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What Are Dog Tumors?

Tumors occur when a cell continues to multiply out of control instead of following the natural cycle that ends in cell death. There are two broad categories of tumors: benign and malignant.

Benign tumors lack the ability to spread or invade other healthy tissue. Although they may need medical attention, these are not cancerous.

Malignant tumors, or cancers, spread to other organs and tissues in a process called metastasis. Depending on the type of tumor and how aggressive it is, cancers can pose serious health risks for your dog. To determine how dangerous a tumor is, your vet will examine it and, depending on the type of tumor, assign a grade or stage to your dog’s tumor.

Canine tumors are staged somewhat differently than human tumors. Depending on the type of tumor, they may be staged numerically with Roman numbers ranging from 0 – IV. A higher number means the cancer has spread further.

Other types of tumors are graded differently, so it’s important to work with your vet to understand how any particular type of canine cancer is staged.

Types of Common Dog Tumors

Although cancer can occur in many organs and tissues, the following are the most common types of tumors in dogs. 

Mast Cell  

Mast cell tumors are malignant tumors that occur in the mast cells in a dog’s skin. Normal mast cells are a type of immune system cell. They play a role in allergic reactions, such as hives and bug bites.

Mast cell tumors may look like a lot of different things, including a simple pimple or cyst. They can also mimic benign tumors such as lipomas. Common breeds susceptible to these tumors include Beagles, Boston Terriers, Boxers, Bulldogs, Bullmastiffs, Golden Retrievers, Labrador Retrievers, Pugs, and Weimaraners.

Lymphoma

Lymphoma is a malignant cancer that arises from white blood cells called lymphocytes. Normal lymphocytes are an important part of a dog's immune system.

A classic sign can of lymphoma is large, firm lymph nodes, usually found around the jaw, in front of the shoulder, or in the back of the knees. Other symptoms are lethargy or a lack of interest in food. Common breeds affected by lymphoma include Golden Retrievers, Boxers, Bullmastiffs, Basset Hounds, Saint Bernards, and Scottish Terriers.

Lipoma

Lipomas are benign growths arising from fat cells. They are typically found in subcutaneous fat, or the fatty layer just under a dog’s skin.

Lipomas are extremely common and can sometimes get very large. They are usually a cosmetic issue (pet parents may not love the look of a lumpy, bumpy pup), but they can sometimes cause problems if they are in a bad location. For example, a large lipoma located under a dog’s leg can cause issues with movement.

The malignant form of this tumor is called liposarcoma and is less common. Common dog breeds with lipomas are Weimaraners, Doberman Pinschers, German Pointers, Springer Spaniels, and Labrador Retrievers.

Osteosarcoma

Osteosarcoma is a type of malignant cancer that arises from bone cells. Osteosarcomas are often painful and can result in bone fractures, limb swelling, and lameness.

Common dog breeds to develop osteosarcoma are generally large dogs such as Boxers, Golden Retrievers, German Shepherds, Great Danes, Great Pyrenees, Greyhounds, Labrador Retrievers, and Rottweilers.

Histiocytoma

These benign tumors arise from histiocytes in the skin. Histiocytes are a type of immune system cell that helps fight infection. They will frequently regress and resolve on their own within a few weeks. Sometimes they can become flat, ulcerated, or red across the top, which has earned them the nickname "button tumors."

Histiocytoma is common in young dogs (typically less than 2 years of age). Any dog breed can be seen with these, but common ones are Labrador Retrievers, Boxers, Shar Peis, Bulldogs, American Pit Bull Terriers, Staffordshire Terriers, and Scottish Terriers.

Hemangiosarcoma

Hemangiosarcoma is a malignant tumor that arises from the cells that line blood vessels. They can pop up anywhere but are most common in a dog’s spleen, heart, and skin.

These tumors are often diagnosed when they rupture, which is an emergency situation that results in internal bleeding. Common dog breeds that get these tumors include Golden Retrievers, German Shepherds, and Labrador Retrievers.

Melanoma

Melanoma is a type of malignant tumor that arises from pigment-carrying cells in the skin called melanocytes. Melanomas in dogs can often be pink or non-pigmented. They can also be flat rather than raised.

Most melanomas in dogs occur in the oral cavity (e.g., oral melanoma) but can also be found in the eye, nail bed, or skin. Common breeds that develop melanomas are Schnauzers and Scottish Terriers.

