The Most Common Tumors in Dogs (With Pictures)

Tumors develop when cells in the body grow uncontrollably. Finding a tumor on your dog can be very frightening, but they are not always detrimental — some are benign, while others are malignant. Benign tumors are not cancerous and do not spread. Malignant tumors, on the other hand, are cancerous and can spread to other parts of the body, causing severe health issues for your dog. 

Both benign and malignant tumors can look the same, so it’s important to talk to your vet if you notice any new growth or abnormality in your dog’s body. Let’s look at some common tumors in dogs, where they come from, and what they look like!

Histiocytoma

What it is: Histiocytes are cells that aid your dog’s immune system as they keep watch for foreign invaders like bacteria, viruses, and fungi. With histiocytomas, however, too many of these cells end up in a single spot and develop into a visible tumor. 

What it looks like: Histiocytoma produces small tumors, usually less than 1 inch in diameter with a button-like shape. They often look like little pink or reddish bumps. Histiocytomas can appear suddenly and grow quickly, but they are more common in younger dogs under 5 years old. They can develop in any dog but are more common in breeds like dachshunds, Great Danes, and boxers.

Dangers and risks associated with histiocytomas: Fortunately, histiocytomas are usually harmless. In fact, oftentimes your dog’s immune system will take care of them, and over a few months the tumor(s) will disappear. However, some histiocytomas can be malignant and cancerous, so you should first have your vet examine your dog. 

Treatment: While some histiocytomas disappear on their own, others may need to be surgically removed. Depending on the size and location of the tumor, these small bumps can be irritating for your dog and may even ulcerate — which means they can break open, bleed, and develop an infection. It’s important to prevent your dog from itching, scratching, licking, or biting the tumor(s). 

Papilloma

What it is: Also known as canine warts, papillomas are small growths that can develop on your dog’s skin. They are similar to the warts that humans sometimes get, appearing or disappearing spontaneously. Papillomas in dogs are caused by papillomaviruses and are species-specific. This means that they can be spread from dog to dog, but not from a dog to a human.

What it looks like: Papillomas are usually flesh-colored, although some may have a whitish or grayish appearance. They are more common in younger dogs but can occur in any age or dog breed. These tiny tumors usually have a cauliflower-shape, but occasionally may appear smooth and rounded or even scaly and flat like a dried-out scab. Occasionally, a papilloma develops under the skin and feels like a hard, internal lump. 

Dangers and risks associated with papillomas: Fortunately, papilloma are almost always benign in dogs. 

Treatment: Papilloma tumors in dogs commonly disappear on their own. However, sometimes they are bothersome for dogs and may need to be surgically removed by a vet. In addition, if your dog has a weak or impaired immune system, they may develop new papillomas. 

Lymphoma / Lymphosarcoma

What it is: Lymphomas are one of the most common tumors in dogs. They are caused by an overdevelopment of lymphocytes (a type of white blood cell in the body). 

What it looks like: These tumors are much trickier to see as they often develop in your dog’s internal organs. However, lymphomas often cause the lymph nodes to swell. These appear under their neck or jaw, behind their knees, under their armpits, in front of their shoulders, or in their groin area. These swollen lymph nodes can move freely beneath the skin with a rubbery and firm feeling. 

Swollen lymph nodes, however, are not a guarantee that your dog has a lymphoma. They can also swell due to other infections and autoimmune diseases, so check with your veterinarian.

Dangers and risks associated with lymphomas: The most common type of lymphoma is multi-centric or systemic lymphoma. At first, the only visible symptoms are swollen lymph nodes, but if untreated your dog may suffer from lethargy, fatigue,  and weight loss. The second most common lymphoma is alimentary, which affects the gastrointestinal tract. Your dog may suffer from gastrointestinal lesions, diarrhea, vomiting, and weight loss.

Mediastinal lymphoma is rare and affects the organs in the chest cavity, typically causing shortness of breath and coughing. Another rare type is extranodal lymphoma, which targets specific organs. It can develop in your dog’s nervous system, eyes, kidneys, skin, or lungs.

Treatment: The most common treatment for lymphoma in dogs is chemotherapy. There is no cure for lymphosarcoma, but fortunately, dogs seem to tolerate chemotherapy better than humans do, so it can often be put into remission. Unfortunately, however, many dogs eventually relapse, and the cancer returns. 

Mast Cell Tumor / Mastocytoma

What it is: Mast cells are part of the immune system. They are a type of white blood cell that helps with allergies and allergic responses. When too many of these cells are generated, however, they can develop into a mast cell tumor — one of the most common types of tumors in dogs. Although they typically develop on the skin, mast cell tumors can also grow internally in the gut, spleen, liver, and intestines. A veterinary analysis is necessary to know whether a mast cell tumor is benign or malignant. 

