Squamous cell carcinoma (SCC) is a locally invasive malignant tumor that arises from a cell type known as the keratinocyte (aka, squamous cell), the primary cell type found in the skin and mucous membranes.
Despite their common cell of origin, SCC tumors display different characteristics in dogs and cats, among them prevalence, preferred sites, behavior, environmental influences, and genetic predisposition.
SCC occurs commonly in dogs and very commonly in cats. In cats, it is considered the fourth most common type of skin cancer and by far the most common type of oral cancer. In dogs, it accounts for 5% of all skin tumors and is considered the second most common type of oral cancer.
In dogs, SCC is classified by its location in the skin (cutaneous), mouth (oral), or nailbeds (subungual). In the mouth, the gums, tongue, or tonsils are most prone. In the skin, locations that are less pigmented, lack hair, or are sparsely haired are most typically affected.
In cats, the skin is most commonly affected at the temples, the outer tips of the ears (pinnae), the eyelids, the lips and the nasal planum (tip of the nose). An oral form also commonly afflicts cats, normally in the gums. An internal form that affects the liver or lungs is considered much less common.
In dogs, SCC behaves differently depending on the site affected.
Unlike many other aggressive cancers, the cutaneous form of Squamous cell carcinoma does not have a high propensity for metastasis (spread). While locally invasive, it is considered relatively slow-growing.
Oral tumors, however, behave somewhat differently. Not only are they locally invasive, but the form that affects the gum (gingival) can be highly destructive of the underlying bone as well. And while gingival SCC do not tend to metastasize, about 20% of tonsilar SCC tumors are known to spread to other organs via the nearby lymph nodes.
The subungual version of SCC affects the nailbeds, sometimes in multiple toes at once. The rate of growth varies. Spread of this tumor to the lungs via local lymph nodes has been reported in one third of subungual SCC patients.
In cats with cutaneous SCC, growth is considered relatively slow and metastasis is considered uncommon. Oral SCC, though highly destructive of underlying bone, also carries a relatively low risk of metastasis. Unfortunately, this latter form of SCC is very fast-moving.
Dogs tend to be diagnosed with SCC between eight and ten years of age. Older cats are more likely to develop SCC.
As some breeds of dogs are overrepresented, a genetic origin for all three forms of the disease can be inferred in this species, though the exact method of inheritance is unknown.
In both dogs and cats, the cutaneous form is associated with exposure to UV radiation. In cats, burns or other forms of physical trauma have also been implicated in the development of SCC. In dark haired cats, a virus is considered a possible factor in the development of cutaneous SCC.
It's postulated that the oral form of feline SCC is associated with chronic inflammatory disease of the mouth (such as periodontal disease). Interestingly, the internal form may be related to exposure to environmental carcinogens like tobacco smoke.
The prognosis for most SCC patients varies widely from good to grave, depending on the location of the tumor, its size, and degree of invasiveness or evidence of metastasis. Early detection is crucial in all cases.
Symptoms and Identification
Cutaneous SCC in cats is first identified upon visual inspection of the distinctive red, irregular, and erosive lesions in characteristic locations (temples, pinnae, tip of nose, etc.).
Oral SCC is more readily identified by observing a cat's discomfort while eating, noticing a foul odor from the mouth, or after examining the oral cavity and finding characteristic erosive or ulcerated lesions. These lesions can become excruciatingly painful once they invade bone.
In dogs, owners will notice characteristic lesions in the skin or nailbeds. Limping may or may not be an issue but pain is a significant factor for these tumors as well as for oral SCC. Oral tumors usually signal their presence either when dogs have a difficult time chewing or when owners notice a foul odor coming from the mouth.
In both species, basic diagnostic testing is always recommended to help determine the extent of the disease and assist in identification of any affected organs. More sophisticated options may be undertaken depending on the invasiveness of the disease, location of the tumor, species, and availability of the diagnostic equipment.
Complete blood count (CBC)
Blood biochemistry panel
Fine-needle aspirate of the local lymph nodes
Fine-needle aspirate and/or impression smears of affected areas
CT scans of the mouth and jaw
However, the disease can only be diagnosed definitively by retrieving a specimen for the affected tissues (biopsy) and evaluating them through histopathology (microscopic analysis of the tissues by a board-certified pathologist).
