How to Determine If Your Dog’s Lump Is Cancerous

Published Jun. 10, 2024
white terrier dog sits on woman's lap.

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In This Article

What Are Dog Lumps?

Lumps and bumps are concerning for pet parents to find on their canine companions. Various types of lumps in dogs can develop, but the only way to know what type  of lump is affecting your pup is to see a veterinarian for a diagnosis.

Lumps on dogs can be benign (non-cancerous) or malignant (cancerous). A veterinarian will conduct testing to make an accurate diagnosis and develop an appropriate treatment plan based on the results.

What Are Dog Lumps?

A dog may develop a lump for various reasons, which range in severity. Normal aging changes can cause benign lumps to form in senior dogs. In more serious conditions, abnormal growth of cells can lead to cancer.

Lumps in dogs can be cysts, warts, tumors, abscesses, or skin tags. Sometimes, a dog develops a lump from an allergic reaction or an injection, such as a vaccine.  

It’s always best to chat with your vet if you notice a new lump on your dog, so they can act swiftly if needed.

What Do Cancerous Lumps Look Like On Dogs?

Cancerous lumps on dogs vary greatly in appearance. In general, cancerous lumps tend to have the following characteristics:

  • Abnormal shape. Cancerous dog lumps usually have irregular borders. They may also have both soft and firm areas.

  • Rapid growth. Pet parents may notice a small lump one day on their dog only to find it has tripled in size just a few weeks later. This is typically seen in fast-growing, aggressive cancers, such as osteosarcoma. Cancerous lumps on dogs will continue to grow without treatment.

  • Ulcerated areas. Many cancerous lumps bleed or have areas of dead tissue within them that may ooze through the skin. There is usually a strong odor to these lumps, and blood or pus may also be present. 

  • Abnormal color. Cancerous dog lumps can be a different color than the dog’s skin. For example, some lumps may be dark brown or black. Others can be red and irritated.

  • Hair loss. Fur may be lost in the lump area, exposing the skin underneath. This is common in cancerous lumps that are itchy or inflamed, such as mast cell tumors, because the dog tends to lick and scratch them. This can lead to trauma to the hair follicles.

  • Attachment to underlying tissue. Cancerous lumps tend to be attached to the bone, muscle, or other tissue below them. This means that when a pet parent or veterinarian grasps the lump, they do not freely move underneath the skin but rather remain firmly in place.

Types of Cancerous Lumps on Dogs

Numerous cancerous lumps can occur in dogs. A few of the most common include:

  • Soft tissue sarcoma. This type of cancerous lump is locally invasive but slow-growing. It forms within soft tissues throughout the body, such as muscles, tendons, and fat.

  • Lymphoma. This type of cancer affects lymph tissues such as the lymph nodes and the spleen. It often spreads to other parts of the body and grows quickly.

  • Osteosarcoma. This cancerous lump forms on a dog’s bone. It quickly spreads to other parts of the body.

  • Mast cell tumor. This lump develops on the skin and can be extremely itchy for dogs, so it tends to bleed. It may stay localized in one area or spread throughout the body.

  • Dogs may also develop other cancerous lumps such as fibrosarcoma, squamous cell carcinoma, basal cell carcinoma, or malignant melanoma.

When Should I Call My Vet About My Dog’s Lump?

Any time you notice a new lump on your dog, it is important to reach out to a veterinarian. An accurate diagnosis is necessary to determine what the lump is and the appropriate next steps to maintain your dog’s health and well-being.

Does Your Dog Have Other Symptoms?

Generally, if a dog has a cancerous lump, he will experience other symptoms due to the cancer. The lump is typically one piece of the cancer puzzle.

These symptoms can vary depending on the type and location of the lump, but may include:

Pet parents should understand that while these are common characteristics of cancerous lumps, not every lump that look likes this will be diagnosed as cancer.

Likewise, not every dog with a lump experiencing these symptoms will be diagnosed with cancer. Seeking guidance from a veterinarian is the best way to determine whether a lump on your dog is cancerous or not.

How Your Vet Will Diagnose Your Dog’s Lump

Anytime you find a new lump on your dog, it is important to contact your vet for a diagnosis so appropriate treatment can begin. If your pup had a previous lump examined and it was diagnosed as benign, but it has since changed or grown, it’s important to flag this to your vet for follow-up care.

Anytime you find a new lump on your dog, it is important to contact your vet for a diagnosis so appropriate treatment can begin.

A veterinarian will start with a thorough physical exam to identify any obvious abnormalities, including lumps and bumps. If a lump is found, the veterinarian will note how it feels, its shape and size, its color, and if it is freely moveable or firmly attached to tissues underneath it.  

A veterinarian may do the following tests to aid in diagnosis:

  • Fine needle aspirate. A small needle is gently poked into the lump and wiggled around to collect a sample of cells. This sample is placed on a slide, stained, and then viewed under a microscope to identify the type of cells present.

  • Biopsy.  A piece of the lump (or sometimes the entire mass) is removed and sent to a special laboratory where various tests are done to identify cells. This may be done under local or general anesthesia. If the lump is determined to be cancerous, this test can also via a grading system determine how aggressive the growth is.  

  • Imaging. Ultrasound, X-rays, and CT scans can all be used to determine whether the lump has spread to other parts of the dog’s body.

A veterinarian may also check a dog’s overall health by completing blood work and urinalysis.

After diagnosis, your vet can determine a suitable treatment plan based on the results. Sometimes no treatment is necessary, while other times more extensive treatment, such as surgery or chemotherapy, is needed.

Treatment varies based on the type of lump present, the dog’s overall health, and the goals of both the veterinarian and pet parent.

Finding any type of lump on your dog can be concerning. However, by promptly working with your veterinarian to determine an accurate diagnosis, you will get back important answers that will guide care decisions going forward for your pup.

Brittany Kleszynski, DVM


Brittany Kleszynski, DVM


Dr. Brittany Kleszynski is a veterinarian and freelance medical writer who specializes in creating meaningful content that engages readers...

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