As Amazon Associates, we may earn from qualifying purchases. See disclosure in sidebar.

Dog Skin Lesions or Lumps Due to Cancer [with Pictures]

Score for Seniors:
Activity Level:
Weight: Pounds

dog with cancer next to owner

This article was updated on May 1st, 2023

vet-approved badge

Many dog parents will see a skin lesion or lump and immediately think of the word cancer. However, in my experience as a veterinarian, many skin lesions or lumps are benign and usually nothing to worry about – it is, therefore important to know the difference. In this article, we will review common dog skin lesions, bumps or lumps due to cancer, with pictures to help understand what they look like.

Similar to cancer in humans, the causes of skin cancer in dogs aren’t completely understood Injury/trauma, sun damage, and certain illnesses may play a role in the development of malignant skin tumors, lesions, or lumps in dogs. Genetics also play their part, and certain breeds are more susceptible to developing specific cancers.

What to Do if You Find a Suspicious Skin Lesion on Your Dog

veterinarian inspecting dog with skin issues

Regularly check your dog for changes in their skin. Run your hands through their coat once a week, and be sure to feel in all the creases and folds for any abnormal lumps or lesions. Make a note of the size of the lesion and the general appearance – a photograph can help you keep a diary in this way.

So, what happens if you find a skin lesion? There are two options: Monitor it for a while and see if it changes or take your dog to the vet for a checkup.  We would always recommend the latter if you’re not sure, particularly if it looks sinister. We will discuss more on how to tell a cancerous lump from a benign one next.

How Do You Know When a Skin Lesion or Lump Is Cancerous?

Cancer is the abnormal and rapid growth of previously healthy cells. Therefore, lesions or lumps that are growing rapidly or changing in appearance quickly may indicate a more sinister underlying cause.

Ulceration, redness, and a firm texture are all common properties of a cancerous lesion, but that doesn’t mean that every lump with these features will be cancerous.

WATCH: 3 Important Tips To Care For an Old Dog [VET VIDEO]

Cancerous, or malignant, skin lesions and tumors can be small or large. They may itch or cause the senior dog some discomfort. They may do neither.

You can never say whether a lesion is cancerous just by looking at it, but there are some clues that can help raise suspicions:

  • Rate of growth: Cancerous lesions tend to grow and spread more rapidly than those that are benign. They will grow into the surrounding healthy tissue, causing damage and inflammation. If a lesion doubles in size over the course of a few weeks, then it’s best to get it checked.
  • Texture: Cancerous lesions and lumps are often harder and firmer to the touch.
  • Shape: Due to their rapid and erratic growth, cancerous lumps tend to appear more irregular in shape.
  • Color: Red, black, or just generally ‘unhealthy’ looking lesions may be more malignant in origin.
  • Discharge: Oozing or discharge from the lesion may occur due to damage and death of the tissue in surrounding areas. While any lump can develop a secondary infection, sinister lumps are more prone to producing pus and bleeding.
  • Irritation: A lump that is itchy or causing discomfort to your dog is more likely to be cancerous.

Skin cancer is more common in older dogs also, and a dog’s skin will naturally change with age so sometimes it can be hard to tell whether changes are due to simple old age or due to a more sinister underlying cause. Therefore, it’s important that your vet has a proper look to make the diagnosis – it’s a good idea to have any new lumps tested by your veterinarian

Skin Lesions or Lumps that Are Cancerous or Often Cancerous

Malignant skin tumors in dogs include mast cell tumors, mammary gland tumors, malignant melanoma, and squamous cell carcinoma. Mammary gland tumors are not skin tumors as such, but they’re seen/felt in skin/tissue around your dog’s teats.

1. Mast Cell Tumors

In healthy dogs, mast cells are the part of the immune system that releases histamine in response to allergies. Mast cell tumors are more often seen in middle-aged and older dogs but can affect younger dogs too. These old dog lumps are not always malignant, but it’s impossible to know which ones are and which aren’t without a fine needle aspirate or biopsy.

mast cell tumor on dog's leg

Below is a cancerous mast cell tumor on a dog (circled with a pen before surgery to remove the tumor):

mast cell tumor

Mast cell tumors can vary a LOT in appearance (view more pictures of mast cell tumors); usually, they’re a smooth, round growth visible on the skin. Other times they can look like a wart, or resemble a lipoma, and sometimes they are red in color. Learn more in our article about “Mast Cell Tumors: a Veterinarian’s Guide for the Dog Owner” (including images).

2. Melanoma

Melanoma tumors are dark and can be small, large, flat, or raised. They can be either benign or malignant, so they shouldn’t be ignored. This picture below also shows a melanoma (black growth) inside a dog’s mouth:

melanoma in dog's mouth

If a dog has malignant melanoma, it’s usually an aggressive cancer that spreads throughout the body quickly, so the lesion needs to be surgically removed as quickly as possible. Only a veterinarian can tell the difference between a benign and a malignant melanoma by doing a biopsy.

Most malignant melanomas in dogs grow in/around the mouth or in other mucus membranes, but they can also be found in other areas.

Disclaimer: This content is not a substitute for veterinary care. Always consult with your vet for health decisions. Learn more.

3. Squamous Cell Carcinoma

Squamous cell carcinomas in dogs are fairly rare, and they are not as aggressive in terms of spreading as melanoma or mast cell tumors are.

squamous cell carcinoma on a dog's leg
Squamous Cell Carcinoma on dog’s legenlarge

These types of malignant tumors are usually found on areas of skin that are bare, or have little hair, and are more common in dogs with light-colored skin. They are almost always on the belly and inner thighs (areas of thin or no hair), but can appear anywhere.

