New scoring system for canine mammary cancer decision-making

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Taking a page from human medicine can help veterinarians determine treatment strategies for dogs with mammary tumors.

That is the finding of a team headed by Karin Sorenmo, DVM, DACVIM, DECVIM-CA, a professor of oncology at the University of Pennsylvania School of Veterinary Medicine (Penn Vet) and head of Penn Vet's Shelter Canine Mammary Tumor Program.

While statistics show that one-quarter of unspayed female dogs will develop a mammary tumor during their lifetime, prognostic tools now in use to help determine tumor stage and the likelihood of progression don't always give veterinarians an accurate portrait of what will come next, or the best course of treatment.

Dr. Sorenmo and her team devised a “bio-scoring” system based on how breast cancers are categorized in humans, and recently published their methods and findings in Veterinary and Comparative Oncology.

The researchers used information from 96 dogs treated for mammary cancer as part of Penn Vet's Shelter Canine Mammary Tumor Program, which treats homeless dogs and then finds them foster or forever families, as well as 31 dogs similarly treated at Norwegian University of Life Sciences in Oslo, Norway.

The researchers used tumor stage and grade, treatments received to date and time to metastasis in developing and evaluating the scoring system.

According to Dr. Sorenmo, veterinarians normally consider tumor stage when determining a treatment course. While that system works for tumors that are more predictable, she said, mammary tumors are different.

Dogs may be classified as having advanced-stage disease, but if the tumor is low grade-and thus slow growing-those dogs may fare better than animals with lower stage disease and advanced tumor grades. In addition, tumor histology can also be predictive of tumor behavior and treatment outcome.

With no existing system putting all those factors together, Dr. Sorenmo and her team of researchers decided to apply a scoring system used to categorize breast cancers in people for use in dogs.

By using a tumor's stage, grade and histologic type, they found, for instance, that no matter how large a grade 1 tumor grows, “the vast majority of the time it doesn't cause any trouble,” Dr. Sorenmo said.

The same was true for lymph node involvement, which is thought to signal metastasis. “Again we found that with a grade 1 tumor it doesn't matter,” she said. “Those dogs didn't develop distant metastasis.”

The bio-score also led the investigators to determine the relative risk for metastasis, which could help in treatment decision making. Dogs with a higher bio-score may be better served with more aggressive treatment such as chemotherapy, whereas surgical intervention may suffice for those with a lower bio-score. 

Dr. Sorenmo hopes to take the group's findings further by singling out the effect of hormones on the tumors and the progression of cancer. She says she is pleased to give veterinarians a workable way to determine a course of treatment that they can use easily and immediately.

“My hope is that this system will be used so that dogs that need systemic therapy after surgery can be identified and, equally important, dogs that have very low risk for metastasis do not have to go through chemotherapy,” Dr. Sorenmo told dvm360. “It is easy to use and with a good pathologist reading the biopsies it should be reproducible.”

Supporting the study was the PennVet Shelter Canine Mammary Tumor Program, the Petco Foundation and the Blue Buffalo Foundation.