Cancer in dogs

Written by Dr Andrew Miller MRCVSDr Andrew Miller MRCVS is an expert veterinary working in the field for over 10 years after graduating from Bristol University. Andy fact checks and writes for Pure Pet Food while also working as a full time veterinarian. Pure Pet FoodPure Pet Food are the experts in healthy dog food and healthy dogs featured in media outlets such as BBC, Good Housekeeping and The Telegraph. Working with high profile veterinary professionals and nutritionists, Pure Pet Food are changing dog food for the better. - Our editorial process

Cancer is a word that no pet parent ever wants to hear, that one word brings on intense feelings of worry, stress, sadness and possibly a glimmer of hope for a cure. It’s a devastating diagnosis, whether that be for us humans or our canine counterparts. Dogs and humans are very similar in the illnesses we share, and unlike other species, our dogs can experience a lot of the same types of cancers as we can.

Cancer comes in various forms with varying degrees of severity. We’re going to give you a broad overview of the disease as a whole, what it actually is, why it happens and the options for treatment. The more awareness owners have of the illness, the more chance dogs have of beating this horrible disease.

What is cancer in dogs?

Cancer is the collective term for several diseases, all of which are caused by the uncontrollable, erratic growth of cells in the body. This strange cell growth has absolutely no purpose, it just causes your dog a lot of pain. Each type of cancer has a different set of symptoms to accompany it, different levels of severity, and different options for treatment.

Cancerous cells grow at a rapid rate, much quicker than normal cells, and they don’t grow in a strict controlled manner, they grow wherever they please. Every single one of the cells in the body can become cancerous. As these nasty cancer cells grow and multiply, they can begin to invade the surrounding healthy cells, infecting them with the disease and spreading it further around the body.

The type of cancer your dog suffers from is usually categorised on what part of the body is infected, whether that be an organ, muscle or skin. Many cancers cause an abnormal mass of tissue, called a tumour, and this will appear as a lump somewhere on your dog’s body. Sadly, if your dog doesn’t receive treatment for their cancer, the cancerous cells will continue to expand, connecting to more systems in the body and producing more tumours.

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Canine cancer is unfortunately the primary cause of death in senior dogs, so as a pet parent you need to have an awareness of what to look out for and what to do if you do suspect this deadly disease. With this awareness, if you can catch the cancer early enough - over half of the cancer types can be treated effectively, allowing your dog to continue living a long, healthy life despite their diagnosis.

What causes cancer in dogs?

Nobody has one definite, good answer for why our dogs develop this dreadful, devastating disease. Sadly, around 1 in 4 dogs will develop cancer in their life, so due to how frequently it arises, it’s important to try and learn everything we can about the illness.

As we know, cancer occurs due to cells that were once healthy, starting to grow, multiply, and spread in an uncontrolled manner. It works in a very similar way in humans as it does with canines.

The body is comprised of loads and loads of cells that work together to create everything we need to function, allowing our organs, skin, bones, blood and muscles to work correctly. Every single cell has a genetic code, its DNA, which tells the cells how they need to operate, where they need to grow, divide, work and eventually die.

Healthy cells will follow this straightforward life span and will be replaced by another round of healthy cells when their time comes to die. However, various anomalies can disturb this cycle, such as infections, hormones, radiation and even aging, triggering the cells to stop working as they should. The old, broken cells that should have died somehow manage to survive and multiply, which is when cancer begins.

Every mammal has the ability to try and fix cellular damage, but as with everything, these systems can sometimes fail to do their duty sufficiently. This can happen due to a dog’s specific breed genetic defects that they’ve inherited, the defence system could have developed a fault over time for no apparent reason or the defence system might not have the capacity to deal with the constant repair of cells due to various environmental factors.

Sometimes, excessive exposure to carcinogens, which is the word for cancer-causing triggers, such as sunlight, smoke and chemicals can have an impact on these defence mechanisms. Cancer can often be linked to obesity too. Whatever causes these damaged cells to continue replicating, the end result is quite often a large mass forming under the skin, which is called a tumour.

