Squamous cell carcinoma of the skin most often occurs on sun-exposed skin, such as your scalp, the backs of your hands, your ears or your lips. But squamous cell carcinoma of the skin can occur anywhere on your body, including inside your mouth, on your anus and on your genitals.

Signs and symptoms of squamous cell carcinoma of the skin include:

• A firm, red nodule
• A flat sore with a scaly crust
• A new sore or raised area on an old scar or ulcer
• A rough, scaly patch on your lip that may evolve to an open sore
• A red sore or rough patch inside your mouth
• A red, raised patch or wart-like sore on or in the anus or on your genitals

Squamous cell carcinoma of the skin occurs when the flat, thin squamous cells in the outer layer of your skin develop errors in their DNA. Ordinarily, new cells push older cells toward your skin's surface, and the older cells die and are sloughed off. DNA errors disrupt this orderly pattern, causing cells to grow out of control, with squamous cell carcinoma of the skin as the result.

Ultraviolet light and other potential causes

Much of the damage to DNA in skin cells results from ultraviolet (UV) radiation found in sunlight and in commercial tanning lamps and tanning beds.

But sun exposure doesn't explain skin cancers that develop on skin not ordinarily exposed to sunlight. This indicates that other factors may contribute to your risk of skin cancer, such as being exposed to toxic substances or having a condition that weakens your immune system.

Factors that may increase your risk of squamous cell carcinoma of the skin include:

• Fair skin. Anyone, regardless of skin color, can get squamous cell carcinoma of the skin. However, having less pigment (melanin) in your skin provides less protection from damaging UV radiation.

If you have blond or red hair and light-colored eyes and you freckle or sunburn easily, you're much more likely to develop skin cancer than is a person with darker skin.

• Excessive sun exposure. Being exposed to UV light from the sun increases your risk of squamous cell carcinoma of the skin. Spending lots of time in the sun — particularly if you don't cover your skin with clothing or sunblock — increases your risk of squamous cell carcinoma of the skin even more.
• Use of tanning beds. People who use indoor tanning beds have an increased risk of squamous cell carcinoma of the skin.
• A history of sunburns. Having had one or more blistering sunburns as a child or teenager increases your risk of developing squamous cell carcinoma of the skin as an adult. Sunburns in adulthood also are a risk factor.
• A personal history of precancerous skin lesions. Having a precancerous skin lesion, such as actinic keratosis or Bowen's disease, increases your risk of squamous cell carcinoma of the skin.
• A personal history of skin cancer. If you've had squamous cell carcinoma of the skin once, you're much more likely to develop it again.
• Weakened immune system. People with weakened immune systems have an increased risk of skin cancer. This includes people who have leukemia or lymphoma and those who take medications that suppress the immune system, such as those who have undergone organ transplants.
• Rare genetic disorder. People with xeroderma pigmentosum, which causes an extreme sensitivity to sunlight, have a greatly increased risk of developing skin cancer.

Untreated squamous cell carcinoma of the skin can destroy nearby healthy tissue, spread to the lymph nodes or other organs, and may be fatal, although this is uncommon.

The risk of aggressive squamous cell carcinoma of the skin may be increased in cases where the cancer:

• Is particularly large or deep
• Involves the mucous membranes, such as the lips
• Occurs in a person with a weakened immune system, such as someone who takes anti-rejection medications after an organ transplant or someone who has chronic leukemia