of these animals have what Moyer calls “a low level of attachment to a household.” Many have been acquired by owners who lack the wherewithal to provide regular veterinary care. Many are strays. Most are older (between eight and 10 years), which is when mammary tumors typically develop.

For these reasons, the incidence of mammary tumors in shelter dogs is much higher than in those with permanent owners, and for these dogs the Penn program is a literal lifesaver. “Without surgery, most of these dogs have a low chance of being adopted,” says Moyer.

Not all shelter dogs with mammary tumors are accepted into the program. “We screen the candidates and don’t do surgery if the cancer has spread to the lungs,” explains Sorenmo. “The dogs also have to have good personalities and the shelter has to agree to tell potential owners about the risks associated with adopting a dog with mammary tumors.” That said, not all mammary tumors are malignant—although veterinary surgeons do remove all (usually 10) mammary glands. “There is no such thing as ‘breast sparing surgery’ in dogs,” says Sorenmo. “The standard of care is to remove all tumors, regardless of how many and regardless of what stage they are in.”

It is precisely the opportunity to examine the development of mammary tumors from non-existent to benign (or pre-malignant) to malignant that makes this study particularly intriguing to oncologists studying human cancers. “In a person, you rarely see progression,” says Domchek. “With the dogs, there is so much breast tissue that you get a snapshot of everything at once. You are able to see the entire spectrum of cancer development and search for patterns of gene expression across the continuum.” While doctors don’t know yet if these are the same genes, and some histology between dog and human cancer is slightly different, the opportunity to see the full range of cancer development is “amazing,” she says.

“Dogs present with multiple tumors in multiple breasts and some are precursors for cancer while others are not,” Vonderheide adds. “Women with breast cancer do not present with the full range of lesions that are common in shelter dogs.” The fact that the cancer in dogs occurs spontaneously (rather than having been induced in a lab setting) makes it even more relevant. “These are tumors that arise in an out-bred, aging mammal exposed to the same environment that we are,” he says. “Histologically, these tumors seem to resemble the tumors that humans get.”

Traditional funding sources have been slow to recognize the value of such research. “What is almost always misunderstood is that this is not a study on lab animals based on the traditional model of animal studies,” says Vonderheide. “We are not inducing cancer; these animals are pets with tumors.” At the very least, he says, this study “seems to make the case for NIH funding in the area of comparative oncology. We can learn so much.”

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FEATURE: A Double Reward by Kathryn Levy Feldman
Photography by Candace diCarlo
©2010 The Pennsylvania Gazette

The vet school’s chief of medical oncology, Karin Sorenmo (right),
who developed the mammary tumor program, and Michael Moyer, director of shelter animal medicine, with a patient.


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Last modified 8/25/10