Tumors on Dogs: Be Prepared As a Pet Parent, Know How to Take Care of Your Dog

September 11, 2023
A dog getting a belly rub from the pet parent

What are dog tumors?

Tumors are one of the most commonly associated medical terms with cancer—though not all tumors are cancerous! 

A tumor is a mass of tissue that forms anywhere in the body when cells grow and divide at a faster pace than they should, or when the cells do not die when they are supposed to, leading to a lump on or inside of the body.1,5 

What causes tumors in dog?

As with most questions surrounding medical causes, the answer is “It depends.” There are so many factors that can lead to a tumor developing in a dog’s body, be it cancerous or benign. And the cause can be from one specific factor, or from hundreds, that determined the body would develop a tumor. 

In general, a tumor develops due to damage to cellular DNA. Examples of ways that damage can occur include:2,4

  • Genetics: some breeds are more likely to develop certain types of tumors than others. Golden retrievers, Bernese mountain dogs, boxers, and rottweilers are known to have a higher probability of developing cancers. 
  • Age: with the rise in the average lifespan length for many dogs, the probability of them developing tumors has also increased. Some veterinarians believe that as their bodies age, their cells degrade somewhat and are more likely to mutate. 
  • Environmental factors: exposure to the sun, chemicals such as herbicides, pesticides, and cleaning solutions, can all lead to a higher likelihood of a tumor developing 

What are signs that your dog might have a tumor?

There are so many different ways that a pet parent might notice something is off with their dog. Some of them can be very noticeable changes in behavior, while others may take a bit more attention to recognize. 

The symptoms that a tumor presents depend on what kind of tumor it is, the location of the tumor, and whether it is benign or malignant. 

For many non-cancerous tumors on the skin, the first symptom will be the tumor itself. Noticing a growth, a lump, or a bump on your dog’s body would be your sign to bring your dog to the vet to get them checked out. And in some cases of cancerous tumors, a pet parent may notice the mass before it starts to affect their pet enough for them to have symptoms. The sooner cancer is caught the better, so finding a mass before any health symptoms start to present and getting it checked out is always the best option. 

Tumors that develop inside the body will often be found due to other health symptoms. 

For example, tumors in the brain may cause seizures, changes in behavior, or signs of confusion. While tumors in the stomach or intestines may lead to changes in eating behaviors, loss of weight, or difficulty using the bathroom.2

In general, tumors can affect the body in many different ways and if you notice changes in your pet’s behavior, interests, activity level, and personality—bring them to the vet as soon as you can. 

Specific symptoms include:2,3

  • Swelling 
  • Loss of appetite
  • Drinking more water than normal
  • Unexplained weight loss 
  • Lethargy and lack of energy
  • Depression 
  • Disinterest in exercise or common play options 
  • Odor 
  • Difficulty with walking and other movements—stiffness
  • Difficulty using the bathroom 

How are dog tumors diagnosed? 

The most important step for diagnosing a tumor is bringing your dog to the vet. Any signs that there is something amiss with your pet’s health should be taken seriously; there are so many different ways that an illness can present itself in your dog that you should always make a plan to visit your vet when something isn’t quite right. 

The way that a vet will diagnose a tumor depends first on the location of the growth. If a tumor is developing on the outside of the body, the first step to a diagnosis will always be a physical examination. Once the tumor is located, the next step would be a fine needle aspirate (FNA). An FNA is a way to sample cells from the tissue by using a small needle with an empty syringe to remove some cells to examine under a microscope. While this examination and evaluation can be done at some vet’s offices, others will need to send the sample out to a laboratory to evaluate it.2 

If an FNA sample comes back inconclusive, the next step would be to perform a biopsy. Some biopsies will be performed after the whole mass of the tumor is removed from the body, or they can be done by removing a small portion of the tumor. A biopsy can provide more information about the growth than an FNA sample can and will often be used when there is more detail required to accurately diagnose and begin treatment.2

If a tumor is inside the body, there are a few other ways that a diagnosis might be reached. The location of the tumor will affect the symptoms that the patient experienced before coming into the vet and that will give the vet some indication of where to look. If the patient is experiencing stomach issues as a symptom, the vet may perform some form of imaging (ultrasound, CT scan, etc.) on the abdomen to identify any causes. If a tumor is identified in the stomach, intestines, or colon, then a biopsy can be performed with an endoscope. If a tumor is located in the spleen or liver, then a surgical procedure to remove and diagnose the tumor will likely be required. For other tumors, some may be able to be diagnosed as cancerous through blood work.3 

Benign or malignant, what does it mean?

