dog sniffs the dandelions its owner holds up

Understanding Dog Cancer, Inside and Out

Cancer is a pretty vast topic, making it hard to navigate and confusing for many.

In this article, this complex topic is going to be simplified a little by dividing it into two categories to help with understanding some basic concepts. We’ll also review signs to look for, when to get in touch with your vet, and some general therapy approaches.

Understanding What Cancer Is

Cancer is a pretty dreaded word in any health conversation, human and pet alike. That’s because it’s typically associated in our minds with aggressive, poorly treatable, and invariably fatal disease.

But there’s an idea in the heads of many folks that cancer is just this one thing, which is entirely untrue. There are as many different types of cancer as there are different types of cells in the body, which is where all cancers originate from.

Every cell type in the body can undergo mutation whether from a dog’s genetic predisposition, environmental stressors, or other causes, that leads to them multiplying at an increased rate. This is what causes a tumor to form. Technically, any type of overgrowth of cells causing the formation of a tumor can be considered cancer.

However, the term “cancer” is most often reserved for the more aggressive types of tumors. Some tumors are benign, meaning they stay in one place and don’t spread to other parts of the body. But others are malignant, meaning that parts of them can break off if you will, travel through the bloodstream, and set up shop in another part of the body. Tumors that spread this way can be more difficult to treat.

Most cell types have both benign and malignant forms of tumors. But all approaches towards cancerous tumors depend on what cell type they’re formed from, as well as their behavior.

For example, the spleen may be affected by a tumor of blood vessel tissue cells called a hemangioma. Hemangiomas are benign, meaning they don’t spread. They still require surgical removal of the spleen, but once the spleen is out, so is the cancer.

Hemangiosarcomas on the other hand, originate from the same tissue but are very malignant, often spreading to other nearby locations like the liver, as well as the lungs. Unlike the hemangioma, there is no cure for hemangiosarcoma. Both surgical removal and chemotherapy are required for the best outcome.

Because there are so many types of tumors and they all have different approaches to diagnosis and treatment, we can’t discuss all of them. Instead, we’re going to divide the cancer discussion into cancer on the outside, such as growths on the skin, and cancer on the inside, like disease of organs like the spleen and liver, and the gastrointestinal tract.

dog leans head on chair and looks up longingly

The Inside

Cancer on the inside of the body is the hardest to detect, simply because it can’t be seen. We have to patch together clues from changes a pet is experiencing at home, combined with physical exam findings and lab work tests.

This is why regular exams with your veterinarian, either annual for most pets, or biannual for senior pets, are so important. Your vet can sometimes detect a tumor on exam while feeling your pup’s belly, even if you haven’t noticed anything yourself.

Most cancers don’t lead to changes on annual lab work with some exceptions being lymphoblastic leukemia and multiple myeloma which affect the bone marrow where affected blood cells are produced. But while there are no general cancer markers we can routinely test for, organ function values will sometimes be altered by the presence of cancer, prompting your vet to investigate the underlying cause further.

At my practice, we also encourage annual screening x-rays for senior pets, as we may identify early signs of a problem long before it truly starts to cause problems.

The reason screening for cancer is so important, is because pets can easily hide signs of a cancerous process going on inside, until it’s severe enough that it causes more noticeable changes. These types of changes are typically very vague and general too and can include digestive upset, poor appetite, weight loss, panting, coughing, fever, and weakness.

Since these signs are vague, it’s important to recognize when a change, even a seemingly minor one, is occurring in your pup, and make sure to have her checked out by your vet.

Because pets can hide signs of an internal problem, the first sign we may get of an internal cancer may appear very suddenly. This is because while the disease had been developing for a long time, it suddenly reached a threshold where it impacted a pet’s body beyond his control.

For example, an older pup may have one of those tumors on the spleen mentioned early on. These tumors can be present, growing for weeks or even months without any outward signs.

But when they reach a certain size, they can become very brittle. If a tumor on the spleen breaks open or ruptures and starts to bleed, a dog will become weak very quickly, sometimes so quickly she may collapse in the middle of what was otherwise a normal walk around the block.

Suddenly, we went from a normal day to a need for an emergency visit.

Or as another example, tumors in the mouth sometimes go unnoticed for a couple of weeks because a resilient dog will simply find a way to work around it by chewing food with the other side of her mouth. It's not until an oral tumor reaches a certain size that a pup may start dropping food and avoiding meals.

Brain tumors are another cancer type that can be developing slowly, but when they reach a certain size and start impacting surrounding brain tissue, we may suddenly see seizures develop, seemingly out of nowhere.

Weight loss can be an important clue to look for and can be an outward sign that at least something very concerning is happening on the inside. Many diseases including heart disease, kidney disease, and certainly cancer, can really suck away energy from the body. In an effort to keep up, the body pulls from its own energy reserves. This leads to a gradual weight loss, especially loss of muscle.

Weight loss usually occurs gradually, over weeks to months. Because it happens so slowly, it may be hard to notice until a more significant amount of weight loss has already occurred. This usually prompts the belief from pup parents that their pooch lost fifteen pounds in the last week or two, but typically, it has been going on longer than that, due to some internal disease process.

