Who Gets It
People who have fair skin, light hair, and blue, green, or gray eyes are at highest risk of developing the disease. But anyone with a history of substantial sun exposure is at increased risk. Those whose occupations require long hours outdoors or who spend extensive leisure time in the sun are in particular jeopardy. Anyone who has had a basal cell carcinoma is also more likely to develop an squamous cell carcinoma, as is anyone with an inherited, highly UV-sensitive condition such as xeroderma pigmentosum.
Squamous cell carcinomas are at least twice as frequent in men as in women. They rarely appear before age 50 and are most often seen in individuals in their 70s.
The majority of skin cancers in African-Americans are squamous cell carcinomas, usually arising on the sites of preexisting inflammatory skin conditions or burn injuries. Though naturally dark-skinned people are less likely than fair-skinned people to get skin cancer, it is still essential for them to practice sun protection.
Chronic exposure to sunlight causes most cases of squamous cell carcinoma. Frequent use of tanning beds also multiplies the risk of squamous cell carcinoma; people who use tanning beds are 2.5 times more likely to develop squamous cell carcinoma than those who don’t. But skin injuries are another important source. The cancer can arise in burns, scars, ulcers, long-standing sores and sites previously exposed to X-rays or certain chemicals (such as arsenic and petroleum by-products).
Chronic infections and skin inflammation can also give rise to squamous cell carcinoma. Furthermore, HIV and other immune deficiency diseases, chemotherapy, anti-rejection drugs used in organ transplantation, and even excessive sun exposure itself all weaken the immune system, making it harder to fight off disease and thus increasing the risk of squamous cell carcinoma and other skin cancers.
Occasionally, squamous cell carcinomas arise spontaneously on what appears to be normal, healthy skin. Some researchers believe the tendency to develop these cancers can be inherited.
Precancers, the First Step
Certain precancerous growths, or precancers, most of them resulting from cumulative sun damage, can be associated with the later development of squamous cell carcinoma.
Actinic, or Solar, Keratoses
These rough, scaly, slightly raised growths, ranging in color from brown to red and from about 1 mm to 1 inch in diameter, are found on sun-exposed areas of the body, most often in older people. They can be the first step on the road to squamous cell carcinoma, and some experts even consider them the earliest form of squamous cell carcinoma. From two to ten percent of untreated actinic keratoses (AK) advance to squamous cell carcinoma, according to different studies. Indeed, 40 to 60 percent of squamous cell carcinomas begin as untreated actinic keratoses.
This form of actinic keratosis occurs most often on the lower lip, causing it to become dry, cracked, scaly and pale or white. Why the lower lip? Because it receives more sun exposure than the upper lip. If not treated promptly, actinic cheilitis can lead to squamous cell carcinoma on the lip.
Arising in the mucous membranes, these white patches on the tongue, gums, cheeks, or elsewhere inside the mouth have the potential to develop into squamous cell carcinoma. They may be caused by sources of chronic irritation, such as habitual alcohol consumption or tobacco use, or rough edges on teeth or dentures. They may even be caused by a long-time habit of biting the inside of the lip; however, leukoplakias on the lips are mainly caused by sun damage.
This is now generally considered an early, noninvasive stage of squamous cell carcinoma. It appears as a persistent red-brown, scaly patch that may resemble psoriasis or eczema. If untreated, it may invade deeper structures. Bowen’s disease is most often caused by exposure to the sun or to arsenic, but other chemical carcinogens, radiation, genetics and trauma also may play a role. The human Papillomavirus (HPV), highly transmissible through sexual contact, has been documented as a cause of one form of Bowen’s disease affecting the genitals. The disease can arise in the mucous membranes of the nose and mouth as well as on the skin. In 2006, the FDA approved an HPV vaccine for use by females aged 9–26; it is considered highly effective in preventing HPV and thereby reducing the risk of both genital warts and cervical cancer as well as Bowen’s disease.