A Pet Parent’s Guide to Dog Tumors: Symptoms, Causes, and Treatment Options

A dog tumor is a mass of tissue that develops when cells grow or divide abnormally. A dog tumor can be benign or malignant. 

Benign tumors are non-cancerous and don’t spread or invade other organs. Dogs can live with benign tumors even without treatment. However, they can cause complications (block blood vessels) or discomfort (limit the dog‘s movement and mobility).     

Malignant tumors are cancerous and can spread to other tissues. They can invade both nearby and distant parts of the body by spreading through the circulatory system. 

Diagnosis of a dog tumor must be completed by a licensed veterinarian (DVM). A pet owner cannot make this diagnosis without appropriate diagnostic tools that are made available at an animal hospital to determine if it is benign or malignant. 

The most common type of tumor in dogs is lymphoma accounting for nearly a quarter of all new cases. Lymphoma is a type of tumor that begins in a type of white blood cells known as lymphocytes. 

Lymphoma is an umbrella term used to define many types of tumors, all of which attack lymphocytes. 

Behind lymphoma, mast cell tumors are the second most common type of tumors in dogs. Other common tumors in dogs include osteosarcoma, melanoma, and fibrosarcoma. 

Fibrosarcomas. This type of malignant dog tumor affects soft tissue cells known as fibroblasts. Fibroblasts are found in connective tissues like tendons and ligaments. 

Fibrosarcomas start as swellings but can progress into open wounds or ulcers, at which point they become prone to infections. 

Large breed dogs (Irish Wolfhounds, Golden Retriever, and Gordon Setters) are most prone to this type of tumor.

Hemangiosarcoma. Hemangiosarcoma is a malignant dog tumor that affects the blood vessel lining. It is a fairly common type of cancer with an incidence of about 5% of all canine cancer cases. 

Since blood vessels are found all over the body, this tumor can occur anywhere, but the most commonly affected organs are the spleen, heart, and skin. 

Hemangiosarcoma is a widespread dog tumor among breeds such as Golden Retrievers, Labrador Retrievers, and German Shepherds. 

Histiocytoma. This is a benign tumor that affects histiocytes, a type of immune cell found in the skin. 

The tumor rarely causes any complications and usually resolves within three months or less. It looks very much like a button (wart) which is why it is sometimes referred to as a button tumor. 

Interestingly, puppies and younger dogs are more prone to histiocytoma than older dogs. 

Lipoma. A lipoma is a benign dog tumor that occurs in the fat cells. Usually, it is found in the subcutaneous fat but can occur in other organs like the stomach as well.

Lipoma rarely causes any complications, but some pet parents choose to remove the tumor for cosmetic reasons. In other cases, the lipoma may be removed because it is affecting the dog’s quality of life, for example, if it is located on a limb or eyelid.  

Lymphoma. Lymphoma is a malignant tumor that affects the white blood cells known as lymphocytes. A typical sign of this dog tumor is the hardening of the lymph nodes. 

Lymph nodes are masses of immune cells found at the base of the jaw, the front of the shoulder, and the back of the knees.

Susceptible breeds include Golden retrievers, Basset Hounds, Boxers, Scottish Terriers, and Saint Bernards. Lymphoma is just as commonly diagnosed in young dogs as it is in seniors.  

Mammary Tumors. Mammary tumors affect the breast tissue of a dog and can be benign or malignant. Interestingly, one dog can have different types of mammary tumors on different breasts at the same time.  

Just like in humans, mammary tumors are more common in female dogs. Spayed dogs are less likely to develop such tumors compared to intact bitches. 

Breeds prone to mammary tumors include English Spaniels, Poodles, and Terriers.

Mast Cell Tumors. As the name suggests, the tumor develops from mast cells – immune cells that play a role in allergic reactions. 

Mast cell tumor dog are the most common type of skin tumor in canines, accounting for about 20% of cases. They can, however, grow in other body parts like the spleen and the gut. Mast cell tumors can be benign or malignant. 

Dog breeds prone to mast cell tumors include Pugs, Labrador Retrievers, Golden Retrievers, Boxers, Bull Terriers, and Boston Terriers. 

Melanoma. Melanoma affects the melanocytes – skin cells that produce the pigment melanin. This tumor can affect different organs, including the eyes, skin, and nail beds.

Melanoma can be benign or malignant. Malignant melanoma is one of the aggressive types of skin cancer with a very high rate of metastasis. 

Breeds prone to melanoma as a dog tumor are Scottish Terriers, Chow Chows, Miniature Poodles, and Schnauzers. 

Oral Melanoma. Oral melanoma is a dog tumor that affects the mouth. Oral melanoma is relatively common and accounts for about 40% of melanoma cases. 

Oral melanoma is very aggressive and affects both the outer and inner layers of the mouth. It can also spread to other parts of the body. Oral melanoma signs include bad breath, difficulty eating, excessive drooling, and tooth loss.

Older dogs are the more prone to oral melanomas, and the most commonly affected breeds are Poodles and Schnauzers. 

Osteosarcoma. Osteosarcoma is a type of malignant cancer that begins in the bone cells. It first presents as temporary lameness but then progresses to painful swelling. 

Osteosarcoma is a very aggressive dog tumor and can cause severe complications. Since it usually affects the limbs, amputation is the treatment of choice. 

The incidence of osteosarcoma is highest in large and giant breed dogs. 