Oral Melanoma

Oral melanomas are melanomas that grow in a dog’s mouth. They are the most common type of oral tumor in dogs, followed by fibrosarcoma and squamous cell carcinoma.

These tumors can be raised or flat and darkly pigmented or pink. You may see them on your dog or you may notice that your dog has bad breath, which is a symptom. They can also be found in routine exams by your veterinarian.

Oral melanomas are highly malignant and have often already spread once they are found. Common breeds with oral melanomas are Cocker Spaniels, Chow Chows, Scottish Terriers, Poodles, Golden Retrievers, and Dachshunds.

Papilloma

Papillomas, also called warts, are benign tumors caused by the canine papillomavirus. The virus is shared via contact from one dog to another. They are common in dogs that play in doggy playgroups, dog parks, or daycares and often occur on the lips, tongue, throat or gums, although they may appear in other locations as well.

This virus is species specific and cannot infect you or any other type of animal in your house. Papillomas will typically resolve on their own within a few weeks.

Mammary Gland Carcinoma

Mammary gland carcinomas are tumors that arise from the mammary or breast tissue of dogs. These tumors most frequently occur in dogs that are not spayed or were spayed after their second heat cycle.

About 50% of mammary gland carcinomas are benign when they are discovered, but this determination can only be made by a pathologist after removal. Common breeds with these tumors include Poodles, English Spaniels, English Setters, and terriers.

Thyroid Carcinoma

Thyroid carcinomas are tumors that arise from thyroid cells in the thyroid gland, which is located in a dog’s throat.

These tumors can be diagnosed by finding a swelling under the skin but are sadly often only found when evidence of spread is noted in other organs. Common breeds to develop these are Golden Retrievers, Beagles, and Boxers.

Symptoms of Dog Tumors

The symptoms of a tumor vary based on the type of cancer, location, and whether the tumor is benign or malignant.

Some types of cancers have symptoms or changes associated with them called "paraneoplastic" syndrome. Paraneoplastic syndrome is caused when tumors excrete hormones or hormone-like substances, or when the dog’s immune system reacts to the tumor and causes symptoms not related to the cancer.

Paraneoplastic syndrome can show up on lab work and in noticeable physical changes in your dog. These symptoms may be the first indication that cancer is present. Symptoms can include altered reflexes, weakness, and partial paralysis, among others. For example, elevated calcium levels are associated with lymphosarcoma and anal gland tumors. You may notice your dog drinking more water than normal, or sometimes the elevation can be found on routine lab work.

Some tumors consume blood cells and can be associated with anemias (low red blood cells), low white blood cells, and/or low platelets. Others can cause the opposite.

Tumors in the brain may cause seizures, changes in behavior, circling, etc. Mast cell tumors have symptoms related to histamine release: swelling, bleeding or difficulty clotting, and vomiting. Tumors associated with the liver can cause increased liver values on lab work.

Causes of Dog Tumors

The cause of cancer in dogs is unknown. Any type of cell can result in a tumor, and we do not always fully understand why a tumor occurs.

Some types of tumors occur more frequently in certain breeds, which suggests there is a genetic link. In a senior dog, cancer is almost always on the list of possible diagnoses when they come into the hospital sick.

Tumors can also be caused when your dog is exposed to known teratogens (substances that can change DNA). Any substance or process that has the potential to alter a cell's DNA can lead to the presence of a tumor.

Even environmental factors, such as UV light exposure, can increase your dog’s risk of developing a tumor. And some types of cancer, such as transitional cell carcinoma in the bladder, have higher incidence in dogs that live around golf courses.

Ongoing research will hopefully provide more information and treatment options in the future.

How Vets Diagnose Tumors in Dogs

One way your veterinarian cannot diagnose a tumor is by looking at or feeling it. Frequently, clients want a veterinarian's opinion on a mass on their dog but then do not approve diagnostics, perhaps because of cost. Watching a mass to see if it changes or bothers your dog is generally a bad idea, as these are not necessarily signs that a mass is benign.

Your veterinarian can have an idea of what they think a mass is but will not be able to give you a definitive diagnosis without sampling. General lab work can often point toward an organ system that needs to be examined closer but rarely provides a confirmed diagnosis of cancer.

Fine Needle Aspirate

Sampling a tumor is often the best way to diagnose a tumor. The larger the sample, the more accurate the diagnosis will be. Tumor samples can be obtained with a procedure called a fine needle aspirate. During this procedure, a small needle is used to remove a small group of cells from a mass. This procedure can frequently be performed at the same appointment and is minimally invasive.

