Squamous Cell Carcinomas in Dogs
What is a squamous cell carcinoma of the skin?
Squamous cell carcinoma (SCC) is a tumor of skin cells. As shown in the illustration, the skin is made up of several layers of cells with the squamous layer at the top. Because this type of cancer arises from squamous cells, tumors can develop anywhere that these cells are present. This can include the nail bed, paw pads, abdomen, back, ears, or the nose, including the nasal planum (top edge of the nose).
Squamous cell carcinomas usually appear as a single, solitary lesion in one location, but there is a kind of SCC called multicentric squamous cell carcinoma (also known as Bowen’s disease or Bowenoid carcinoma) that presents as many (two or more) lesions in multiple locations on the body. Multicentric SCC is very rare in dogs.
What causes this type cancer?
The reason why a particular dog may develop this, or any cancer, is not straightforward. Very few cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary.
As with SCC in humans, exposure to ultraviolet rays/sunlight has been attributed to the development of these tumors. Exposure to papilloma-like viruses appears to contribute to multicentric SCC in the mouth and other areas of the skin where squamous cells are present.
Certain breeds are known to have an increased incidence of SCC, including Scottish Terriers, Pekingese, Boxer Dogs, Poodles, and Norwegian Elkhounds. SCC of the skin is also more common in dogs that are sparsely haired and have light-colored hair and skin (e.g. Dalmatians, Beagles, Whippets, and white Bull Terriers). In contrast, large-breed dark-coated dogs (such as Labrador Retrievers and Rottweilers) are more prone to SCC of the toes (digits).
What are the signs of SCC?
Typically, these lesions are found in light-skinned areas and can be highly variable in their appearance. It may look like a small area of irritated, red, or ulcerated skin. Alternatively, there could be plaques or crusts that develop over the region. SCC lesions of the toe or nail bed tend to be red, irritated, bothersome, and ulcerated. Dogs may even lose nails on the affected toes.
Lesions of the skin or nose may become dry, irritated, and bothersome for your pet. The lesion may get larger, spread, and ulcerate which can be very painful as well. Your pet should not be allowed to scratch, lick, or bite the area.
SCC of the toe (digit) can be very painful. Your pet may be reluctant to go for walks, and you may notice blood on the floor or on your dog’s toes. Your dog may attempt to lick or chew the affected toe(s) aggressively and you may notice missing toe nails. These lesions are typically painful, and your veterinarian may prescribe pain medications. Secondary infection is also possible for which antibiotics may be required.
How is squamous cell carcinomas diagnosed?
Depending on where the tumor develops (i.e., the skin or nail bed), a diagnosis may be achieved with a fine needle aspiration (FNA) or a biopsy. FNA involves taking a small needle with a syringe and suctioning a sample of cells directly from the tumor and placing them on a microscope slide. A veterinary pathologist then examines the slide under a microscope.
In some cases, results from FNA may not be entirely clear and biopsy may be necessary. A biopsy is a surgical excision of a piece of the tumor. Pieces of the tumor are then examined by a veterinary pathologist under the microscope. This is called histopathology. Histopathology is not only helpful to make a diagnosis but can indicate how the tumor is likely to behave.
How does this cancer typically progress?
In the skin form of the disease, although spread (metastasis) to nearby lymph nodes or other organs is possible, it is very uncommon. With SCC of the nose, spread may occur to the lymph nodes under the chin (the submandibular lymph nodes).
Dogs with multicentric SCC often develop new lesions in other sites after surgical removal of lesions.
The digit (or toe) form of SCC is far more aggressive. It can spread to the local lymph nodes and beyond. For this reason, your veterinarian may recommend staging (searching for potential spread to other locations in the body). Staging may include bloodwork, urinalysis, X-rays of the lungs, and possibly an abdominal ultrasound. If any lymph nodes appear to be affected (enlarged), samples may be taken via FNA to determine if the tumor has spread into them.
What are the treatments for this type of tumor?
The most well-described treatment for SCC of the skin, including the nose, is surgery. Surgery appears to provide the best long-term control of the disease as long as the tumor can be completely removed, meaning no cancer cells are left behind. Radiation therapy does not appear to be very effective for the treatment of SCC of the skin and nose in dogs, though it may be used in combination with surgery if the tumor is not completely removed or if complete removal is not possible. The role of chemotherapy is still up for debate and should be discussed with your veterinarian.
Similar to SCC of the skin, SCC of the digit is typically treated with surgery, the only treatment currently shown to improve outcomes. This may include toe amputation, though larger tumors may necessitate amputation of an entire limb. The role of chemotherapy and/or radiation therapy for digit SCC remains controversial and should be discussed with your veterinarian.
Multicentric SCCs may be treated with surgery and/or immune-response modifiers.
Is there anything else I should know?
As UV rays/sunlight may play a role in the development of SCC, it is best to limit your dog’s exposure to the sun, especially between the hours of 10am and 3pm, and especially in dogs with thin haircoats. Your dog should not be allowed to rub, scratch, lick, chew, or bite the affected area, as this may cause trauma and increase the risk of secondary infection. Secondary infections are common and treated with antibiotics as recommended by your veterinarian. Your veterinarian may also recommend medications for pain.
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