What is a tumour?

Cancer and the terminology surrounding it can be tricky to get to grips with, due to the diverse forms of the disease. Tumours are when those damaged cells that shouldn’t have survived turn into fleshy, solid lumps of tissue.

If you’re ever giving your dog a pet and a stroke, you may sometimes notice a bump under their skin - this could potentially be a tumour. You might instantly think the worse, that the tumour must be cancer, but this isn’t always the case. Tumours can be both cancerous and non-cancerous, so it’s important to get all lumps and bumps checked out by the vet as soon as you spot one. To further differentiate between the various types of tumours, they can be defined as being either benign or malignant.

Benign tumours grow in one localised area, remaining within the tissue where they initially formed and growing in a relatively orderly fashion, rather than spreading rapidly all over the body. These types of tumours usually grow slowly, are non-cancerous and can be solved easily.

On the other hand, malignant tumours are much more vicious, they’re likely cancerous and will have deadly consequences if they’re not treated promptly. These tumours will invade and attack surrounding areas of the body far away from the point where the cancer began, with the goal of binding themselves to other tissues totally unrelated to the initial cancer. As a result, this causes secondary growths, spreading the horrible disease all over the body. This ability to travel and spread is called metastases and tumours that have spread like this will need urgent treatment.

Not every tumour will be visually evident, you might only be able to feel it, sometimes you can’t even feel them because they’re that deep in the dog’s body. It’s important to routinely check your dog over for any strange lumps, and consistently visit the vets for annual check-ups. Early detection and treatment of a tumour provides your dog with a much better chance of beating cancer and living a long, happy life.

What are the benign types of tumour?

Benign lumps and bumps mean that they’re non-cancerous, but it doesn’t necessarily mean that they won’t need some kind of medical attention. A few benign tumours your dog might experience are lipomas, warts, cysts, abscesses and even bumps left from a parasite bite.

Lipomas are very common, one of the most frequently reported type of random lump or bump a dog will experience. They’re just fatty lumps that grow slowly, usually impacting overweight or senior dogs the most. Surgical removal or any other kind of treatment will rarely be required, it’ll only really be necessary if they’re causing your dog some kind of discomfort, for example, if they were disturbing bodily functions by blocking passageways, limiting movement if they were growing somewhere like behind the leg or if they become a constant thing for your dog to lick.

Warts and cysts are other small skin growths, not really having much impact and will probably be left completely alone by the vet unless they’re causing any irritation. Abscesses are another type of benign lump, created by an amassing amount of pus growing underneath the skin. The vet will probably recommend draining these.

Even a pesky parasite such as a tick could create a lump. A tick will bite your dog with the purpose of feeding from your dog’s blood and can sometimes leave a small crater afterwards, presenting as small lumps on the skin’s surface.

Various lumps and bumps that appear on your dog’s skin can be completely nonthreatening, they’ll often look a lot worse than they actually are. However, if you ever feel a bump on your dog’s skin, you must always get it checked out by the vet due to the possibility of it being something sinister.

What are the signs and symptoms of cancer in dogs?

Cancer comes in various forms, and each type comes with different symptoms. The symptoms created by each form of cancer will usually correlate to the part of the body the cancer is affecting, for instance, a limp or change in walk might be associated with bone cancer or bad breath and puddles of drool might indicate a type of mouth cancer.

Although, there are many mutual symptoms that many types of cancers share, with the most common one being evidence of a tumour, which we detailed earlier. Other signs and symptoms to look out for are:

  • Sores that fail to heal

  • Persistent or abnormal swelling

  • Strange smells coming from mouth, ears or other body parts

  • Sudden weight loss

  • Loss of appetite

  • Abnormal discharge and bleeding

  • Difficulty eating or swallowing

  • Lethargy

  • Lack of stamina

  • Lameness and stiffness

  • Difficulty urinating, defecating or breathing

  • Changes in bathroom habits

  • Evidence of pain

These are all classic warning signs of the horrible disease, they’re pretty much identical to the symptoms that humans with cancer will experience. In several cases of canine cancer, the dog can show either no symptoms whatsoever or very few, until the disease has advanced. Therefore, any time your dog is feeling unwell, or you notice any of the changes detailed above, you must inform your vet.

