A Primer on Canine Cancers

A lump, a limp, a loss of appetite. Notice one of these in your dog and your first thought is likely to be, “Huh. Wonder what’s causing that?” Your second thought may be, “Could it be cancer?” It’s a reasonable question, especially if your dog is middle-aged or older. According to the American Veterinary Medical Association (AVMA), one in four dogs will develop some form of cancer during their lifetimes.

What Is Cancer?

When cells begin replicating uncontrollably, the growth is called a neoplasm (literally, new growth), or tumor. A neoplasm can be either benign or malignant; only malignant neoplasms are considered cancers. As the AVMA notes on its website, “Benign neoplasms tend to grow slowly; displace, but do not tend to invade, the surrounding body tissues; and do not spread throughout the body. Malignant neoplasms, on the other hand, can be unpredictable and grow at various rates (sometimes rapidly); invade the tissues around them; and spread, or metastasize, to other parts of the body. The word tumor or mass is often used to describe the actual swelling or other physical appearance of a neoplasm.”

While we use the word as a singular, there are many types of cancers, categorized by the organs or systems they affect.

• Carcinoma: Begins in the skin or tissues that line or cover the internal organs.
• Sarcoma: Begins in bone, cartilage, fat, muscle, blood vessels, or other connective or supportive tissue.
• Leukemia: Begins in blood-forming tissue.
• Lymphoma: Begins in the cells of the immune system.

A cancer metastasizes, or spreads, when cells break away from the main tumor and enter the bloodstream or lymphatic system, which carry fluids around the body—basically, the cells use these systems as highways to find new places to grow. While, theoretically, all cancers can metastasize, not all cancer cells have the capacity on their own to do so, or may lie dormant for months afterward moving away from their source.

Dogs tend to develop most cancers later in their lives, but some breeds are known to be at higher risk for specific types of cancers, like osteosarcoma, often at a younger age. A breed’s genetic vulnerability isn’t the only factor, however. Individual dogs of any breed (or mix) have higher or lower sensitivities that may affect whether or not they develop cancer, and how that cancer behaves.

Based on research in both humans and animals, we know that cancer is a complex process that can be triggered by hereditary, environmental and nutritional factors. In both people and animals, cancer development is a complex process that proceeds gradually through the interaction of multiple factors.

—Merck Veterinary Manual

What Are the Treatment Options?

How a canine cancer is treated depends on where it is, what system(s) it affects and how that particular cancer is known to respond to the various available options. Often, more than one approach is used. Barring the possibility of completely eliminating the cancer, veterinary specialists look for ways to put it into remission and/or help dogs and their owners cope with the challenges it creates.

Surgery tops the list of the most common strategies. Removal of a malignant tumor with complete margins—encased in healthy tissue—offers a hopeful prognosis. Surgery is also used to improve a dog’s quality of life. For example, in the case of leg-based bone cancer (appendicular osteosarcoma), amputation of the affected limb can significantly reduce the dog’s pain. While it isn’t necessarily a cure, it helps a dog be more comfortable. Surgery may be followed by chemotherapy and/or radiation, or they may be used on their own.

Chemotherapy uses drugs to kill or slow the growth of cancer cells, and may be used to treat a cancer that doesn’t lend itself to surgical intervention, or to reduce the size of a tumor before surgery is attempted. It’s also used to eliminate microscopic cancer cells before they can spread to other parts of the body. Unlike people, dogs rarely experience the side effects common to humans; 70 percent suffer few, if any, side effects.

Radiation therapy destroys tumor cells’ DNA and is most often used with solid tumors—carcinomas or sarcomas. The goal is to provide long-term control by shrinking tumors, and, possibly, a cure. Radiation is delivered as either external beam radiation therapy (also known as teletherapy) or stereotactic radiation therapy (SRT). SRT is particularly useful for brain and nasal cancers and can require only one to three treatments instead of the 15 to 20 needed for teletherapy. Other types of tumors, such as soft tissue sarcomas of the limbs, can be treated with repeated low doses of radiation, often resulting in a cure.

Another approach, cryosurgery—or more accurately, cryotherapy, since no cutting is involved—uses liquid nitrogen to freeze small skin and subcutaneous (under the skin) tumors; it reportedly has a very high success rate. Swelling, reddening, and occasionally pain and lameness are among the possible side effects. At the upper end of the temperature scale, hyperthermia uses heat to damage cancer cells, and is almost always paired with other forms of therapy rather than used as a standalone. On its own, it has few side effects.

Many of these approaches have been used for decades, and their limits and therapeutic potential are well known. However, in recent years, more options are being tested in vet school laboratories and clinics across the country. Among them, immunotherapy—a way of weaponizing a dog’s immune system to take action against a cancer by recognizing the danger and destroying the cancer cells—and genetic manipulation have had encouraging results.

