Cancer is common in dogs. About one in four will develop cancer at some point during its lifetime—and that proportion rises to an estimate of nearly 50 percent after a dog passes its 10th birthday. A new analysis of thousands of dogs finds that traits such as size, breed and whether an animal has been fixed are associated with how soon our furry friends might get diagnosed with the disease.

Researchers have examined the age of cancer diagnosis in dogs in the past—but those studies have mostly looked at specific breeds or cancer types. Some have been conducted in Europe, where the common breeds, as well as other factors such as neutering or spaying practices, differ from those in the U.S., says veterinary oncologist Andi Flory, co-founder and chief medical officer of PetDx, a California-based pet diagnostics company. “We really wanted to develop something that was applicable to the population of dogs that we have here [in the U.S.],” she says.

To determine what factors were associated with age of cancer diagnosis, Flory and her team at PetDx evaluated previously collected data from 3,452 dogs in three separate groups. Two of those groups of samples came from academic sites within the U.S.: one from the University of California, Davis, and another from a consortium that included Colorado State University, the Ohio State University, the University of Wisconsin–Madison and others. Samples from the third cohort were collected in a prior study by PetDx, which was carried out at 41 different sites in several countries, including the U.S., Canada and Brazil.

The researchers found that, overall, the dogs’ median age at cancer diagnosis was 8.8 years. Males were diagnosed at a younger median age than females (8.4 versus nine years). And neutered and spayed dogs were spotted later than those that had not been fixed (8.9 versus 7.9 years in males and nine versus 7.3 years in females). The team also found that purebred dogs tended to be detected at younger median ages than mixed-breed ones (eight versus 9.5 years) and that larger size was associated with earlier diagnosis. Dog breeds with the youngest median age of cancer diagnosis (seven years or earlier) included mastiffs, Saint Bernards, Great Danes and bulldogs. Those on the other end of the spectrum included bichon frises, West Highland white terriers and American Staffordshire terriers, all of which had a median age of diagnosis of 10.5 years or more. The results were published this week in PLOS ONE. 

Chart shows that the median age of cancer diagnoses varies widely among dog breeds, with mastiffs having the youngest age: five years.
Credit: Jade Khatib; Source: “Age at Cancer Diagnosis by Breed, Weight, Sex, and Cancer type in a Cohort of More Than 3,000 Dogs: Determining the Optimal Age to Initiate Cancer Screening in Canine Patients” by Jill M. Rafalko, et al., in PLOS ONE, Vol. 18, No. 2, Article No. e0280795. Published online February 1, 2023

“I’m happy to see more larger-scale studies like this in veterinary medicine because it wasn’t very long ago that we really didn’t have this kind of research,” says Lisa Moses, a veterinarian and bioethicist at Harvard Medical School, who was not involved in this work. “It certainly adds to what we know about the onset and diagnosis of cancer in different purebred dogs, which is certainly interesting—and, in the future, will be important to people like me, who are clinical veterinarians.”

Based on these findings, as well as evidence regarding tumor size and progression from prior studies in humans and canines, the authors of the study recommend that all animals start cancer screening at age seven and that breeds with a lower median age of diagnosis get screened earlier. Being able to detect cancer earlier can enable veterinarians to provide families with more options for treatment and care, Flory says. PetDx offers a blood-based canine cancer test called OncoK9, which works by searching for cancer-associated mutations in DNA floating outside of cells. The test, which is available in North America, costs around $500. Liquid biopsy assays “are considered multicancer detection tests,” says Jill Rafalko, director of scientific communications at PetDx. “We’re looking for any genomic alterations in the blood, which could signal a variety of different cancer types that could be present in that dog.”

But experts who are not involved in this study are wary of the cancer screening recommendations. A key problem, according to Cheryl London, a veterinary oncologist at Tufts University, is the lack of effective early canine cancer screening tools. “If we’re talking about true cancer screening, we don’t have the tools in veterinary medicine to actually do that yet,” London says. Tools commonly used in veterinary practices, such as ultrasounds and x-rays, are not sensitive enough to detect most early cancers—and based on the data published by PetDx to date, neither are liquid biopsies, she adds. (London is on the scientific advisory board of One Health Company, a California-based company focused on diagnostics and personalized therapies for canine cancer.)

PetDx published a validation study of OncoK9 in PLOS ONE in 2022. It reported an overall sensitivity (the ability to pinpoint true cases) of 54.7 percent and a specificity (the ability to avoid detecting false positives) of 98.5 percent. In general, the test’s detection rate was much higher for larger and more advanced cancers: while the rate for large metastasized cancers was 87.5 percent, the one for small, localized cancers was just 19.6 percent.

Another issue is that the team based its recommendation to screen two years prior to the median age of diagnosis based solely on the modeling of physical characteristics of tumors, Moses says. As a veterinarian, “I’m way more concerned about issues like morbidity and quality of life,” she adds. What’s missing, according to Moses, are studies showing that early screening will lead to tangible benefits for dogs’ health and well-being. “As a clinician, I need a lot more information about whether or not knowing this information sooner is going to help me help dogs,” she says.

London notes that while the data from this study are useful for confirming previously identified patterns in canine cancer diagnoses, the fact that a company selling a screening test for cancer is making a recommendation as to when to start using the test is a “major of conflict of interest.” Based on the information currently available, “we have no ability to make a screening recommendation,” London says. “You’re asking people to spend a lot of money. The consumer has to be aware of what the tests can and can’t do. And right now they cannot detect cancer early—period.”