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Health & Wellness

How to know if your dog's tumor is cancerous

The difference between benign and malignant tumors in dogs.

We do certain things regularly for our pups, like daily walks, checking their fur for ticks or cleaning their ears. Another activity you’ll want to consider adding to your care list is feeling for lumps on their skin. Not only are you giving your dog some more quality petting time, but you’re also searching their skin as a preventative health measure. If you find a lump on your dog while petting their skin, use this article as a guide for what to do next. 

What does a tumor look like on a dog? 

According to Dr. Elizabeth Devitt, DVM, a veterinary consultant for Fetch, tumors vary greatly in appearance depending on their type — and what they look like alone shouldn’t be used to classify what kind of growth they are. 

Tumors can be hard or soft, moveable or immovable and can differ in color and size. 

What if I find sudden lumps under my dog's skin?

The best next step after finding a lump on your dog is to call your veterinarian. Although there’s always a chance that a growth is harmless, you’ll want to have them confirm the prognosis. While waiting for your appointment, keep an eye on the lump for any changes and monitor your dog’s behavior. 

“Don’t panic, but definitely take note of where you found the lump,” Dr. Devitt explains. “Take a photo or put a dab of red nail polish on it, unless it’s near their eyes. Nothing’s more frustrating than being unable to locate the lump when you get into the veterinary exam room.”

Certain factors, like the length of time a lump has been on your pup, any pain associated with it or rapid growth, aren’t reliable factors for determining a tumor's severity, so it’s important to get your vet’s opinion on every lump, Dr. Devitt adds.

How do I know if my dog’s lump is cancerous?

To determine whether a lump is cancerous, you’ll need to take your dog to your veterinarian’s office for a checkup. Although some bumps can be identified through a fine needle aspirate (using a needle to collect a sample of cells), others may require a biopsy. 

If you’re unfamiliar with a biopsy, it’s when a veterinarian removes the whole (or part of a) tumor and sends it to a laboratory for review. Depending on the results, your veterinarian will work with your dog to determine the next steps or might refer you to an oncologist. 

What are the signs of cancer in dogs?

Dogs show different cancer symptoms depending on the type, stage and location. Some common signs of cancer include weight loss with a healthy hair coat, poor appetite, vomiting, lack of excitement or noticeable energy changes. Unfortunately, not all dogs present symptoms the same way, making it challenging to catch cancer early.

“Maybe the most heartbreaking thing about many canine cancers is that our beloved pets may not show any specific signs until they can no longer hide the impact of cancer,” Dr. Devitt says.

RELATED: Types of cysts and bumps on dogs

What are the types of dog tumors?

Various types of tumors can affect our pups. Some dogs may even have more than one type of tumor at a time.

Rest assured that not all tumors are malignant, meaning they aren't all cancerous or prone to spread. So, let’s dive deeper into some of the most common non-cancerous and cancerous tumors seen in dogs.

Types of benign tumors on dogs

Some tumors are harmless unless they grow in a spot that makes things uncomfortable for your pup. The most common benign (or non-cancerous) tumors include:


Lipomas are tumors made up of fat cells and are commonly referred to as fatty tumors. They typically sit under the skin and feel soft to the touch. These growths are usually round and easy to move, and dogs may grow multiple lipomas, especially if they’re genetically predisposed.

Unless a lipoma grows large or causes discomfort, they don’t usually need to be removed.


A lump of collagen is called a fibroma. One of the most common causes of a fibroma is trauma on pressure points, such as the elbows. Fibromas are slow growing and generally don’t cause discomfort.


Adenomas are tumors made up of hepatoid glands (aka modified sweat glands). They’re particularly common on the skin of small dogs and are wart-like.

Although benign, adenomas are easily irritated by grooming clippers or brushes and can rupture and become painful.


Sometimes extra blood vessels in the skin can form a mass called a hemangioma. Most commonly, these tumors look like bruises or blood blisters, and they may sometimes rupture, bleed and cause discomfort.