This makes regular weight checks very important, either at your vet, or even at home. If you’re not trying to get your pup to lose weight, but we’re seeing a reduction in weight on, say, a monthly basis, this could be the sign of a problem to take a closer look at.

With the exception of the couple of blood cell cancers previously mentioned, internal cancers are usually detected best using x-rays, especially if there’s a bone that’s being affected. For the abdomen, x-rays are still the best screening tool to use first.

But because they only give a very general picture of the belly, it can be tough to determine where a large abdominal mass may be originating from. That’s why it’s common to follow up abdominal x-rays with ultrasound, which can provide a more narrow, focused picture of a specific area. Ultrasound can not only be helpful in narrowing down the specific organ involved, but a fine needle sample aspirate and sometimes a needle biopsy may be obtained using ultrasound guidance.

Some fine needle aspirate samples designed to get a cell sample to look at under the microscope may provide an answer. But full tissue biopsies are often required for many internal types of cancer to be properly diagnosed.

If our simple, less invasive methods of diagnosis don’t get an answer, more advanced imaging, like a CT scan or MRI, may be needed. For some lesions, like brain tumors, x-rays and ultrasound are not the best detection methods, and MRI imaging is always needed for the most accurate diagnosis.

In the end, we may sometimes decide to surgically remove a tumor depending on its location and the organ it’s associated with, as this may both provide a potential cure, as well as a diagnosis on the tumor type, helping us to figure out what type of follow-up care may be required. This is especially true with tumors of the spleen and liver.

Depending on the tumor type and location, chemotherapy and/or radiation treatments may be required to get the best outcome.

The Outside

The good thing about cancerous growths on the outside of the body is that they’re far more noticeable and can be caught early.

It’s common for pup parents to detect a new growth maybe while brushing or petting their dog, prompting them to bring their furry friend into the vet to have it checked out.

A select few number of external tissue growths can be identified visually or by feeling them on exam. Many readers out there may be familiar with the soft, fatty growths called lipomas that are commonly found on older dogs. Older pets also commonly get little “wart-like” masses, which are not true warts most of the time but are instead little benign tumors of glands in the skin.

A majority of external growths cannot be simply identified visually, even by a vet, and so it is common to start with a fine needle aspirate sample to collect some cells from the growth and check it out under the microscope.

Your vet may be able to identify the sample right there in the clinic, or may elect to send it out to a lab. Either way, this sample can often provide some information on what needs to happen next.

Surgical removal of external growths is common. Sampling them first can provide an idea of whether they are likely to be benign or malignant, affecting the plan of whether the risks of anesthesia and surgery are outweighed by getting the growth off.

For example, in many cases, lipomas, which are benign growths, tend to stay small, and surgically removing them, especially in a much older pup, may not outweigh anesthetic risks or financial restrictions.

But skin growths like melanomas and mast cell tumors, which are typically straightforward to diagnose with a fine needle aspirate, require surgical removal as they can be malignant. Soft tissue sarcomas, also straightforward to diagnose, often don’t spread to other parts of the body, but they can grow so quickly and invasively that they won’t be treatable if not addressed ASAP.

In some cases, a needle aspirate sample may not be conclusive, and this does unfortunately happen from time to time. Some growths simply don’t “shed” their cells well. In these cases, your vet may recommend surgically removing the growth so that it can be biopsied, thus addressing both the need to remove it as well as to find out what it is.

Some pet parents question the value of sending a growth out for biopsy if it's already been removed. After all, it's gone, so what's the point?

There are two main advantages to biopsying. The first, is that if we find out a growth is malignant, like a high grade mast cell tumor, these do have the potential to spread through the body, even if they were located on the skin. Without a biopsy, you can't differentiate a high grade tumor from a low grade one and malignant growths often require follow-up treatments for the best outcome.

The second advantage is understanding if the whole growth was removed, which is called getting ""margins"". Many tissue growths can have microscopic little tendrils that spread out from the main tumor. This is why we try to get up to 3cm margins around a growth.

But some locations, like a leg, the tail, or the face, may not allow for such a wide area of tissue to be removed, meaning that a growth will have to be removed with very narrow margins.

If removed incompletely, some growths will return. Getting a biopsy to confirm good margins is therefore important because if we don't have good margins, a revision surgery to remove more tissue may be needed. Or at the very least, we can establish the expectation that a growth may return, requiring that the location is monitored more closely.

If you and your vet decide to leave a growth alone and monitor it, either because it’s suspected to be benign or is too small to sample, make sure to keep some guidelines in mind. Growths that change color or that increase in size within a short time, like a month or less, can be more concerning. If a small growth doubles in size or changes in appearance, this warrants an immediate recheck with your vet.

small dog lays on the ground looking sad

What is a Veterinary Oncologist? When Do I Need One?

There are certain types of cancers that a general practitioner may be comfortable addressing, depending on the cancer type and the wishes of the pup’s parent. For example, a small soft tissue sarcoma is likely within an everyday GP’s skills to remove and send out for biopsy.

