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Dog Tumors

What Are Dog Tumors?

Dog tumors arise when cells undergo uncontrolled proliferation, failing to adhere to the natural cycle of cell death. They generally fall into two categories: benign and malignant.

Benign tumors lack the capacity to infiltrate surrounding healthy tissue or spread to other parts of the body. While they may necessitate medical attention, they are not cancerous.

Malignant tumors, also known as cancers, have the ability to metastasize, spreading to distant organs and tissues. Depending on the tumor type and its aggressiveness, cancers can pose significant health risks to your dog. A visit to the veterinarian is essential to assess the nature and potential threat of any tumor.


Here are the most common types of tumors found in dogs:

  • Mast Cell Tumor: These malignant tumors develop in the mast cells of a dog’s skin, which are part of the immune system. They can resemble various skin abnormalities and may even be mistaken for benign growths.
  • Lymphoma: This cancer originates from lymphocytes, a type of white blood cell crucial for immunity. Lymphoma often manifests as enlarged, firm lymph nodes, and affected dogs may display signs of lethargy or reduced appetite.
  • Lipoma: Lipomas are benign fatty growths typically located beneath the skin. While usually harmless, large lipomas can cause mobility issues if situated in inconvenient spots.
  • Osteosarcoma: This malignant bone cancer can be extremely painful, leading to limb swelling, fractures, and lameness in affected dogs.
  • Histiocytoma: Benign tumors originating from skin histiocytes, these growths often resolve on their own within weeks, though they may undergo changes in appearance.
  • Hemangiosarcoma: Arising from blood vessel lining cells, hemangiosarcomas are often diagnosed after rupturing, leading to internal bleeding.
  • Melanoma: These malignant tumors stem from melanocytes, pigment-carrying skin cells, and can occur in various locations, including the oral cavity, eye, nail bed, or skin.
  • Oral Melanoma: The most prevalent oral tumors in dogs, oral melanomas are highly malignant and may present as raised or flat lesions, often accompanied by halitosis.
  • Papilloma: Benign growths caused by the canine papillomavirus, papillomas typically disappear within weeks and commonly affect areas like the lips, tongue, throat, or gums.
  • Mammary Gland Carcinoma: Originating from mammary tissue, these tumors are frequently found in intact or late-spayed female dogs.
  • Thyroid Carcinoma: Tumors developing from thyroid gland cells, thyroid carcinomas often go unnoticed until metastasis occurs in other organs.

Understanding these common types of tumors in dogs is crucial for early detection and effective management of canine health issues.

Symptoms and Types

The symptoms of tumors in dogs vary depending on factors such as the type of tumor, its location, and whether it is benign or malignant.

For instance, brain tumors may manifest as seizures, behavioral changes, or a tendency to circle. Mast cell tumors often exhibit symptoms related to histamine release, including swelling, bleeding, difficulty clotting, and vomiting. Liver-associated tumors can result in elevated liver values in laboratory tests.

Certain types of cancers may induce paraneoplastic syndrome, characterized by symptoms or changes unrelated to the cancer itself. This syndrome occurs when tumors secrete hormones or hormone-like substances, or when the dog’s immune system reacts to the tumor. These symptoms, which may indicate the presence of cancer, can be observed through lab work and noticeable physical changes in the dog. They include altered reflexes, weakness, partial paralysis, and other manifestations. For example, lymphosarcoma may lead to elevated calcium levels, causing increased thirst in dogs or showing up in routine lab tests.

Additionally, some tumors can deplete blood cells, resulting in anemia (low red blood cells), decreased white blood cell count, and/or low platelets. Conversely, other tumors may cause an increase in these blood cell counts.


Understanding the causes of cancer in dogs involves recognizing the complexity of the disease, which ultimately stems from cellular damage.

Cells in the body undergo growth, division, and replacement as part of their natural processes. However, when the DNA within a cell becomes damaged, it can lead to mutations or incorrect divisions. Although the body has mechanisms to correct or eliminate mutated cells, some may persist and develop into tumors or cancer.

Various factors can contribute to cellular damage, often through a combination of influences. These may include:

  • Genetics: Certain breeds, such as Golden Retrievers, Boxers, Bernese Mountain Dogs, and Rottweilers, are more prone to specific types of tumors. While a genetic or hereditary link is suspected, further research is required to identify which dogs are at higher risk of developing particular cancers.
  • Age: Cancer incidence tends to rise in older pets. The relationship between cancer and aging is not fully understood but is believed to be related to the weakening of the immune system with age. As the body ages, the likelihood of mutated cells evading the body’s defenses and causing cancer increases.
  • Environment: Environmental factors and chemical exposures can elevate the risk of cancer in both humans and their pets. Examples include various herbicides, pesticides, nickel, uranium, radiation, asbestos, and prolonged exposure to sunlight and UV radiation, all of which have been linked to carcinogenesis.

Ongoing research endeavors aim to enhance our understanding of cancer in dogs, potentially leading to improved treatment options in the future.