For example, an experimental vaccine made from a dog’s individual tumor has been used to treat bone cancer. In a study at the University of Missouri, the treatment plan for dogs with osteosarcoma was amputation of the affected limb followed by a vaccine made from the dog’s tumor. According to Jeffrey Bryan, DVM, MS, PhD, DACVIM/Oncology, “It’s the first time that dogs with osteosarcoma have experienced prolonged survival without receiving chemotherapy, which is really exciting.”

Another recently developed approach involves sequencing a tumor’s DNA to identify cancer-causing mutations and then using a customized drug therapy to attack the cancer cells; the treatment is delivered in pill form and can be administered at home.

And on the horizon, metabolomics, the large-scale study of very small molecules. At the Cornell College of Veterinary Medicine, a study of a type of lymphoma common to both people and dogs began in mid-2020. To look for metabolic changes similar to those found in people with lymphoma, blood from healthy dogs is being analyzed and compared to samples from dogs who’ve been diagnosed with this cancer. According to a story in the Cornell Chronicle, “If the project is successful, it may one day allow oncologists to use a patient’s metabolic profile—as well as the DNA of the tumor—to design a precision medicine treatment plan. Genetic data shows what mutations a tumor has, but metabolomics reveal what is actually happening inside the cells.”

Who Manages Treatment?

Diagnosing and treating canine cancer may involve a three-part team: the dog’s general-practice veterinarian, a veterinary oncologist and, sometimes, a university teaching hospital. The initial diagnosis is most often made, or at least suggested, by the dog’s family veterinarian. Many vets will then refer their patient to a specialist to confirm the diagnosis and map out a treatment plan. Some of those specialists practice at university teaching hospitals, where the options and clinical capacities tend to be more extensive than at individual practices. Independent specialists may refer their patients to these facilities for treatment for the same reason.

What Are the Goals of Treatment?

The ideal and best-case scenario is to completely eliminate, or cure, the cancer. While this is sometimes possible, remission is usually a more realistic goal. Remission, or a regression of the cancer, can be partial—meaning that the disease’s burden on the body has been reduced by at least 50 percent—or complete—meaning that the cancer is undetectable by any standard screening test. Remission is rarely permanent, and dogs will usually experience a relapse at some point. If achieved, a second remission is likely to be shorter, since over time, cancer cells develop an immunity to the drugs used to fight them.

Cancers are wildly variable in their behaviors and responses to treatment; some, either because they’ve advanced too far or are too aggressive, are unlikely to go into remission. In that case, your veterinarian will treat your dog’s symptoms, including pain, with the goal of keeping him comfortable and maintaining his quality of life as long as possible.

What Are Clinical Trials and Why Should I Look for One?

Clinical trials—research studies aimed at evaluating a medical, surgical or behavioral intervention for a particular health problem—offer participants potential for a longer and better quality of life as well as an opportunity to help others (in this case, dogs) who may develop the same problem in the future.

Many are run by academic institutions; each has very specific criteria for participation; and they can go on for weeks, if not months. Depending on how they’re funded, they may be free or a fee may be required. It’s not always easy, or even possible, to find one within reasonable driving distance that would be a good fit for your dog’s cancer.

Regardless, once you have a diagnosis, it can be worth the effort to look for one that’s appropriate for your dog’s condition. The AVMA maintains a searchable animal health studies database, and the Veterinary Cancer Society has an accessible database that lists both universities and other organizations running trials. The Comparative Oncology Trials Consortium maintains a list of trials underway at vet schools across the nation. The nonprofit Morris Animal Foundation also provides a list of health studies that can be sorted by species and health issue.

Here’s an example that highlights the process. At the University of California, Davis, human oncologist Robert Canter and veterinary oncologist Michael Kent launched a clinical immunotherapy trial to test their theory that NK (natural killer) cells will attack tumor cells and stop them from creating new tumors. In June 2020, a chocolate Lab named Josie with oral melanoma who had been given two to four months to live was infused with NK cells from a healthy dog. While it’s too soon to tell if the theory will hold, six months later, Josie was back doing what she loved: retrieving ducks.

Cancer is a cellular glitch, an aberration, a system malfunction. Genetics may play a role and certainly, there are environmental stressors: tobacco smoke, chemicals associated with almost every aspect of everyday life and other things we probably aren’t even aware of yet. At any rate, very few cancers have a single known cause or restrict themselves to a single age, gender or breed type.

Statistics can be useful when considering various treatment options, but they don’t predict how an individual dog, or the cancer he’s dealing with, will respond to whatever approach is taken. Some cancers can be successfully treated, some can be put into remission for a time, and some are just too aggressive to overcome.

Whatever cancer challenge your dog may face, keep in mind that there are things you can do to improve the quality of his life and, ideally, extend it. Ask questions, listen carefully to the answers and make decisions based on what best serves your dog. Your veterinarian is your partner in figuring out how to compassionately go forward.