In dogs, the breeds affected depend largely on the location of the tumor:
Cutaneous: Though any breed of dog can be affected -- especially light-colored, short-coated breeds that are exposed to UV light -- the breeds that are at an increased risk include the Keeshond, Standard Schnauzer, Basset Hound, and Collie. Interestingly, the Boxer is believed to be at a decreased risk.
Oral: Oral SCC is most commonly seen in large breeds of dogs. While no breed predisposition has been shown for oral SCC of the gums, the form of SCC that affects the tongue may be more common in Poodles, Labrador Retrievers, and Samoyeds.
Subungual: The Giant Schnauzer, Gordon Setter, Standard Poodle, Standard Schnauzer, Scottish Terrier, Labrador Retriever, Rottweiler, Dachshund, and Miniature Schnauzer are all at an increased risk of developing SCC. Breeds at lower risk include the Golden Retriever, Boxer, Lhasa Apso, Collie, Basset Hound, Beagle, and Shetland Sheepdog. Dark haired breeds and a dark coat color has been associated with the development of this tumor type.
In the cat, there is no known breed predisposition.
SCC is considered a highly treatable disease, but identification and intervention must arrive early.
Surgery is the initial treatment of choice for all SCC tumors. Unfortunately, radical resection may be the only option for some patients. If identified in its later stages, however, the tumor may be deemed inoperable. This is more often the case with destructive oral tumors or when metastasis is already evident.
Wide surgical resection of tumors is necessary to ensure the complete removal of all tumor cells. This means that cats with SCC of the eartips must lose the entire pinna, dogs with subungual SCC require toe amputation, and all patients with oral lesions must undergo removal of any nearby affected bone, if possible. So it is that owners must be made aware that treatment often results in less-than-cosmetic outcomes.
In cases where surgery is considered incomplete or the area less amenable to wide surgical resection (as with tumors on the face), radiation therapy is typically recommended. Systemic treatment with chemotherapy may also be in order, depending on the tumor location, species, and degree of invasiveness or presence of metastasis.
Cryotherapy, a technique that destroys the tumor by freezing it is sometimes employed. So, too, is a technique in which the tumor is directly injected with chemotherapeutic substances. For oral SCC, a combination of surgery and radiation therapy may be successful and result in excellent control, but this is typically only the case if the cancer is detected early on.
All cases are amenable to a variety of adjunctive treatments aimed at relieving pain, inflammation, and/or infection, when needed.
As with many cancers that require surgery for diagnosis and treatment as may well require radiation therapy and chemotherapy for best effects, the cost of this disease can be impressive and even prohibitive.
The cost of diagnosis and treatment depends on many factors, including geographic location (cost of living), standard of care (lower vs. higher standards of veterinary care), whether specialty hospitals are employed (higher quality equipment, certified personnel, and board-certification for veterinarians specialized in the fields of surgery and oncology).
Depending on the location of the tumor, definitive diagnosis and treatment may typically cost anywhere from $500 (for a very small skin tumor) to more than $5,000, should a tumor require expensive imaging (CT scan) and advanced surgical treatment in a specialty hospital.
Treatment of the tumor post-operatively can be very expensive as well should radiation therapy or chemotherapy be elected. The high price of the equipment and drugs and the close monitoring these procedures entail means caretakers may see invoices totaling anywhere between $3,000 and $10,000 or more, depending on the modalities employed.
Sadly, many owners feel compelled to forgo treatment for their pets due to cost and/or because they've elected to euthanize as a result of their pet's suffering and the disease's poor prognosis.
There is no known method for the prevention of SCC in dogs except for the cutaneous form associated with exposure to UV radiation. Dogs with light hair who are lightly pigmented and/or have sparse haircoats should have limited exposure to sunlight. Application of a pet-appropriate sunscreen to pale or thinly-haired areas of the body may be recommended. This is especially recommended for dogs of predisposed breeds.
For cats, cutaneous SCC among light haired individuals can be largely prevented by restricting exposure to UV light. Pet-specific sunscreens may be of some benefit. Managing periodontal disease and maintaining good oral health may be effective in preventing or delaying the onset of oral SCC in cats. Eliminating exposure to tobacco smoke is doubtless of some benefit as well.
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