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. Given that this type of cancer arises from squamous cells, tumors can develop anywhere that these cells are present. This can include the nail bed, ear tips (pinnae), nose, and corner of the eyes.
Squamous cell carcinomas usually present 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, including the mouth. Multicentric SCC is rare in cats.
What causes this cancer?
The reason why a particular cat 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. Exposure to ultraviolet rays/sunlight has been attributed to the development of SCC in cats. Exposure to papilloma-like viruses also appears to contribute to multicentric SCC in the mouth and other areas of the skin where squamous cells are present.
White and light-colored cats are more likely to get these tumors. A decreased risk has been reported in Siamese, Himalayan, and Persian breeds, likely related to their protective pigment.
What are the clinical signs of SCC?
Typically, SCC tends to arise in sparsely-haired and lightly-pigmented areas of the skin, particularly the eyelids, top edge of the nose (nasal planum), lips, and ears, and other areas that have greater exposure to ultraviolet rays/sunlight. SCC can be highly variable in appearance. Tumors may appear as a shallow or deep sore (ulceration), a raised, reddened area, or a cauliflower-like growth.
Multicentric SCCs arise as pigmented areas on the skin which become ulcerated (break open) and bleed. These areas are painful and can become scabby in appearance. They can expand across the skin and reach 1½ inches (4 cm) in diameter.
Although SCC most commonly occurs on the facial (face) area in cats, it can occur anywhere, including the toes. SCC of the toe(s) represent about 25% of all digital tumors in cats. Tumors of the toes, especially the nail bed, can cause swelling, pain, loss of the nail, and lameness. Cats may lick or chew the area and cause self-trauma.
A condition called hypercalcemia (a higher than normal calcium level in the blood) is occasionally seen in cats with SCC. When cats are affected by hypercalcemia, they may urinate or drink excessively and permanent kidney damage may occur if not addressed by your veterinarian.
How is this cancer 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). 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.
In cases of tumors of the toe(s), X-rays may be especially helpful with making a diagnosis. In cats, about 90% of carcinomas of the toes are actually cancer that had spread to the toe from the lungs. Lung cancer in cats tends to selectively spread (metastasize) to the nailbeds and toes. This is called lung-digit syndrome. X-rays of the chest, therefore, are necessary to determine if the tumor in the toe area is truly SCC of the toe (i.e. primary) or if it is secondary to a lung cancer.
How does this cancer typically progress?
SCC and multicentric SCC of the skin tends to remain localized, meaning that the tumor does not tend to spread to other areas of the body. In cats with SCC of the nose, however, the tumor may spread to the local lymph nodes under the cat's chin (the submandibular lymph nodes). Metastasis to the lungs or other lymph nodes is also possible. For this reason, your veterinarian may recommend staging.
Staging (searching for potential spread to other locations in the body) is highly recommended for malignant tumors, as they tend to spread. This 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 to determine if the tumor has spread into them, or possibly further.
Up to 75% of cats with multicentric SCC will develop new lesions in other locations after surgical removal of the lesions.
What are the treatments for this type of tumor?
The course of treatment will depend on how large your cat's tumor is and how many tumors there are. The most well-described treatment for SCC of the skin 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. If the tumor is small and has not spread to other organs, it may be removed by cryosurgery (a freezing procedure) or various other options.
Tumors of the toes require amputation of the affected toe, and tumors of the nose may require a partial removal of the nose. Treatment of SCCs of the nose is often successful with radiation therapy. This therapy involves using a probe to deliver a high dose of radiation to a very small area that only penetrates a few millimeters of skin. This allows for effective tumor control without making your cat sick from radiation.
Multicentric SCCs may be treated with surgery and/or immune-response modifiers.
The role of chemotherapy, including injecting the chemotherapeutic medication directly into the tumor, remains controversial. Your veterinarian will discuss this option with you.
Is there anything else I should know?
It may be helpful to limit the amount of time that your cat spends in the sun. If your cat spends a lot of time on the window sill, you may consider placing a window shade to block the UV rays. Your cat 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.