But there are certainly many cancer’s well out of the scope of a generalist’s comfort level and expertise. Some external growths may be too large or in too difficult a location to remove with basic surgery skills, prompting the need to bring in a boarded veterinary surgical specialist.

And some tumor types, especially of the “inside” variety, may require diagnostic tests and treatments beyond a GP’s capabilities.

Generally, if the type of cancer requires advanced diagnostics or treatment with chemotherapy or radiation, your vet may refer you to a veterinary oncology specialist.

A veterinary oncologist is a veterinarian who has completed a 3 year residency of training specifically in treating pets with cancer. Some further specialize in radiation oncology as well. Chemotherapy and radiation treatments both have very complex protocols that require special training.

Even if you decide you don’t want to continue with chemo or radiation for your pet, consulting with an oncologist can still be beneficial to get more specific information on a type of cancer, associated prognosis, and expectations.

A veterinary oncologist’s opinion and insight can be so useful in fact, that a lot of general practitioners like myself have one or two that we can consult with by phone to get some special advice on a patient’s case.

So if you find yourself in discussions with your vet about your pet having a type of cancer, make sure to ask if involving a veterinary oncologist could be helpful.

How is Treating Dog Cancer Different vs. People?

There are some differences between addressing cancer in people and cancer in humans. Most of us know at least one person in our lives afflicted by a cancer, and this knowledge can sometimes alter our perception of cancer in pets, making it really important to know the differences.

First, it can be completely acceptable to monitor a benign skin growth and leave it alone. While a person would want any kind of growth removed for appearances and aesthetic reasons, pets don’t have the same kind of vanity.

In fact, some of these benign wart-like growths or fatty growths bother the pet parents more than the pet. If a pet has other diseases she's dealing with, like liver, kidney, or heart disease, removing a small wart or tiny eyelid growth may not be worth it. But if removing a growth for aesthetic reasons would not put the pet at great risk, you and your vet might elect to go ahead.

In a somewhat similar but opposite vein, taking “extreme” measures like amputating a limb affected by a sarcoma or a bone tumor can be tolerated by a dog far better than many might think and should be pursued in appropriate cases. It’s more often well-meaning pet parents who will have an issue with the idea of amputation because of how it might look or if their pup will be able to adjust.

Unlike people, dogs adjust to amputation really well because they still have three other legs to bear weight on. And in most cases, if we’re talking about amputating a limb, it’s likely that the pet is already in chronic pain from the tumor, not using the limb well to begin with, and will be in less pain once the affected limb is removed.

While “phantom limb syndrome” has been described by human amputees, it’s unclear if pets experience the same kind of tingling or “phantom” sensation of a limb still being there after it’s been removed. But one thing is certain. If a pet has a painful cancerous growth on a limb, any discomfort caused by removing that limb will be temporary and minimal compared to the chronic pain of living with the cancerous leg.

Chemotherapy can also have a bad connotation in the thoughts of many folks. Chemo is very difficult on people, leading to hair loss, weight loss, a terrible appetite, and often emotional depression.

But in pets, they tolerate most chemo protocols far better. Because their hair growth is different from ours and is not continuous, pets don’t lose their hair. It’s true that chemo can cause some digestive upset, but this is often mild compared to what people experience. And because pets can’t comprehend what cancer is and can’t have foreboding feelings about the distant future, true emotional depression on their part is unlikely.

Chemo also doesn’t exclusively mean injections and weekly hospital visits, another thing that pet parents may dread. For some types of cancers there are oral medication options that can be given at home. And while injectable chemo treatments can be very expensive, if oral chemo is an option, it may be far more affordable.

Chemotherapy can in many cases extend a good quality of life for a pet for several months, or even over a year for some cancers, so make sure not to count it out completely before discussing with your vet or a veterinary oncologist.

Last, and this is most important, we want to make sure that any pet with cancer has a good quality of life. If that quality of life deteriorates despite our best efforts, it’s important to discuss euthanasia with your vet.

Pets live in the present. And while there can be some up and down days when treating cancer, if most of those days aren’t filled with good quality, continuing on just isn’t fair to them, especially if they’re in pain.

Understanding if a pet is in pain or is suffering can be hard. But your vet can help you understand if your pup might truly be in pain and what measures can help.

dog lays outside staring at the ground

Closing on Cancer

Cancer can be very difficult to face, both for we humans, as well as for our pets.

But it’s important to recognize that cancer isn’t just a single disease. It represents a spectrum of tumor types ranging from more benign and easily treatable, to more malignant, aggressive, and very difficult to treat.

This variability highlights the need to firmly diagnose the type of cancer involved so that treatment recommendations are as specific as possible.

This also highlights the importance of regular visits and discussions with your veterinarian. Routine exams and health screenings can “catch” some cancerous processes early before they become too big to handle.

You know your dog best, which is why it is also important to have your pup checked out if you’re noticing any changes in her general health, like energy level, appetite, or weight. Signs of internal cancer can be vague and often share signs with other disease processes.

And if you notice something like a new lump or bump on your pup, it’s best to have it checked out sooner rather than later, just to be safe.

While there is no cure-all or miracle preventive for cancer, the combination of diligence and regular health screening can sometimes make the biggest difference between a good outcome and a poor one.