Can We Detect Cancer Before Dogs Show Symptoms? The Genetics Revolution in Veterinary Oncology.

By Steve Mehler, DVM, DACVS

Cancer is one of the leading causes of death in older dogs. In fact, approximately one in four dogs will develop cancer during their lifetime, and the risk increases dramatically as they age.

For decades, veterinary medicine has relied on a familiar approach: wait until a dog develops clinical signs, find a mass during a physical examination, or discover abnormalities on routine bloodwork or imaging. By the time many cancers are diagnosed, they have already progressed.

But what if we could identify cancer before a dog ever becomes sick?

A growing body of research suggests that we are entering a new era of veterinary oncology—one driven by genetics.

Cancer Is a Genetic Disease

When many people hear the word "genetics," they think about inherited diseases or breed predispositions.

Cancer genetics is a little different.

Most cancers develop because individual cells accumulate genetic mutations over time. These mutations alter how cells grow, divide, repair DNA, and eventually evade the body's normal defenses.

Some of these mutations are inherited, explaining why certain breeds are more susceptible to particular cancers. Others are acquired during a dog's lifetime due to aging, environmental exposures, or simply random errors that occur when cells divide.

These genetic changes happen long before a tumor becomes large enough to feel or causes obvious clinical signs.

That creates an exciting opportunity.

If we can detect those genetic alterations early enough, we may be able to identify cancer before it becomes clinically apparent.

Breed Matters

One of the most fascinating aspects of canine oncology is that dogs naturally develop many of the same cancers as people—but certain breeds carry dramatically different risks.

For example:

• Golden Retrievers have increased risk for lymphoma and hemangiosarcoma.

• Boxers are predisposed to mast cell tumors.

• Rottweilers have higher rates of osteosarcoma.

• Bernese Mountain Dogs are at increased risk for histiocytic sarcoma.

These breed-specific risks have made dogs an incredible model for cancer genetics research.

By understanding why certain breeds develop particular cancers, researchers hope to improve prevention, diagnosis, and treatment—not only for dogs but potentially for people as well.

What Is a Liquid Biopsy?

Traditional cancer diagnosis usually requires finding a tumor and obtaining tissue through aspiration or biopsy.

A liquid biopsy works differently.

Cancer cells constantly shed tiny fragments of DNA into the bloodstream.

These fragments are known as cell-free DNA (cfDNA).

Using next-generation sequencing (NGS), laboratories can analyze these DNA fragments and look for genetic mutations associated with cancer.

Rather than looking for a tumor directly, the test looks for the genetic fingerprints that tumors leave behind.

Think of it like detecting smoke before you ever see the fire.

The CLASSiC Study

One of the largest prospective studies evaluating this technology in dogs is the Cancer Lifetime Assessment Screening Study in Canines (CLASSiC).

Researchers enrolled 725 client-owned dogs from 24 veterinary hospitals across the United States and Canada, with most dogs considered high-risk because of age and/or breed. Dogs underwent routine wellness examinations combined with next-generation sequencing–based liquid biopsy screening.

For the interim analysis, 417 dogs had sufficient follow-up for evaluation.

What Did They Find?

During the study:

• 51 dogs developed cancer 

• Overall cancer incidence was approximately 12% over the observation period.

The liquid biopsy demonstrated:

• Sensitivity: 56.9%

• Specificity: 98.9%

• Positive predictive value: 87.9%

• Negative predictive value: 94.3%

Those numbers deserve some explanation.

Understanding the Numbers

A sensitivity of 56.9% means the test detected a little more than half of the cancers that eventually developed.

That also means it missed some cancers.

This is important.

A negative result does not guarantee that a dog is cancer-free.

However, the specificity of nearly 99% means false positives were uncommon.

When the test identified a cancer signal, there was a high likelihood that cancer truly was present.

Like any screening test, it should never replace a physical examination or appropriate diagnostic workup.

Instead, it serves as another tool that veterinarians can use alongside traditional diagnostics.

Earlier Detection

Perhaps the most impressive finding involved preclinical detection.

Using routine veterinary care alone, only 6 of the 51 cancers (12%) were identified before dogs developed obvious clinical disease.

When liquid biopsy was added, 28 of the 51 cancers (55%) were detected during the preclinical period—a 4.6-fold increase in early detection.

The study also found that adding liquid biopsy to routine care doubled the total number of cancers detected in this high-risk population.

That is potentially practice-changing.

Which Cancers Were Found?

The study detected a wide variety of cancers, including:

• Hemangiosarcoma

• Lymphoma

• Osteosarcoma

• Hepatocellular carcinoma

• Prostatic carcinoma

• Histiocytic sarcoma

• Soft tissue sarcoma

• Mast cell tumors

• Mammary carcinoma

• Brain tumors

The technology appeared particularly useful for cancers that occur internally—such as those affecting the liver, spleen, heart, and lungs—where tumors can be difficult to identify during a routine physical examination.

Does Earlier Detection Improve Survival?

This is the question everyone wants answered.

The honest answer is:

We don't know yet.

Human medicine has repeatedly shown that early detection can improve outcomes for many cancers.

In veterinary medicine, earlier diagnosis often creates more treatment options, but the CLASSiC study was not designed to prove that screening extends survival. Instead, it demonstrated that cancers can be detected earlier and more frequently when liquid biopsy is added to routine wellness care.

Future studies will need to determine whether those earlier diagnoses ultimately translate into longer survival or improved quality of life.

Is This Test for Every Dog?

Probably not.

Current evidence suggests this technology may be most valuable for:

• Senior dogs

• Dogs from breeds with elevated cancer risk

• Dogs undergoing annual or semiannual wellness screening

• Owners interested in proactive cancer detection

For younger dogs or breeds with relatively low cancer risk, routine physical examinations remain the cornerstone of preventive care.

As more research becomes available, recommendations will likely continue to evolve.

The Bigger Picture

Cancer isn't one disease.

It is hundreds of different diseases, each driven by unique genetic changes.

That realization is transforming veterinary oncology.

Instead of waiting for tumors to become large enough to see or feel, veterinarians are increasingly looking for the earliest genetic evidence that cancer exists.

This shift mirrors what has happened in human oncology over the past decade.

Genetic sequencing, precision medicine, targeted therapies, and liquid biopsy are rapidly changing how we think about cancer—not just how we treat it, but how we detect it.

Final Thoughts

The CLASSiC study represents an exciting milestone in veterinary oncology.

While liquid biopsy is not a replacement for physical examinations, imaging, or biopsy, it appears to be a valuable addition to comprehensive cancer screening for appropriately selected dogs.

Perhaps the most important message is this:

The future of veterinary oncology isn't just about better chemotherapy.

It's about understanding the genetics of cancer well enough to find disease earlier, personalize treatment, and ultimately improve outcomes for our canine patients.

As genomic medicine continues to evolve, the next decade may fundamentally change how veterinarians diagnose—and perhaps someday prevent—many forms of canine cancer.