Skin cancer — the abnormal growth of skin cells — most often develops on skin exposed to the sun. But this common form of cancer also can occur on areas of the skin not ordinarily exposed to sunlight.

There are three major types of skin cancer: basal cell carcinoma, squamous cell carcinoma and melanoma. Basal and squamous cell carcinomas are slow growing and highly treatable, especially if found early. Melanoma is the most serious form of skin cancer. It affects deeper layers of the skin and has the greatest potential to spread to other tissues in the body.

All three types of skin cancer are on the rise — but most skin cancers can be prevented by limiting or avoiding exposure to ultraviolet (UV) radiation and by paying attention to suspicious changes in your skin. If caught early enough, most skin cancers can be successfully treated.

Skin cancer develops primarily on areas of sun-exposed skin, including the scalp, face, lips, ears, neck, chest, arms and hands, and on the legs in women. But it also can form on areas that rarely see the light of day — the palms, spaces between the toes and the genital area.

A cancerous skin lesion can appear suddenly or develop slowly. Its appearance depends on the type of cancer.

Basal cell carcinoma
This is the most common skin cancer, accounting for nearly 90 percent of all cases. It's also the most easily treated and the least likely to spread. Basal cell carcinoma usually appears as one of the following:

• A pearly or waxy bump on your face, ears or neck
• A flat, flesh-colored or brown scar-like lesion on your chest or back

Squamous cell carcinoma
Squamous cell carcinoma is easily treated if detected early, but it's slightly more apt to spread than is basal cell carcinoma. Most often, squamous cell carcinoma appears as one of the following:

• A firm, red nodule on your face, lips, ears, neck, hands or arms
• A flat lesion with a scaly, crusted surface on your face, ears, neck, hands or arms

Melanoma
This is the most serious form of skin cancer and the one responsible for most skin cancer deaths. Melanoma can develop in otherwise normal skin or in an existing mole that turns malignant. Although it can occur anywhere on the body, melanoma appears most often on the upper back or face in both men and women.

Warning signs of melanoma include:

• A large brownish spot with darker speckles located anywhere on your body
• A simple mole located anywhere on your body that changes in color, size or feel or that bleeds
• A small lesion with an irregular border and red, white, blue or blue-black spots on your trunk or limbs
• Shiny, firm, dome-shaped bumps located anywhere on your body
• Dark lesions on your palms, soles, fingertips and toes, or on mucous membranes lining your mouth, nose, vagina and anus

Less common skin cancers
Other, less common types of skin cancer include:

• Kaposi's sarcoma. This rare form of skin cancer develops in the skin's blood vessels and causes red or purple patches on the skin or mucous membranes. Like melanoma, it's a serious form of skin cancer. It's mainly seen in people with weakened immune systems, such as people with AIDS and people taking medications that suppress their natural immunity, such as people who've undergone organ transplants.
• Merkel cell carcinoma. In this rare cancer, firm, shiny nodules occur on or just beneath the skin and in hair follicles. The nodules may be red, pink or blue and can vary in size from a quarter of an inch to more than 2 inches. Merkel cell carcinoma is usually found on sun-exposed areas on the head, neck, arms and legs. Unlike basal and squamous cell carcinomas, Merkel cell carcinoma grows rapidly and often spreads to other parts of the body.
• Sebaceous gland carcinoma. This uncommon and aggressive cancer originates in the oil glands in the skin. Sebaceous gland carcinomas — which usually appear as hard, painless nodules — can develop anywhere, but most occur on the eyelid, where they're frequently mistaken for benign conditions.

Precancerous skin lesions, such as an actinic keratosis, also can develop into squamous cell skin cancer. Actinic keratoses appear as rough, scaly, brown or dark-pink patches. They're most commonly found on the face, ears, lower arms and hands of fair-skinned people whose skin has been damaged by the sun.

Not all skin changes are cancerous. The only way to know for sure is to have your skin examined by your doctor or dermatologist.

Your skin consists of three layers — the epidermis, dermis and subcutis. The epidermis, the topmost layer, is as thin as a pencil line. It provides a protective layer of skin cells that your body continually sheds. Squamous cells lie just below the outer surface. Basal cells, which produce new skin cells, are at the bottom of the epidermis. The epidermis also contains cells called melanocytes, which produce melanin — the pigment that gives skin its normal color. When you're in the sun, these cells produce more melanin, which helps protect the deeper layers of skin. The extra melanin is what produces the darker color of tanned skin.

