An invasive cancer which can be found in several areas of the cat's anatomy, including but not limited to the lips, esophagus, or the mouth and jaws of cats. Because it is so malignant, it is usually treated aggressively by radiation and/or surgery.
Probably one of the most widely-known instances of squamous cell carcinoma is found on the ear tips and nose leather of white cats or cats with light-colored ears and pink noses. Possibly caused by excessive exposure to the sun, the cancer is often treated by surgical removal of the ear tips with a good margin away from the injured area. Cryosurgery is sometimes used in small, delicate areas, such as the rim of the eye. Chemotherapy is also mentioned as a possible treatment in this article by Dr. Mike.
The best way to avoid squamous cell carcinoma in a cat is to keep all cats indoors and out of the sun. If allowed outside in enclosed areas, plenty of shade should be available, and a sunscreen, such as zinc ointment may be applied to the nose and eartips of white cats.
Arthur, the cat in the photo, developed squamous cell carcinoma at the age of 18, after many years of being allowed outdoors. The cancer first presented as black spots, such as fleas, or dirt, then gradually progressed to crusty black lesions.