Skin Cancer in Dogs

What is Skin Cancer in Dogs?

Skin cancer is an abnormal growth of skin cells. While the term cancer has come to be used as a synonym of malignancy, fortunately not all skin cancer is malignant or cancerous. Tumors that are not malignant are called benign.

Malignant tumors are those that spread to other parts of the body through the blood stream or lymphatic system. Benign masses are those that do not have any harmful effects beyond the tumor itself.

As it does with humans, sun exposure (UV radiation) can increase the risk of skin cancer in many species. Dogs with lighter colored fur and skin are at higher risk of developing certain types of skin tumors.

Beyond the sun exposure risk, further study is needed to figure out what causes skin tumors to form. There is a genetic link with some types of tumors, like the high prevalence of mast-cell tumors in Boxers, Bulldogs, Boston Terriers and Pugs. There are even growths that can be caused by viral infections.

If you come across a lump on your pet, don’t immediately panic. There are many possible causes for lumps, and thankfully many are benign or easily treatable. The most important thing for you to do is to be diligent about watching your pup for any lumps or bumps, and let your vet know about any that you find.

Types of Skin Cancer in Dogs

Benign Skin Cancer in Dogs

Lipomas: This is a tumor that generally lies just under the surface of the skin. These masses originate from fat/lipid cells. Lipomas tend to be soft and squishy (fluctuant) and are generally not attached to any underlying structures. Lipomas can be quite small (grape sized) but can grow to be the size of a watermelon.

Histiocytomas: These tumors are generally found on the limbs of younger dogs, generally those less than 2 years old. They originate from a cell type called a Langerhans cell, which is part of the immune system. These tumors are often called “button tumors” as they tend to be small, round, raised, hairless pink masses. These tumors are fairly common in larger-breed dogs like Labrador Retrievers, American Staffordshire Terriers and Boxers. A true histiocytoma usually goes away (regresses) on its own after about three months.

Papilloma: These are wart-like growths that commonly occur around the mouth or eyes of younger dogs or dogs with immature or weaker immune systems. They are caused by the canine papilloma virus or CPV1. The virus is transmitted through the pet's environment, usually by food or water bowls. It takes about 1 to 2 months for the tumors to appear, and they generally go away in that same period of time.

Sebaceous Adenomas: These are tumors that begin in oil glands in the skin. These masses tend to look like a raised, hairless bumpy mass and are generally smaller — usually the size of a pea or blueberry. They do tend to be more prevalent in older, light colored dogs and particularly small dogs (especially Poodles, Shih Tzus, and Maltese).

Malignant Skin Cancer in Dogs

Mast Cell Tumor: A mast cell is part of your immune system and is also involved in allergic reactions. As part of an allergic response, mast cells release histamine, a common mediator of allergies. However, mast cell tumors (MCT) can release large amounts of histamine at once, a phenomenon called mass degranulation. This causes swelling, itchiness, irritation and even life-threatening allergic reactions. These masses can vary in appearance but are typically a raised red lump that may be ulcerated (having an open sore) and display swelling of the surrounding tissue. Mast cell tumors are the most common skin tumor in dogs, and the average age of occurrence is around 10 years old.

Mast cell tumors are most common in Boxers, Boston Terriers, Bulldogs, Pugs, Staffordshire Terriers, Labradors, Golden Retrievers, Rhodesian Ridgebacks and Weimaraners.

Squamous Cell Carcinomas: This is a type of tumor that originates directly from the epidermis or skin cells. These tumors tend to form in older animals, especially those with lighter skin and/or shorter fur. It is also common on the nose of pets that spend a large amount of time outside. The appearance of a squamous cell carcinoma (SCC) can vary widely. It is also the second most common oral tumor in dogs and can occur on the gums or tongue. When on the skin, these masses tend to first appear as red or ulcerated skin. Crusting and erosion can also occur.

Melanomas: This type of skin tumor arises from melanocytes, which are the cells that give our skin color with the pigment melanin. These tumors are generally pigmented (colored), mostly black or brown and are found in a region of the dog’s body that has less hair, especially around the mouth, feet and eyes. Melanomas of the toes (or digits) is prevalent in black dogs and may start as a swelling around a toenail. Melanomas are also the most common oral tumor in dogs. These tumors unfortunately grow quickly and have a high tendency to spread.

Fibrosarcoma: This type of tumor develops from the connective tissues of the skin and beneath the skin. These are most found on the legs of middle-aged or older dogs. They usually appear as a firm lump on the skin or under it. There may be multiples of these small lumps in one place. There may be pain and swelling associated with these masses which can bleed, open and become infected. These tumors can also be found around the mouth and nose.

Symptoms of Skin Cancer in Dogs

A change in your pet’s skin is the most common sign of skin cancer in dogs. Fortunately, this means that there is a highly visible change and lumps can be noticed early.  

This change is generally a new growth, lump or bump but it can also potentially appear as a sore that refuses to heal. All new lumps and bumps regardless of size, color, location, consistency or fur should be evaluated by a veterinarian.

Causes of Skin Cancer in Dogs

Skin cancer is, by definition, a mass that grows without normal cell regulations — from any of the many cell types that make up your dog’s skin.

We do not know all the reasons why a cell begins to replicate without the normal restrictions that the body has in motion to regulate cell growth. As it does with humans, sun exposure (UV radiation) can increase the risk of skin cancer in dogs, especially those with light skin colors.