Normally, skin cells within the epidermis develop in a controlled and orderly way. In general, healthy new cells push older cells toward the skin's surface, where they die and eventually are sloughed off. This process is controlled by DNA — the genetic material that contains the instructions for every chemical process in your body. But when DNA is damaged, changes occur in these instructions. One result is that new cells may begin to grow out of control and eventually form a mass of cancer cells.

The role of UV light
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. UV light is divided into three wavelength bands: ultraviolet A (UVA), ultraviolet B (UVB) and ultraviolet C (UVC). Only UVA and UVB rays reach the earth. UVC radiation is completely absorbed by atmospheric ozone.

At one time scientists believed that only UVB rays played a role in the formation of skin cancer. And UVB light does cause harmful changes in skin cell DNA, including the development of oncogenes — a type of gene that can turn a normal cell into a malignant one. UVB rays are responsible for sunburn and for many basal cell and squamous cell cancers. But UVA also contributes to skin cancer. It penetrates the skin more deeply than UVB does, weakens the skin's immune system and increases the risk of cancer, especially melanoma.

Tanning beds deliver high doses of UVA, which makes them especially dangerous. What's more, occasional exposure to intense UVA puts you at greater risk of skin cancer than spending long hours in the sun does. An initial high dose of UV radiation will severely damage melanocytes, but not destroy them. When these damaged cells are subjected to further intense bouts of UVA light, they have little capacity to repair their DNA. This makes them more likely to become malignant.

Other factors that may contribute to skin cancer
Sun exposure doesn't explain melanomas or other skin cancers that develop on skin not ordinarily exposed to sunlight. Heredity may play a role. Skin cancer can also develop from exposure to toxic chemicals or as a result of radiation treatments.

These factors may increase your risk of skin cancer:

• Fair skin. Having less pigment (melanin) in your skin provides less protection from damaging UV radiation. If you have blond or red hair, light-colored eyes, and you freckle or sunburn easily, you're much more likely to develop skin cancer than a person with darker features is.
• A history of sunburns. A sunburn is your body's attempt to heal itself from the sun's damaging rays. Every time you get sunburned, you damage your skin cells and increase your risk of developing skin cancer. If you had one or more severe, blistering sunburns as a child or teenager, you have an increased risk of skin cancer as an adult. Sunburns in adulthood also are a risk factor.
• Excessive sun exposure. Anyone who spends considerable time in the sun may develop skin cancer, especially if your skin isn't protected by sunscreen or clothing. Tanning also puts you at risk. A tan is your skin's injury response to excessive UV radiation.
• Sunny or high-altitude climates. People who live in sunny, warm climates are exposed to more sunlight than are people who live in colder climates. Living at higher elevations, where the sunlight is strongest, also exposes you to more radiation.
• Moles. People who have many moles or abnormal moles called dysplastic nevi are at increased risk of skin cancer. These abnormal moles — which look irregular and are generally larger than normal moles — are more likely than others to become cancerous. If you have a history of abnormal moles, watch them regularly for changes.
• Precancerous skin lesions. Having skin lesions known as actinic keratoses can increase your risk of developing skin cancer. These precancerous skin growths typically appear as rough, scaly patches that range in color from brown to dark pink. They're most common on the face, lower arms and hands of fair-skinned people whose skin has been sun damaged.
• A family history of skin cancer. If one of your parents or a sibling has had skin cancer, you may be at increased risk of the disease. Some families are affected by a condition called familial atypical multiple mole melanoma (FAMMM) syndrome. The hallmarks of FAMMM include a history of melanoma in one or more close relatives and having more than 50 moles — some of which are atypical. Because people with this syndrome have an extremely high risk of developing melanoma, frequent screening for signs of skin cancer is crucial.
• A personal history of skin cancer. If you developed skin cancer once, you're at risk of developing it again. Even basal cell and squamous cell carcinomas that have been successfully removed can recur in the same spot, often within two to three years.
• A weakened immune system. People with weakened immune systems are at greater risk of developing skin cancer. This includes people living with HIV/AIDS or leukemia and those taking immunosuppressant drugs after an organ transplant.
• Fragile skin. Skin that has been burned, injured or weakened by treatments for other skin conditions is more susceptible to sun damage and skin cancer. Certain psoriasis treatments and eczema creams might increase your risk of skin cancer.
• Exposure to environmental hazards. Exposure to environmental chemicals, including some herbicides, increases your risk of skin cancer.
• Age. The risk of developing skin cancer increases with age, primarily because many skin cancers develop slowly. The damage that occurs during childhood or adolescence may not become apparent until middle age. Still, skin cancer isn't limited to older people. Basal cell and squamous cell carcinomas are increasing fastest among women younger than 40.