What it looks like: Mast cell tumors typically look like a bump or a raised lump on your dog’s skin. Some may feel like a small bump under your dog’s skin. However, some tumors appear swollen and red, sometimes with scabs or cracked skin. The tumors can also become aggravated, which may cause them to fluctuate and grow larger or smaller.

Dangers and risks associated with mast cell tumors: Mast cell tumors can be malignant or benign, so your vet will need to examine your dog. Malignant tumors, called mastocytomas, can be very aggressive and spread to surrounding tissue quickly. Unfortunately, they can be fatal. However, if caught in time (and depending on how quickly the tumor grows) some dogs can survive this type of cancer.

Treatment: If they have not metastasized or spread to other parts of the body yet, mast cell tumors can be surgically removed. Removing the tumor is the best treatment, as mastocytomas can be very invasive and aggressive. Vets will often combine surgery with radiation therapy, chemotherapy, intratumeral injections, or oral medication.

Melanoma

What it is: Melanocytes are cells that produce pigment in the body. Occasionally, however, these cells can produce abnormally and form nodules, lesions, and masses. Known as melanomas, these tumors can be malignant or benign. Malignant melanomas can spread quickly to other parts of the body. 

What it looks like: Melanomas typically look like small, dark lesions (similar to a bad mole). They may be raised like a bump or flat. Some melanomas don’t have any pigment and will look like pink or red patches. As they grow, these tumors can also ulcerate and bleed.

Dangers and risks associated with melanomas: Melanomas in dogs are common tumors that typically develop in the skin, eyes, nail beds, or mouth. You may notice things like coughing or difficulty breathing, lameness, limping, or dark skin patches on your dog. 

Oral melanoma (melanoma of the mouth) is very aggressive and spreads quickly. It may cause difficulty eating, lack of appetite, bad breath, tooth loss, weight loss, unusual chewing behavior, and excessive drooling. If you notice any of these symptoms in your dog, you should take them to the vet right away. 

Treatment: Depending on when it is diagnosed and what stage it is in, melanoma can be fatal. This type of tumor in dogs is extremely aggressive and spreads quickly. However, sometimes melanomas can also be treated with surgery, radiation therapy, chemotherapy, immunotherapy, and a melanoma vaccine. 

Osteosarcoma (Bone Cancer)

What it is: Osteosarcoma causes tumors in the bone. These tumors are aggressive and break down and eat away the bone. 

What it looks like: You likely won’t be able to see osteosarcomas on your dog since these tumors develop inside the bone. However, bone tumors are very painful, so you may notice that your dog is limping or isn’t interested in walking or playing. They may not be as hungry as usual, have unusual swelling, or seem extra tired. In addition, the area where the tumor is located may be swollen and/or warm to the touch. 

Dangers and risks associated with osteosarcomas: If osteosarcoma weakens the bone enough, it can cause breaks and fractures. Unfortunately, osteosarcoma is very hard to detect, and in most cases, it has already spread throughout the body by the time it is diagnosed.  

Treatment: Osteosarcomas can be treated with surgery, which often means the affected limb must be amputated. Dogs usually do well after amputation, and can still run and walk on three legs. Your vet may also suggest chemotherapy after surgery to help control the cancer and keep it from spreading. 

Fibrosarcoma 

What it is: Fibrosarcomas are a common type of tumor found in dogs, particularly in larger breeds. They develop when too many fibroblasts (cells in the connective tissue) are produced. They usually grow slowly, but tumors in the legs are more aggressive.

What it looks like: Most of the time fibrosarcomas develop as a single lump or bump, either on or beneath your dog’s skin. They can also break open and bleed. These tumors are most common in your dog’s mouth, intestines, chest wall, and legs. However, since they can develop deep within the tissue, they can be very hard to detect. A full physical examination by your veterinarian is the best way to discover and diagnose a fibrosarcoma. 

Dangers and risks associated with fibrosarcomas: Fibrosarcomas often cause swelling in the surrounding area, resulting in a dog who does not want to be touched. In addition, a dog suffering from fibrosarcoma may refuse to eat or play, struggle with walking, and may have trouble getting up or lying down. Fibrosarcomas in the mouth and nasal cavities can also cause eye or nasal discharge, bleeding, sneezing, and snoring. In the mouth, they can bleed and make it hard to eat, or cause bad breath and even tooth loss.

Treatment: If they are detected early and treated quickly, dogs with fibrosarcomas tend to do very well. The primary treatment is to remove the fibrosarcoma surgically. However, these tumors are very invasive. They can destroy surrounding tissue and bones, making them difficult to remove completely. Amputation is required in some cases, and fortunately, dogs tend to do quite well afterward. Although they are aggressive and malignant, fibrosarcomas do not typically spread to other parts of the body. 

Hemangiosarcoma 

What it is: Hemangiosarcomas occurs in the lining of the blood vessels, so these tumors can develop anywhere in your dog’s body. However, tumors in dogs are most common in the skin, heart, and spleen. Hemangiosarcomas are highly invasive and can be deadly. They are known as “silent killers” because dogs don’t always show symptoms until it is too late. 