Papilloma. This benign tumor is caused by the Canine Papillomavirus that can be contracted from other dogs (in dog parks and daycare centers). This dog tumor can grow anywhere, but it is most common on the lips, tongue, throat, and gums. 

Papilloma presents as outward swellings on your dog’s skin. It can regress on its own within a couple of months, but surgery may be necessary in some cases. 

Sarcomas. Sarcoma is a broad term used to describe tumors that start in the bones or soft tissues like muscles, tendons, or blood vessels. 

Since there are so many types of soft tissues and therefore many types of sarcomas, the signs and symptoms of sarcomas vary widely. 

Large breed dogs seem to be especially prone to sarcomas. Age is also a risk factor as they are more common in older dogs.  

Sebaceous Gland Tumors. This tumor type is called an adenoma and attacks the sebaceous glands. An adenoma is a general term referring to a gland tumor. 

Adenomas or cysts are usually benign and don’t require treatment unless they affect a dog’s quality of life. Malignant adenomas are rare and, even if they develop, are highly treatable. 

The exact cause of a dog tumor is very hard to identify. There are, however, risk factors that can play a role in determining how likely a dog is to get a tumor. Some of these include:

Signs and symptoms of tumors vary widely in dogs. They heavily depend on the location and type of tumor. Some benign tumors like lipoma may not show any signs or symptoms except for a lump. 

Common signs and symptoms of a tumor in a dog include: 

In most cases, a cancerous dog tumor is firm in comparison to a benign dog tumor which is often soft. They are also usually immovable under the skin.

As cancer progresses, the dog tumor may ulcerate and form wounds. The wounds may have fluid discharges like pus or blood. 

There is no way to know if a dog tumor is cancerous just by looking at a dog’s body. Even a qualified veterinarian will use advanced tests like a fine needle aspiration and biopsy to diagnose your dog. 

It depends – a dog tumor can be painful or painless for dogs. A benign dog tumor like the mast cell tumor or lipoma tends to be painless, but it can cause discomfort in some dogs, depending on the location. 

In some cases, secondary injuries on the dog tumors like lesions, ulcerations, and rashes can be painful even though the tumor itself is not. 

A thorough physical examination by your vet should let you know how your dog’s tumors are affecting her well-being. 

During diagnosis, your vet will often start with a physical exam if the tumor is visible. The vet will then progress to more advanced tests like blood tests, x-rays, needle aspirations, and biopsies. 

A biopsy is a procedure that involves taking a sample of your dog’s tissue for examination. It is usually the best way to diagnose tumors. 

For easily accessible and superficial tumors, fine needle aspiration is sufficient. It involves the vet extracting a sample of the dog’s tumor for examination. 

Your vet may also need to perform a scan like an MRI or ultrasound to determine if the cancer has spread to other organs. 

If your dog is found to have a tumor, you will probably be referred to a veterinary oncologist. They are specialists in oncology and perfectly suited to help tumor diagnosis and treatment.

The mainstream treatments for a dog tumor are chemotherapy, radiation therapy, surgery, and since recently, intratumoral injections. Let’s take a closer look at the different treatments.

Chemotherapy. Chemotherapy is the use of anti-cancer drugs to kill off tumor cells. There are several types of chemo drugs. Which drug will be used depends on the type of cancer, location, and budget.

Common chemotherapy medications include Prednisone, Prednisolone and Chlorambucil, Cyclophosphamide, Carboplatin, and Doxorubicin (Dox). They are highly effective but also cause side effects like gastrointestinal distress and a change in bone marrow function. 

Radiation Therapy. Radiation is a form of treatment that uses targeted x-rays to shrink or destroy cancer cells. The goal of radiation is to damage cancer cells to prevent them from multiplying. 

Radiation can be used after other treatments like chemotherapy and surgery to deal with remaining cancer cells or before treatment to reduce the tumor to a more manageable size. 

Surgery. Your vet will be able to determine if your dog’s tumor qualifies for surgical removal. Surgical removal of tumors is a very effective means of tumor treatment, especially if the cancer is caught early. 

Unfortunately, not all tumors can be addressed with surgery. Tumors that have spread to other organs are usually not good candidates. In some instances, the risk of the surgery may outweigh the benefit, for example, if the tumor is on a vital organ or the dog is too old. 

Intratumoral Injection. This cancer treatment involves the injection of special medications directly into the tumor. The goal of the injection is to trigger the body’s immune system to fight the tumor.

If looking into ways how to shrink a dog’s tumor, Stelfonta is the answer. Stelfonta is an FDA-approved therapy for treating non-metastasized mast cell tumors. Amazingly, as much as 87% of mast cell tumors treated with Stelfonta resolve after only two treatments, and 89% of them have no tumor recurrence after one year. 

The time your dog has to live depends on the type and stage of the tumor. The promptness and quality of treatment your dog receives also play major roles in determining the survival time. 

A dog tumor is graded in stages marked by Roman numerals, just like humans. The stages go from 0 to IV. Advanced stages, particularly stage IV, have the worst prognosis, with most dogs passing within one year. 

Stage II cancers have a better prognosis, but 24 to 36 months seems to be the average life expectancy. Stage 0 and I have the best prognoses, and many dogs in these stages go on to recover fully. 

Cancer in dogs progresses very differently. Your vet should be able to tell you what to expect after diagnosis.