What are the different types of cancer in dogs?

Cancer has several forms, degrees of severity and stages, just like it does in humans. There are different things to look out for in each type of cancer, so it’s a good idea to make yourself aware of some of the most common, universal signs and symptoms detailed above. We’re going to detail a few of the most common types of canine cancer, but this is by no means a definitive list.


One of the most frequently diagnosed canine cancers, lymphoma is the cancer that impacts the lymph nodes and lymphatic system, it’s triggered from the white blood cells in the immune system, which work to fight off infections, becoming abnormal. The most prominent symptom you’ll notice with lymphoma is swollen lymph nodes, which are located in the neck, behind the knees, chest, armpits and groin.


Osteosarcoma accounts for over 80% of skeletal tumours, it’s the most common type of bone cancer that dogs encounter. Although it can occur in various areas of the body, the bones around the shoulders, wrists and knees are usually the most commonly impacted, leading to swelling and lameness. Unfortunately, osteosarcoma is an aggressive, malignant cancer, with studies suggesting it has a 90% metastasis rate, meaning it has a high rate of spreading rapidly to other areas of the body even after treatment.

Mast cell tumours

Mast cell tumours are really common skin tumours that dogs encounter, usually forming as a lump on the skin, however they can occur elsewhere in the body. They can spread and affect various parts of the body, however this isn’t too likely, they’re usually relatively benign and can often be fully removed through surgery.


Sadly, hemangiosarcoma is an aggressive cancer with a poor prognosis. It’s a cancer that derives from the cells that line the blood vessels, called endothelial cells. As blood vessels are throughout the whole body, hemangiosarcoma can occur anywhere, however it commonly impacts the spleen, heart and liver. Symptoms often don’t make themselves apparent until the later stages of the disease, making it even more threatening. The most common sign will be an enlarged spleen.


One of the most common oral cancers to impact our dogs is melanoma, which is a cancer that originates from skin cells called melanocytes. These cells are in charge of producing pigment, so this disease is most common in dogs that have darker skin. Typically, a malignant melanoma tumour will have already spread quite far and fast throughout the body by the time they’re noticed and is sadly incurable.

How is cancer in dogs diagnosed?

The quicker cancer is diagnosed, the quicker your dog can receive the necessary treatment, resulting in a better outlook for your dog’s life. Diagnosing cancer can take some work, partly due to all the various forms of the disease. However, if your pet is experiencing any of the symptoms detailed earlier, especially ones in complete relation to a specific body part, the vet will probably quickly have their own suspicions from these signs.

Various tests such as blood tests and x-rays to detect internal tumours (including ones that have experienced metastasis) may be necessary if there isn’t a visible tumour. If your vet is aware of the tumour, they will need to take some samples from it to detect what type of tumour is causing the cancer.

This sample could be taken using a fine needle aspiration, which is essentially the process of using suction to remove cells for testing. The cells will be examined under a microscope, in a process called cytology, which has provided many accurate diagnoses of tumour types in the past.

However, most samples will need to be taken through a biopsy, which is the complete removal of a patch of cells. Biopsies usually provide an accurate analysis, identifying what the type of tumour is, however, it can sometimes be tricky and require another sample to be taken, as the samples often don’t contain enough quality material to get an accurate assessment of. The sample will be sent off to a laboratory for histopathology, which is the examination of these cells. Histopathology helps the vet understand if the tumour is benign or malignant, judging whether or not the tumour is cancerous. All of these tests work to find out the type of the tumour, how far it has spread and how aggressive it is. From this, the vet can work out the best treatment plan for your dog.

Further tests such as MRI and ultrasound scanning may be used to help determine if the cancer has spread to various other parts of the body, and these procedures are referred to as ‘staging’. Also, these scans can provide an assessment of your dog’s overall health, allowing the vet to judge if they’ll be able to tolerate certain treatment methods.