The downside of a fine needle aspirate is that your sample may not be fully representative of the type of tumor being sampled, and the veterinarian cannot get any information about the layout of the cells, which can help with diagnosis. 

Biopsy

Tumor samples can also be obtained with a biopsy. A biopsy can be excisional (the entire tumor) or incisional (a small portion of the tumor). A biopsy will typically require general anesthesia or heavy sedation but can provide more information to help with diagnosis and can sometimes remove the tumor completely. 

Biopsies can sometimes be achieved externally but more often are performed surgically when the veterinarian performs an exploratory surgery.

Other Methods

Endoscopy can be used to collect samples as well, similar to a colonoscopy in people. Bladder tumors can be sampled via a urinary catheter or with a routine urinalysis.

Internal tumors are sampled in similar ways but with more complicated methods. Tumors in the abdomen (liver, spleen, kidney, bladder, etc.) can be sampled with a fine needle aspirate that is guided by ultrasound. 

Treatment for Tumors in Dogs

Different types of cancer have different treatments. The treatment recommended for a dog’s tumor depends on several factors:

  • How aggressive you want to be with treatment

  • Whether the tumor is one that typically spreads to other organs or stays locally invasive

  • How advanced or large the tumor is (e.g., the stage)

  • How serious the tumor-related symptoms are

Treatment options include surgical removal, radiation, immunotherapy, and chemotherapy. Several herbal remedies have also been used successfully to help either with the symptoms of a tumor or with its growth. Even changes in diet can help with cancer treatment in dogs. 

Recovery and Management of Tumors in Dogs

Some types of dog tumors can be cured. These are usually tumors that are locally invasive and can be completely removed surgically.

Types of cancer that spread or metastasize to other parts of your dog’s body can be managed with treatment but are unfortunately not usually curable.

The good news is that cancer treatment in dogs is often well tolerated with minimal side effects. For example, chemotherapy in dogs does not typically cause hair loss and has only mild gastrointestinal side effects. You can even administer some chemotherapeutic drugs at home, which decreases stress for your pet.

When your pet is diagnosed with cancer, it is best to get as much information as possible before you start making decisions. A consult with a veterinary oncologist can help provide answers around your dog’s possible outcomes and what to expect. They can explain treatment options in terms of what each one means as far as longevity and quality of life for your pet.

Even if you ultimately choose not to proceed with chemotherapy or radiation, the more knowledge you are equipped with, the more at peace you will be with your decisions.

Dog Tumors FAQs

How do you tell the difference between a cyst and a tumor on a dog?

A cyst and a tumor are differentiated by a fine needle aspirate or biopsy. Cysts are usually filled with fluid or waxy debris, whereas tumors are usually more solid.

Can dogs live with mast cell tumors?

Mast cell tumors that are low grade may be present for years without being detected. Mast cell tumors can look like just about any other type of tumor, so a fine needle aspirate is needed to help diagnose them.

High-grade mast cell tumors can spread, invade healthy tissue, and are fatal in the long term. The only way to determine a high-grade versus a low-grade mast cell tumor is through removal and testing with a pathologist.

What does a tumor look like on a dog? What does a benign tumor look like on a dog?

Tumors can present in lots of different ways on your dog: a lump or bump on the skin, a change in coloration, or even a change in the consistency of the skin. Tumors are frequently found on routine physical exam by your veterinarian, which is one of the many reasons you should have your dog checked by the vet regularly. The only way to determine whether a mass is benign or malignant is to take a sample of the tumor for testing.

Are cancerous tumors in dogs hard or soft?

Cancerous or malignant tumors can be hard or soft. The feel of a mass and whether it bothers your dog has little to do with whether it is cancerous or not.

How can you tell the difference between a tumor and a fatty tumor on a dog?

The only sure way to determine whether a tumor is a fatty tumor (lipoma) or something else is with a fine needle aspirate or biopsy. Lipomas are very common in dogs, and while it can be tempting to diagnose them based on feel, this can end up missing a more dangerous diagnosis. Any new lump or bump on your dog should be checked by your veterinarian.

What do skin tumors look like on dogs?

Skin tumors in dogs can come in all shapes, sizes, and colors. The bumps vets get more concerned about are ones that are in the skin layer versus on top of it like a skin tag or wart-like growth common in older dogs.

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