UV radiation from the sun can contribute to the development of these tumors. Squamous cell tumors in dogs can be raised lumps or nodules, or flatter areas of ulcerated skin. They can sometimes resemble warts.

4. Mammary Gland Tumors

Mammary gland tumors can be cancerous or benign. They are often located next to or beneath the nipple and may extend between multiple mammary glands. They are firm and may have ulcerated skin overlaying or be abscessed and bleeding. Due to the lymphatic drainage that links mammary glands together, these tumors can spread quickly to the other mammary glands and even to the rest of the body. Therefore, quick surgical removal is recommended. Learn more about mammary gland tumors in dogs.

closeup of a dog mammary tumor
Close-up picture of a cancerous mammary tumor in a dogenlarge

Read more: Mammary Tumors in Dogs (With Pictures)

5. Sarcoma Tumors

Hemangiosarcoma is a malignant cancer of the cells that line the blood vessels of the body. Therefore, hemangiosarcoma can occur anywhere in the body where blood vessels are found. In the case of hemangiosarcoma, the cells that line the blood vessels start dividing uncontrollably and in an unhealthy manner. This leads to the development of a mass that is very prone to rupture and can even cause a dog to bleed out internally. Read more: Hemangiosarcoma in Dogs: A Vet Explains.

hemangiosarcoma lump in a dog

Skin Issues That Can be Cancerous (Although They are Mostly Benign)

Warts on dogs are most often benign. Occasionally they can become cancerous so any wart that is a long-term issue, or that changes in color/size/look needs to be investigated by a vet.

See images below for example warts on dogs:

Also, check out our article featuring Common Skin Lesions in Dogs.

How to Diagnose and Treat Cancerous Skin Lesions

The diagnosis of skin lesions will involve your vet taking samples and analyzing them under a microscope in a process known as histopathology. Histopathology is performed by a board-certified veterinary pathologist in a laboratory so the sample will need to be sent in for analysis.

Different sampling techniques can be performed including:

  • Fine needle aspirate – a needle with a syringe attached is inserted into the lesion, and a sample is taken by pulling back on the plunger of the syringe. This aspirates some cells that can be placed on a microscope slide for analysis. The advantage of this technique is that many dogs will tolerate having it done consciously. However, it won’t always achieve a diagnostic sample as only a pinprick sample is taken. Furthermore, not all lumps will readily release cells for aspiration, so this technique doesn’t always work.
  • Biopsy –  a biopsy is a wedge of tissue that is taken from the lesion and then sent for analysis. This is performed under a general anesthetic and is the gold standard for getting a diagnosis of a skin lesion. Often the edge of the lesion will be taken to allow the histopathologist to compare normal to abnormal tissue.

Treating Malignant Skin Tumors in Dogs

The best way to treat most malignant or cancerous lumps in dogs is to remove them surgically, and as quickly as possible. The smaller the lump the easier the surgery and the less tissue which must be removed. A malignant lump will often be removed with a margin of normal-looking tissue around it – this is to ensure that any microscopic spread of the cancer is also removed.

Acting fast also helps to reduce the chances of the cancerous cells metastasizing (traveling) into other tissues, organs, and lymph nodes. Sometimes radiation therapy, chemotherapy, or other treatments are recommended in addition to removing the tumor. Your vet may wish to refer your dog to an oncologist to aid in this treatment. There are other treatments, such as injectable medications for tumor removal as well as cancer vaccines.

The exact treatment options will be decided by your vet and consider the size and location of the tumor and whether it has spread to other areas of the body.


Is skin cancer the only cancer that can cause skin lesions and issues?
While cancerous skin lesions can look very sinister, there are many other benign lesions that can look just as bad. These include dermatitis, hot spot lesions, and ringworm. It’s important therefore to have any unusual lesions tested as they may not be cancerous at all. This article covers the most common types of skin lesions in dogs.

Related Posts:


  • Dr Alex Crow, Veterinary Surgeon

    Alex Crow, VetMed MRCVS, is an RCVS accredited Veterinary surgeon with special interests in neurology and soft tissue surgery. Dr Crow is currently practicing at Buttercross Veterinary Center in England. He earned his degree in veterinary medicine in 2019 from the Royal Veterinary College (one of the top 3 vet schools in the world) and has more than three years of experience practicing as a small animal veterinarian (dogs and cats).


  1. We have a 12-year-old mastiff/ lab mix and she has a huge red growth on the outside of her upper lip. Just like the first photo but on the lip. It is quite large. Is that a cancerous tumor?

    • These lumps or growths are not necessarily malignant. However, you should get your dog checked out by a vet as they will be able to do further testing to determine the true nature of the growth. It’s really challenging to make an accurate diagnosis just by looking at a lump. If your dog needs treatment, the sooner you act, the better.

  2. A dog was basically dropped off at my driveway, seemed in good health, not underweight, no fleas and a healthy appetite. After further inspection I saw a soft/firm or slightly firm growth on on leg above paw. Hairless,painless and about the size of a charry tomato, a large cherry tomatoe but still about that size. Doesn’t seem to cause pain. We have an appointment On Monday after the weekend. Hoping it isn’t cancer. Can’t find anything online that looks like what he has. I have been looking for a dog for soo long. And was so excited to see one just walk right into my life. Fingers crossed it will be something treatable.

Leave a Reply

Your email address will not be published.


This site uses Akismet to reduce spam. Learn how your comment data is processed.