Histiocytomas are another common benign tumor made up of histiocytes, which are immune cells protecting the skin.

“While histiocytomas and hemangiomas are benign, their appearance may be similar to mast cell tumors or hemangiosarcomas, which are cancers that require medical and surgical intervention,” Dr. Devitt shares. Therefore, it’s important never to assume you know the tumor type without a trip to the veterinarian.

Types of cancerous tumors on dogs

Mast cell tumors

Mast cell tumors are cancerous and made up of mast cells, which are a type of white blood cell. They often form as a nodule on or underneath the skin with a hairless or ulcerated surface, but they can also affect the organs. 

These tumors can vary in size and appearance, and handling one of these growths can cause it to suddenly shrink, which is a potentially serious medical situation because of what’s inside of them. 

“When histamine granules are released into the bloodstream, they can cause a sudden drop in blood pressure, itchiness, bruising or stomach ulcers,” Dr. Devitt warns.

Mast cell tumors’ severity is graded on their potential to metastasize or spread throughout the body. Low-grade mast cell tumors are less likely to spread than high-grade variations.

Boxers, Labrador Retrievers and Golden Retrievers are at a higher risk of developing mast cell tumors. 

Mammary tumors

Mammary tumors — tumors arise from the cells that make up the mammary glands — can be benign or malignant.

About half of these tumor types are benign, but the only way to confirm is through analyzing the cells in the lump, ideally through a biopsy. Unfortunately, female dogs who aren’t spayed are far more likely to develop these tumors. Obesity can also pose a risk to dogs developing these tumors. 

“The exact cause for how mammary tumors develop isn’t fully understood, but there’s a link to estrogen and progesterone hormones,” Dr. Devitt says. “Studies have shown that dogs spayed before their first heat have 0.05% risk of developing a malignant mammary tumor, an 8% risk if spayed after the first heat and a 26% risk if spayed after the second heat.”

Malignant mammary tumors are usually firm and either within or next to a nipple. It’s essential to check your dog regularly for lumps and to take every bump you find seriously. The most common type of mammary tumor is carcinomas, a variety of cancer that starts in epithelial cells in the skin or in tissue that lines organs, such as the liver or kidneys.

Brain tumors

Brain tumors, although they may sound frightening, aren’t always malignant or terminal. A meningioma is one common example of a benign tumor that originates from the meninges, the membranes covering the brain.

Unlike the other tumors we’ve discussed, brain tumors aren’t visible to the naked eye and will need to be diagnosed via magnetic resonance imaging (MRI) or computed tomography (CT). These are specialized scans used to take detailed images of the inside of the body.

According to Dr. Devitt, a dog with a brain tumor may show behavioral signs such as a head tilt, circling in one direction or falling to one side. However, all these signs can also indicate a common and benign condition called vestibular disease, so it’s important to have your dog evaluated by a veterinarian because the following steps will vary depending on the diagnosis.

What are dog cancer treatment options?

Before deciding on a treatment plan, your veterinarian will likely want to stage the cancer to determine how much it’s spread. 

Depending on the type, stage and location of cancer, there are different treatment options your veterinarian might recommend, including:

  • Surgery to remove the cancer
  • Radiation therapy to kill cancer cells
  • Chemotherapy which can be injections and/or oral medication
  • Immunotherapy uses the dog’s immune system to fight cancer cells

Navigating through a cancer diagnosis is hard, but remember that hope isn't lost. It’s best to get your dog into your vet’s office as soon as you notice any lumps and to avoid shrinking the tumor at home. Your veterinarian will be able to guide you through the best treatment options to help get your pup’s health back on track. 

The Dig, Fetch Pet Insurance's expert-backed editorial, answers all of the questions you forget to ask your vet or are too embarrassed to ask at the dog park. We help make sure you and your best friend have more good days, but we’re there on bad days, too.

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