Skin Cancer in Cats and Dogs

The skin is the largest organ of the body and the most obvious to see.

Along with these accolades, skin is also the most common site of cancer in cats and dogs — but this is not quite as grim as it sounds.

Skin cancer is responsible for a third of all tumors in our furry friends, but only a third of these are classed as malignant, which means they have the potential to behave in an aggressive way and spread to other parts of the body.

The other side of the coin is that two-thirds of skin cancer occurrences pose no real threat to the animal.

Skin is a wonderful thing with its ability to stretch, heal itself, grow with the animal and keep him warm. It is a complex structure made up of layers of different types of epidermal and dermal cells, plus pigments cells, sweat glands, oil glands and hair shafts.

However, each of these building blocks has the potential to become cancerous — hence the large number of tumor types linked to skin.

Here is a list of the most important types of skin cancer:

• Papilloma — caused by a virus and tends to go of its own accord; affects young dogs
• Sebaceous gland tumors — often referred to as “warts”; common in older dogs and cats
• Squamous cell carcinoma (SCC) — a serious, erosive skin cancer caused by UV damage; tends to occur in cats with white ears or noses because they lack protective pigment
• Melanoma — a tumor of the pigment cells and a serious cancer, although not that common; it represents 4 to 6% of canine and 1 to 2% of feline skin tumors
• Mast cell tumors (MCT) — should always be treated seriously, but there is a wide range in aggressiveness of MCT from relatively harmless to life-threatening; responsible for about 20% of dog skin cancers, and certain breeds have a genetic predisposition such as Boxers and Golden Retrievers
• Cutaneous lymphoma — a rare but serious generalized skin cancer
• Histiocytoma — despite the scary name, this skin cancer is benign (unlikely to spread) and often disappears, if given time, as mysteriously as it appeared

Symptoms

Mention skin cancer and most people think of a lump.

This is true for the majority of skin cancers in dogs and cats. However, there are also cancers, such as SCC, that eat away at tissue, such as an ear tip or a pink nose, rather than forming a lump.

Also, some of the rare, generalized skin cancers can mimic either a skin allergy — with raised red areas — or a severe infection that makes the top of the skin sticky. When it comes to skin cancer, keep an open mind.

Lumps that are not likely to spread or endanger the pet’s health are called “benign.”

These lumps have certain characteristics, such as:

• Slow to grow
• Do not bother the animal/are not itchy
• They are “superficial” in that you can pinch a finger and thumb around the lump to lift it away from the tissues beneath

There are also aggressive lumps that could spread to other organs, such as the liver and lungs, to endanger life.

These are termed “malignant” and have different habits:

• Grow quickly
• May be darkly colored (not all malignant lumps are dark)
• Invasive, which means they anchor themselves to tissue beneath the skin
• Sometimes (but not always) are itchy and irritate the pet

Causes of Skin Cancer

Any cancer, including skin cancer, can occur without rhyme or reason.

However, certain lumps are linked to certain risk factors.

Perhaps the most obvious is SCC and the link to UVB damage to the skin where it lacks the protection of pigment or fur. Thus, the classic place for SCC is the ear tips of white cats.

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Another group of lumps with a known cause are papillomas caused by the papilloma virus. Although dogs can grow impressive crops of lumps, these usually resolve of their own accord after several months and do not need treatment.

The most common malignant skin tumor, MCT, has a genetic link. Some breeds, such as Boxers, Golden Retrievers, Staffies and other bristly coated dogs, seem more at risk.

In the video below, Dr. Karen Becker, DVM, discusses mast cell tumors in dogs and cats:

Diagnosis

The gold standard tool for diagnosis is histology, where a technician looks at stained slices of the lump under a microscope to identify the cell types.

To this end, most veterinarians will perform an “excisional biopsy” of solitary lumps, which means surgically removing the lump with wide skin margins around it (a precaution in case serious results come back) and sending it off to the lab.

Sometimes, especially with multiple lumps, the clinician will perform a fine needle aspirate (FNA) from several of the lumps. This procedure can be done without sedation in friendly animals and is the collection of cells from the lump using an ordinary hypodermic needle.

There is an element of luck as to whether representative cells are harvested, but the results can be useful if a diagnostic sample is obtained.

If your veterinarian is worried about an aggressive skin cancer, she may also check lymph nodes for signs of spread, perhaps X-ray the pet’s chest and scan the liver (to search for secondary spread).

Treatment

For the majority of skin tumors, the pet’s best chance of a full recovery is surgical removal of the complete lump along with plenty of tissue and skin around and beside the lump.

Very few skin tumors respond to chemotherapy, and although a range of options for treatment exist for MCT, the science is still very new and the benefits are limited to a small percentage of carefully selected cases.

Prevention

The best candidate for prevention is SCC. Cats, and indeed dogs, with white ears or pink noses should either have a pet-safe, waterproof sunscreen applied to pink areas during summers or should be kept out of the sun.

Reference

• Small Animal Oncology. Morris and Dobson. Publisher: Blackwell Science.

This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed Dec. 17, 2018.