Visiting Vet: Liquid biopsy

Advanced blood tests allow for better cancer screenings, diagnosis, and monitoring.

A dog can undergo cancer screening tests that require just a blood sample. — Karsten Winegeart

One of my patients is a sweet Great Pyrenees–Bernese mountain dog mix named Sol. His owner recently told me that Sol‘s brother had died suddenly from cancer. “Is there any way you can check Sol?” they asked. Until recently, the answer would have been “No, not really.” If an animal was not showing clinical signs of illness, and/or there was no obvious tumor, options were limited. Folks are often surprised to learn that pets with cancer can have completely normal values on many standard blood tests. Imaging such as radiographs, ultrasound, MRI, and CT scan are useful, but prohibitively expensive and time-consuming for routine cancer screening in pets, not to mention the cost and risk of repeatedly anesthetizing animals for what may be unnecessary procedures.

This year, however, I was able to tell Sol’s owner that they had options. There are now two new cancer screening tests available for dogs that require just a blood sample. That’s right. Just a blood sample. But wait! Before you rush to the vet, let’s discuss pros and cons, and benefits and limitations of these tests. First, you need to learn a little science.

Cancer: It’s complicated. What you need to know is that cancer is a disease of alteration of the genome. In other words, something goes wrong with the DNA in some cells somewhere in the body. Remember DNA from biology class? That long double-helix polymer made up of a bunch of nucleotides? Cancer occurs when the DNA changes. It can be anything from a change in a single nucleotide to insertion or deletion of hundreds of nucleotides. The next thing to know is that even healthy cells die in the body all the time. It’s normal. New healthy cells grow to replace the old. When cells die, they break down and release DNA. This is called cell-free or cf-DNA. If the cells that are breaking down have genomic alterations, that can be identified in the DNA being released, which is now called circulating-tumor or ct-DNA.

Those of you sleeping in the back row, wake up. PetDx is a laboratory now offering a test called OncoK9 Liquid Biopsy, which looks for circulating-tumor, ct-DNA, in the blood. The test is very specific, meaning that if the lab finds Sol has ct-DNA, there is a 98 percent chance he has a tumor somewhere. On the flip side, however, a negative test is not a guarantee Sol is cancer-free. Some types of cancer “shed” more ct-DNA than others, and many shed very little in the early stages. Overall, OncoK9 will pick up about half of all cancers. So you can trust a positive result really does mean cancer, while a negative one may be false, and warrant repeat testing if suspicions are high.

Should Sol be tested? One in three dogs in the U.S. will develop cancer at some point during their lifetime. The odds increase greatly after the age of 8 years and, just like in people, early detection saves lives. It is not unreasonable to run this test annually on all dogs 7 and older, if owners can afford it. There’s the rub. It is expensive, running $500 to $600 or more. And remember, it only says if Sol has a high probability of having cancer right now. It does not predict if he might develop cancer later in life.

There are circumstances in which dogs might benefit from being screened younger, starting at 4 years old, particularly in breeds known to be particularly cancer-prone: golden retrievers, boxers, Labrador retrievers, German shepherds, and most giant breeds, whose shorter lifespan may predispose them to cancer earlier. In addition to using the OncoK9 Liquid Biopsy just as a screening tool in healthy dogs, it would be a great test for any individual case in which we are suspicious of cancer due to clinical signs of illness. It can also be used to evaluate response to cancer treatments, and monitoring for recurrence.

If Sol’s owner decides to proceed with the OncoK9 Liquid Biopsy, here’s how it works. First I have to order special tubes from the lab. Then we draw 15 milliliters of blood from our patient. That’s a tablespoon — a bigger sample than we generally draw for other types of tests. It is a safe amount of blood to take, unless the dog is very small or very anemic. That’s all there is to it, at least to start. The veterinarian ships the samples in the special tubes to PetDx, where the DNA is extracted, amplified, and sequenced, looking for ct-DNA. Results are back in 10 to 14 days or less.

Now comes the tricky part. What do we do with those results? The Liquid Biopsy can pick up ct-DNA of about 20 different kinds of cancer, but our “result” is just a “cancer signal detected” or “not detected.” In some cases, the report may include an “origin prediction,” suggesting what type of cancer it may be. At that point, an owner is faced with deciding how much to spend on additional testing to try to track down the tumor(s). If we locate the cancer, treatment can be initiated quickly. If the cancer cannot be pinpointed immediately, then we can monitor the dog closely, with a thorough physical exam every few months. PetDx emphasizes that “the result should not be used as the sole basis for making important decisions such as treatment or euthanasia.”

There is another much less expensive canine cancer screening, called NuQ, available through Idexx Laboratories. This one looks for things called nucleosomes in the blood. I will spare you another science class. NuQ checks for seven common cancers. It is not as able to identify cancer origins, and is less accurate in dogs who are suffering concurrently from other illnesses and chronic conditions. NuQ may be worth doing when finances are limited, but the OncoK9 Liquid Biopsy gives us significantly more information. Together, they make exciting new additions to the veterinary diagnostic toolbox in our fight against cancer.