What it looks like: Cutaneous hemangiosarcomas develop on the surface of the skin, which can make them easier to see and diagnose. These tumors appear as a black or red mass or growth on your dog’s skin. Visceral hemangiosarcoma, however, develops internally and spreads very quickly. 

Dangers and risks associated with hemangiosarcomas: Signs and symptoms of hemangiosarcoma often don’t show up until the cancer is in its later stages. Visceral hemangiosarcoma, for example, is usually detected after the tumor has already ruptured. This requires emergency surgery to stop the bleeding. 

Dogs with light-colored, thin, or short fur have a higher risk of developing cutaneous or dermal hemangiosarcoma with sun exposure. In addition, some breeds may be more susceptible to hemangiosarcoma, including boxers, Portuguese water dogs, golden retrievers, and German shepherds. 

Treatment: If the cancer is detected before the tumor ruptures, your vet may treat it with surgery, chemotherapy, or radiation therapy.

Lipoma / Liposarcoma

What it is: Lipoma is a slow-growing tumor in the fat cells. Fortunately, these common tumors are usually benign and rarely cause problems for dogs. However, malignant lipomas, or liposarcomas, are cancerous. 

What it looks like: Generally, these tumors grow slowly and may look like a small and soft, moveable lump beneath the skin. 

Dangers and risks associated with lipomas: Lipomas are rarely dangerous, but your dog will need to be examined by a vet to be sure. Malignant lipomas, or liposarcomas, are cancerous and need to be treated. Fortunately, they do not tend to spread to other parts of the body. 

Overweight dogs are more likely to develop lipomas. In addition, breeds like Labrador retrievers, miniature schnauzers, golden retrievers, cocker spaniels, Doberman pinschers, and Weimaraners are also more prone to developing lipomas. 

Treatment: Most of the time, lipomas develop in subcutaneous fat under your dog’s skin. Occasionally, they may also occur in organs like the stomach. In some cases, the tumor may need to be removed because of its location (such as if it is causing your dog to limp or if it is affecting a vital organ). In addition, some owners may wish to have the tumor removed for aesthetic or cosmetic reasons. If the tumor is cancerous, it will also need to be removed and your dog may need chemotherapy or radiation therapy. 

Mammary Tumor / Mammary Carcinoma

What it is: Similar to human breast cancer, mammary tumors affect the mammary glands (milk-producing glands) and breast tissue in dogs. Many are benign, but malignant tumors — known as mammary carcinomas — can be dangerous to your dog’s health. 

What it looks like: Typically a mammary gland develops as a mass on your dog’s abdomen. It is usually firm and may feel warm to the touch. It can also be quite painful for your dog, and the skin covering the tumor may break and bleed. 

Dangers and risks associated with mammary carcinomas: 50% of all mammary tumors are malignant and cancerous, and they can spread to other parts of your dog’s body. Your dog may have swollen glands, pain in their abdomen, and gland discharge. They may eat less and lose weight, cough, or have trouble with breathtaking. 

Certain breeds appear to be more prone to mammary tumors, including poodles, Brittany spaniels, cocker spaniels, Chihuahuas, English setters, Yorkshire terriers, dachshunds, Doberman pinchers, and boxers. 

Treatment: Fortunately, when female dogs are spayed before their first heat, they only have a 0.5% risk of developing a mammary tumor. After that, however, the risk increases. Dogs spayed after their first heat, for example, have an 8% risk, and dogs spayed after their second heat have a 26% risk. 

Dogs with just one tumor can have it removed with surgery. However, dogs with multiple tumors may need their mammary glands removed completely, and they may need chemotherapy and radiation therapy. 

Thyroid Carcinoma

What it is: The thyroid gland in your dog’s neck produces hormones to help the body. However, in thyroid carcinoma, abnormal cells in the thyroid grow out of control and develop into a mass or tumor. If the tumor is benign, it is called an adenoma. A malignant thyroid tumor is called a carcinoma or adenocarcinoma.

What it looks like: Thyroid carcinomas are internal tumors, so sometimes you can’t see much from the outside. However, in some cases, you may notice a lump on your dog’s neck. 

Dangers and risks associated with thyroid carcinomas: Unfortunately, most thyroid tumors in dogs are malignant and often spread to other parts of the body. In addition, thyroid tumors can press on your dog’s throat, making it difficult to breathe and swallow. Certain breeds, including Siberian huskies, beagles, golden retrievers, German longhaired pointers, and boxers seem to be more prone to this type of cancer. 

Treatment: Unfortunately, by the time a thyroid tumor is diagnosed in a dog, it may have already spread to the rest of the body. However, depending on the severity of the tumor, it may still be treated with chemotherapy, radiation therapy, surgery, or radioactive iodine therapy.