What To Do if You Find a Lump on Your Dog

If you discover a lump or bump on your dog, remain calm and schedule an appointment with your veterinarian to have the growth examined.

Take note of when you first observed the lump, its location, any changes in size or texture, and whether your pet is exhibiting any behaviors like licking or chewing at it. It can also be beneficial to measure the mass with a tape measure or capture a photo for reference during the vet visit.


Veterinarians cannot definitively diagnose a tumor simply by observing or palpating it. Accurate diagnosis typically requires sampling.

Fine Needle Aspirate:

The initial step in determining the nature of a growth involves a fine needle aspirate (FNA). During this procedure, a needle is inserted into the mass to collect cell samples. The needle, often smaller than those used for blood draws, facilitates a relatively quick and straightforward process.

The collected cells are placed on a slide and sent to a laboratory for analysis, providing insights into the type of growth present. In cases involving internal organs like the liver or spleen, ultrasound-guided FNAs may be recommended.

However, FNAs have limitations as they only provide cell samples, occasionally lacking sufficient information for precise diagnosis by pathologists. In instances of cancer diagnosis, additional data may be necessary to determine optimal treatment strategies.


If an FNA proves inconclusive or if more detailed information is required, a biopsy may be suggested. Biopsies involve the removal of either the entire tumor (excisional biopsy) or a small portion (incisional biopsy). While typically performed under general anesthesia, biopsies offer comprehensive insights into the tumor and may result in complete tumor removal.

For tumors affecting the gastrointestinal tract, endoscopic biopsies using a camera inserted through the mouth or rectum may be conducted. In cases where internal organ tumors require additional sampling, surgical procedures like exploratory laparotomy may be necessary.


The staging of canine tumors differs somewhat from that of human tumors. Depending on the type of tumor, staging may involve Roman numerals ranging from 0 to IV, where a higher number indicates further spread. Other tumors are graded differently.

The choice of treatment for a dog’s tumor depends on several factors:

  • The level of aggressiveness desired in treatment.
  • Whether the tumor is prone to spreading to other organs or remains locally invasive.
  • The stage and size of the tumor.
  • The severity of tumor-related symptoms.

Treatment options encompass surgical removal, radiation therapy, immunotherapy, and chemotherapy.

Living and Management

Certain types of dog tumors can be effectively cured, particularly those that are locally invasive and can be entirely excised through surgery.

However, for cancers that spread or metastasize to other areas of the dog’s body, curative options are limited. Nonetheless, these tumors can often be managed with treatment, even though a complete cure may not be achievable.

The positive aspect is that cancer treatments in dogs are generally well tolerated and come with minimal side effects. For instance, chemotherapy seldom causes hair loss in dogs and typically only induces mild gastrointestinal discomfort. Some chemotherapy drugs can even be administered at home, reducing stress for the pet.

Upon receiving a cancer diagnosis for your pet, it’s crucial to gather as much information as possible before making decisions. Consulting with a veterinary oncologist can offer insights into potential outcomes and what to anticipate. They can elucidate treatment options and their implications for your pet’s longevity and quality of life. Even if you opt against chemotherapy or radiation therapy, having a thorough understanding of the situation can bring peace of mind and aid in decision-making.

Dog Tumors FAQs

How can you distinguish between a cyst and a tumor in dogs?

A cyst and a tumor are discerned through a fine needle aspirate or biopsy. Cysts typically contain fluid or waxy debris, while tumors tend to be more solid in nature.

Can dogs live with mast cell tumors?

Low-grade mast cell tumors may remain undetected for years. However, as mast cell tumors can mimic various other types of tumors, a fine needle aspirate is necessary for diagnosis. High-grade mast cell tumors have the potential to spread, infiltrate healthy tissue, and pose long-term fatality risks. Differentiating between high-grade and low-grade mast cell tumors requires removal and testing by a pathologist.

What do tumors look like on dogs? How about benign tumors?

Tumors on dogs can manifest in various ways: as lumps or bumps on the skin, alterations in coloration, or changes in skin texture. Routine veterinary examinations often uncover tumors, emphasizing the importance of regular check-ups. Differentiating between benign and malignant tumors necessitates sampling the tumor for testing.

Are cancerous tumors in dogs firm or soft?

Cancerous or malignant tumors can exhibit either firm or soft characteristics. The texture of a mass and its impact on the dog do not reliably indicate malignancy.

How can you differentiate between a tumor and a fatty tumor in dogs?

Determining whether a tumor is a fatty tumor (lipoma) or another type requires a fine needle aspirate or biopsy. Lipomas are prevalent in dogs, and although it may be tempting to diagnose them based on feel, doing so risks overlooking potentially dangerous conditions. Any new lump or bump on a dog should be evaluated by a veterinarian.

What do skin tumors appear like on dogs?

Skin tumors in dogs vary in appearance, encompassing a range of shapes, sizes, and colors. Veterinarians express more concern over bumps within the skin layer compared to those on the surface, resembling skin tags or wart-like growths common in older dogs.

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