How is cancer in dogs treated?

Cancer treatment for dogs will differ from dog to dog, some forms of cancer treatment won’t be suitable for every dog. Overall, the aim for each type of cancer treatment is to totally eliminate the cancer so it doesn’t return, but sadly, this goal is usually not totally possible. However, even if treatment can’t cure the cancer completely, it’ll help your dog live a happier life free from too much pain and distress.

If your dog’s quality of life can be significantly improved through cancer treatment, your vet will recommend which is the best option for your dog. Factors that need to be considered when choosing the treatment are:

  • The dog’s age

  • Type of cancer

  • What stage the cancer is at

  • Location of the cancer

  • How far the cancer has spread

  • The overall, general health of the dog

Your dog’s general health is a key thing to consider before going ahead with cancer treatment, as some cancer treatments can be too intense for a dog who has other health issues. A dog who is already unhealthy probably won’t be able to tolerate the intensity of cancer treatment.

Of course, this is the last thing you’ll want to think about, but if you’ve got a senior dog, you’ll need to consider how much time you think you have left with them anyway. A slow-developing tumour in a senior dog might mean that the negative treatment side effects will overshadow the possible benefits of undergoing treatment. The treatment needs to work as a balancing act between destroying the cancer and making sure that the treatment itself doesn’t have too much of a negative impact on the dog’s quality of life.

Possible treatment options that the vet will choose from for your dog are surgery, chemotherapy and radiation therapy. Each of these treatments can be used alone or alongside each other. Again, this all depends on the factors listed above, the vet will work with you and your dog to decide what the best route to take is. Research and development into cancer treatment is constantly underway to find the best options, so as time continues, treatment will always improve.


Several cancers can be removed surgically, and if so, surgery proves to be one of the most effective forms of cancer treatment. Surgery will be suggested for tumours that are in accessible sites, for example, mast cell tumours and breast cancers. Once removed, the tumour will be sent off for analysis to determine if it might have spread. If the lump hasn’t spread further than its original site, surgery might be enough to completely cure the cancer.

Unfortunately, it isn’t always that simple. Surgery won’t have any impact on some non-tumour cancers, such as leukaemia, so other treatment types must be provided here. Also, many tumours are in totally inaccessible locations, the tumour might be too large, or the cancer might have metastasized (spread) to other locations, so it makes total surgical removal basically impossible.

However, even if surgery isn’t enough to completely rid your dog of the cancer, your vet might still decide to go ahead with surgery to remove part of the tumour, which is a procedure referred to as debulking. Although it won’t completely cure the cancer, debulking does have a lot of advantages.

For example, if the tumour is large and in an awkward location that hinders your dog’s mobility and creates pain, partially removing the tumour will reduce its size and hopefully make your dog feel more comfortable all round. Debulking can also work effectively alongside other cancer treatments, such as chemotherapy or radiation therapy. Basically, the less cancer cells present within the body, the best possibility treatment has for succeeding.


Chemotherapy is probably the treatment type most people will instantly think of when it comes to treating cancer. Chemotherapy is the overarching term for utilising drugs to battle the cancer, and it can actually be used in various ways - your dog can receive it orally or by injection into the vein, under the skin, into a muscle, directly into the tumour or into a body cavity.

The treatment works to destroy the cancer cells, shrinking them, inhibiting the growth of the cells and ideally completely destroying the cancer. As with the majority of treatments and illnesses, chemotherapy will work the best in the earliest stages of the disease. Chemotherapy doesn’t usually fully eliminate the cancer, but it does work to slow the spread and alleviate the symptoms your dog experiences. Also, it sadly doesn’t always work. Vicious cancers don’t always respond to the treatment at all, depending on the size of the cancer, the spread and location of it.

Chemotherapy is helpful for treating many cancer types, mostly being used to control the cancer and how it spreads. Therefore, it’s the best treatment option for cancers that can’t be removed surgically and affect the whole body, such as lymphoma and leukaemia, which both impact the dog’s white blood cells.