Beyond the sun exposure risk, further studies are needed to figure out what causes skin tumors to form in dogs. Additional known causes include genetic links, like mast cell tumors and viral infections, like papillomas.

How Veterinarians Diagnose Skin Cancer in Dogs

Any lump that visibly grows, irritates a pet, bleeds, changes size or color, is larger than a pea, or has been present for a month or more should be checked by your pet’s primary care veterinarian.

Different diagnostic procedures may be recommended. Many tumors look alike on the outside, so it’s very important for your veterinarian to check all masses and determine a plan of treatment.

A needle biopsy is generally the first step toward a diagnosis, but other testing may be recommended at the same time, or at a future visit. A select group of masses or dogs, like a young dog with a suspected histiocytoma or papilloma, may call for monitoring before further testing is done.

Needle Biopsy

A needle biopsy (also called an FNA or Fine Needle spirate) is a way of collecting a sampling of cells from within a mass for evaluation. A needle is inserted into the mass, and cells are either compressed into the needle as it is inserted or pulled into the needle through a syringe. These collected cells are then placed onto a slide and examined under the microscope.

Most needle biopsies are sent to a laboratory where a veterinarian pathologist evaluates the sample. This sample of cells is a part of the mass, and while this procedure will often give you an exact diagnosis, it is possible that the cells collected will not provide a complete overview of the mass.

Some patients may require sedation for a needle biopsy depending on the location of the mass and the patient's temperament (personality).

Punch/Tissue Biopsy

In some cases, there is the need to evaluate a larger part of the tissue. When this is needed, a section of the mass or the entire mass (if it is about 8 mm or less) can be removed and sent to a lab for evaluation. Patients may need local anesthesia (like Novocain at your dentist's office), sedation or full anesthesia for a biopsy to be done, depending on the patient and the tumor location.

Radiographs/X-rays

With tumors that are suspected to be cancerous/malignant, there is the risk of spread to other portions of the body. The lungs are a very common place for spread (or metastasis) of tumors, so radiographs are often recommended to monitor for signs of malignant cells spreading.

Advanced Imaging

Advanced imaging includes diagnostic testing like CT (computerized tomography), MRI (magnetic resonance imaging) or ultrasound. These tests are often used to determine the extent of a tumor that is deeper than skin level or they may be used to examine lymph nodes around a mass to see if a malignant tumor has spread to those areas.

Treatment of Skin Cancer in Dogs

Since there are quite a few different types of tumors that can affect the skin, each case should be treated individually based on your pet and the tumor type, stage and location. Many types of skin tumors are treatable, especially if caught early. Generally, treatment will include one or more of the following: surgery, chemotherapy, radiation or immunotherapy.

Surgery

For most skin tumors, especially tumors that are small and in easily accessible areas of the body, surgical removal is often the first treatment step. This removes most, if not all, cancer cells from the area. Surgery can be curative, meaning that removal of the mass should be the only treatment necessary (like with small benign masses, such as sebaceous adenomas), or it can be part of a large treatment plan (like with melanomas where spread/metastasis is common).

Chemotherapy

Chemotherapy is a medication that is either given intravenously (IV) or by mouth. These drugs are toxic to cells that divide rapidly, like those involved in tumor growth. Chemotherapy is often used in certain types of cancer present in multiple locations in the body — or after surgical removal of a mass that is already suspected of spreading.

Radiation

Radiation therapy uses a focus-radiation beam to target specific tumors, generally in areas which make surgical removal difficult. Radiation is often used in combination with surgery or chemotherapy.

Immunotherapy

Immunotherapy is very much like a vaccine. For a very select number of cancer types (like melanomas), an immunotherapy vaccine is available. This vaccine contains killed parts of a cancerous cell. These cell pieces travel through your pet's system stimulating its own immune system to fight off cancerous cells. Immunotherapy is also generally used in conjunction with one or more of the other treatment types.

Recovery and Management of Skin Cancer in Dogs

Recovery and management of skin cancer is highly dependent on the type of tumor present. Your veterinarian will review any diagnostic findings and discuss your pet's recommended treatment and prognosis. They may choose to refer you to a veterinary oncologist. 

After surgery, your dog may feel sore and your veterinarian will likely prescribe pain medication. Please strictly follow all medication directions. Many patients will need crate rest. Your veterinarian will give you a time frame for return to normal activity levels.

If chemotherapy is part of your pet’s treatment plan, it can cause nausea, vomiting, diarrhea and low energy levels. Your vet may prescribe anti-nausea medications, or other supportive therapies, while your pet is undergoing chemotherapy. It is also important to ensure that your pet is getting high-quality food to aid in a quick recovery.

Prevention of Skin Cancer in Dogs

When possible, limit your pet's exposure to the sun, especially during peak UV hours. Pets with light colors or short fur may benefit from dog sunscreen.

Early intervention is extremely important for many types of skin tumors. It is very important to monitor your pet for the formation of lumps and bumps on their skin and for you to be an active advocate in its care by seeking an evaluation of the mass by your vet promptly.

Any mass, even one that has previously been diagnosed as benign, should be evaluated if a change in size, shape or color is noted — or if there is any bleeding. Many cancerous or malignant masses can often recur in the same area, so it is important to watch your dog closely for any other masses and report them to your vet.