The hope for pet parents who find a growth on their pet is that it will be a benign growth, meaning that there is no cancer. Benign growths are unable to spread to the rest of the body. They will still need to be addressed by your vet to ensure your pet has the best quality of life that is possible—some locations may affect your dog more than others—but they have a bit more leniency on the timeline for taking care of them than their malignant counterparts.3,6

Malignant tumors are the bad news: malignant means that the cells are cancerous. It is very important for you to get your dog’s malignant tumors addressed as early as possible. While there is always room for optimism, a mass growing on your pet is a situation that calls for caution. Any growth should be treated as if it is cancerous until it is proven that it is not, or you are otherwise told by your vet that you needn’t worry.3,6 

Common types of tumors found in dogs

Mast Cell tumors

These are one of the most common forms of tumors found in dogs. It is a type of skin tumor that can grow on the skin, subcutaneous tissue, or inside the body. These tumors may look like many different things on the skin, such as a cyst or a pimple. They are known to grow quickly and can be itchy, and since it is the job of a mast cell to alert the body to any allergens when a tumor grows from these mast cells the symptoms often mimic those of an allergic reaction. Mast cell tumors are one of the few tumors not correlated with older-aged dogs.2,3,4,5 

You can read more about mast cell tumors here. 


Lipoma is the official name for benign fatty tumors. These are very common in many dogs, especially older and/or overweight dogs. They can range in size and sometimes can get very large, but many of them are simply a cosmetic issue for the dog rather than an actual hindrance. Treatment by removal is only required if the dog has mobility issues due to the location of the growth, though some pet parents may want them removed if the procedure won’t cause their dog any excess health stresses for the aesthetic aspects of it.2,3,4,5,6  


Osteosarcoma is a tumor that develops in a dog’s bones. It is a type of cancer that can result in limping, limb swelling, or bone fractures. Osteosarcoma is often very painful and is a common cancer found in large dogs, especially greyhounds and great Danes. Treatment for this kind of tumor if it’s in a limb is often amputation; thankfully dogs have an easier time adapting to the loss of a limb than their human counterparts. Along with amputation, chemotherapy will follow as this tumor is very aggressive with a high rate of spread to other parts of the body.2,3,4

You can read more about osteosarcoma here.


Histiocytoma is a benign tumor that develops on the skin in the histiocyte cells. These cells are part of the immune system and they help fight infection. Many of these tumors will go away on their own within a few weeks, though some may be recommended for removal because they bother your dog. Often referred to as “button tumors”, these are common in young dogs under 2 years old. Histiocytoma is very commonly seen in breeds like English bulldogs, Scottish terriers, greyhounds, boxers, Boston terriers, and Shar Peis.2,3,4  


This is a cancer that develops from the cells that line blood vessels. A hemangiosarcoma tumor can develop anywhere in the body, but they are most commonly found in the dog’s heart, spleen, and skin. Unfortunately, these tumors are often diagnosed after they rupture, which would make it an emergency situation that involves internal bleeding and can be very quickly fatal if not treated immediately. Treatment usually starts with addressing the internal bleeding, surgical removal of the spleen, and then chemotherapy.2,3,4,5 

You can read more about hemangiosarcoma here.


Melanoma is a type of cancerous tumor that comes from cells that produce pigment called melanocytes. Some melanomas can be pink or non-pigmented, as well as dark in color depending on the dog’s skin tone under their fur. The most common melanomas develop in the mouth, eye, nailbed, and other locations on the skin. Oral melanoma is the most common form in dogs and is often very aggressive with a high chance of spread. Treatment often includes a combination of surgery, chemotherapy, radiation therapy and immunotherapy.2,3,4 

You can read more about melanoma here.


Papilloma is more commonly referred to as warts. These are benign growths that are caused by a contact virus from one dog to another. These may be found in dogs that have playgroups, go to dog parks regularly, or spend time in daycare. A papilloma growth will look like cauliflower developing on your dog’s skin, often found on the lips or inside the mouth and around the eyes. Severe infections may cause difficulty eating. Though most of these infections will go away after a few weeks, some can take months. If they are creating an impact on your dog’s life, either through discomfort or by causing difficulty with eating, your vet may recommend surgery to remove it. If your dog has a papilloma, it is best to keep them away from other dogs as this is a viral infection that you won’t want to share with others.2,3,4 


Lymphoma is a cancer that develops in the lymphocyte cells, which are a major part of your dog’s immune system, and dogs with lymphoma will commonly have swelling in the lymph nodes as a sign of the cancer’s development. There are multiple different kinds of lymphomas—you can read more about the different types here. As it is a cancer that affects the whole body, the most common treatment for lymphoma is through multi-agent chemotherapy protocols. Learn more about chemotherapy treatments for lymphoma.

What to do if my dog has a tumor?

At any point when you find a tumor, growth, lump, or bump on your dog you should take them to your vet as soon as possible, though keep in mind that it’s not always a reason to panic. There are plenty of non-cancerous reasons that your dog may develop a tumor. 

Call your vet, schedule an appointment, and let them know if you’ve noticed any other symptoms besides the growth so they know if there’s anything else they’ll need to worry about. 

Keep an eye on the size of the growth and pay attention to if it changes or gets larger or smaller, and let your vet know these details when you get in. 

Treatment options vary based on what the cause of the tumor is, but trust that your vet is looking out for your dog when they come up with a treatment plan for them.

Read more on lumps and bumps on dogs.

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