The same type of drugs used for canine chemotherapy are basically identical to those used for human chemotherapy, the dosages are just adjusted to suit a dog’s size. Animals typically seem to have a better tolerance to chemotherapy than humans do, experiencing very few side effects or none at all because of the small dose.

Potential side effects include a loss of appetitevomiting and diarrhoea, but these usually aren’t too extreme. Hair loss rarely occurs in dogs as it does for people, it only really occurs in breeds such as Poodles, due to the way their hair constantly grows. However, the worst possible side effect that chemotherapy can have on dogs is the potential drop in white blood cell count, which can in turn make your dog more susceptible to other infections.

Radiation therapy

Radiation therapy, often referred to as radiotherapy, is another cancer treatment that works to kill the cancer through use of radiation. Cancer cells divide a lot quicker than normal cells, but they struggle to recover from radiation damage as well as healthy cells would.

Radiotherapy aims to supply just the right amount of radiation to eliminate, or at least hinder the cancerous cells to prevent them from multiplying, but it needs to be careful not to destroy any of the healthy, surrounding tissues. Usually, radiation therapy will be used for cancers in the brain, nasal areas and other tumours around the head and neck, and even for cancers of the spine or pelvis. For these types of cancer, radiotherapy may be the only viable treatment option. However, radiation therapy is most commonly used simultaneously with surgery and chemotherapy.

Radiation therapy is delivered in really small doses over many weeks, it’s quite a lengthy process. By sticking to this schedule, the cancers cells can be killed and any negative impacts to the healthy cells are lessened, as it gives the good cells some time to repair themselves after the radiation exposure. Overall, radiotherapy typically doesn’t fully cure cancer alone, but it helps to prevent the spread of the cancer and give your dog some pain relief from the tumour.

Can cancer in dogs be prevented?

Cancer can’t necessarily be prevented completely, it does just sadly happen in some dogs seemingly out of nowhere. However, there are a few environmental aspects that contribute to cancer that owners can pay attention to, to reduce the risk of their dog developing this deadly disease.

Environmental factors include:

  • Keeping your dog at a healthy weight, as obesity is linked to cancer

  • Limit your dog’s exposure to carcinogens such as smoke, pesticides and other chemicals

  • Visit your vet for annual check ups and routinely look out for any strange lumps

  • Spaying and neutering can both increase the risk of some cancers or decrease the risk

For example, by castrating your dog, it takes away the risk of testicular cancer. However, it’s believed that spaying your female dog increases the risk of hemangiosarcoma. Spaying or neutering your dog has both benefits and risks.

What is the outlook for my dog with cancer?

Following treatment, your dog’s outlook can’t be predicted with total accuracy. It will differ for every dog, some may be totally cured after surgery if it was caught early enough, but if the cancer is malignant, a full cure isn’t always possible. If so, the aim with treatment is to help your dog live comfortably and happily for as long as possible.

Unfortunately, if no permanent cure has been found, there will come a time where your dog’s quality of life will be deteriorating - you never want the treatment to be worse than the disease. The cancer type and its stage will provide your vet with some idea of your dog’s lifespan, but cancer doesn’t follow a dedicated, set path.


Even the dogs with the nastiest of cancers can experience a long period of remission. Remission is the phase where the cancer cells are undetectable within the body, and the symptoms have pretty much cleared up. However, remission means that the cancer is still present and not completely cured, so it will eventually come back.


The word cancer is enough to send any pet parent into a spiral of stress and worry, but if you can seek treatment quickly, you’ll be able to provide your dog with a restored vibrancy and increase your time together. Research into cancer and its treatment is ever-growing, meaning veterinary expertise needed to manage and cure the cancer will constantly improve.

If your dog does get the unfortunate diagnosis, the best thing you can do is learn as much as you can about the type of cancer and how you can help. If you’re proactive in your pet’s condition, you’ll be able to give them the best care that they deserve.