If you notice any suspicious change in your skin, consult your doctor right away. As with most cancers, early detection increases the chances of successful treatment. Don't wait for the area to start hurting — skin cancer seldom causes pain.

See your doctor if you notice a new skin growth, a bothersome change in your skin, a change in the appearance or texture of a mole, or a sore that doesn't heal in two weeks. Your doctor may suspect cancer by simply looking at your skin. But to properly diagnose skin cancer, your doctor or dermatologist will need to take a small sample of your skin (biopsy) for analysis in a lab. A biopsy can usually be done in a doctor's office using local anesthetic.

Skin cancer is generally divided into two stages:

• Local. In this stage, cancer affects only the skin.
• Metastatic. At this point, cancer has spread beyond the skin.

Because superficial skin cancers such as basal or squamous cell carcinoma rarely spread, a biopsy often is the only test needed to determine the cancer stage. But if you have a large growth or one that's existed for some time, your doctor may do further tests to determine the extent of the cancer.

Treatment for skin cancer and the precancerous skin lesions known as actinic keratoses varies, depending on the size, type, depth and location of the lesions. Often the abnormal cells are surgically removed or destroyed with topical medications. Most treatments require only a local anesthetic and can be done in an outpatient setting. Sometimes no treatment is necessary beyond an initial biopsy that removes the entire growth.

If additional treatment is needed, options may include:

• Freezing. Your doctor may destroy actinic keratoses and some small, early skin cancers by freezing them with liquid nitrogen (cryosurgery). The dead tissue sloughs off when it thaws. The treatment may leave a small, white scar. You may also need a repeat treatment to remove the growth completely.
• Excisional surgery. This type of treatment may be appropriate for any type of skin cancer. Your doctor cuts out (excises) the cancerous tissue and a surrounding margin of healthy skin. A wide excision — removing extra normal skin around the tumor — may be recommended in some cases. To minimize or avoid scarring, especially on your face, you may need to consult a doctor skilled in skin reconstruction.
• Laser therapy. A precise, intense beam of light vaporizes growths, generally with little damage to surrounding tissue and with minimal bleeding, swelling and scarring. A doctor may use this therapy to treat superficial skin cancers or precancerous growths on lips.
• Mohs' surgery. This procedure is for larger, recurring or difficult-to-treat skin cancers, which may include both basal and squamous cell carcinomas. Your doctor removes the skin growth layer by layer, examining each layer under the microscope, until no abnormal cells remain. This procedure allows cancerous cells to be removed without taking an excessive amount of surrounding healthy skin. Because it requires special skill, the surgery should be done only by specially trained doctors.
• Curettage and electrodesiccation. After removing most of a growth, your doctor scrapes away layers of cancer cells using a circular blade (curet). An electric needle destroys any remaining cancer cells. This simple, quick procedure is common in treating small or thin basal cell cancers. It leaves a small, flat, white scar.
• Radiation therapy. Radiation may destroy basal and squamous cell carcinomas if surgery isn't an option.
• Chemotherapy. In chemotherapy, drugs are used to kill cancer cells. For cancers limited to the top layer of skin, creams or lotions containing anti-cancer agents may be applied directly to the skin. Topical drugs can cause severe inflammation and leave scars. Other types of chemotherapy can be used to treat skin cancers that have spread to other parts of the body.

Treatments for skin cancer under study include:

• Photodynamic therapy. This treatment destroys skin cancer cells with a combination of laser light and drugs that makes cancer cells sensitive to light. Photodynamic therapy for precancerous skin lesions is currently available by prescription.
• Biological therapy (also called immunotherapy). Interferon and interleukin-2 are under study to treat melanoma and nonmelanoma skin cancers. These immunotherapy drugs stimulate your immune system to fight the cancer. Other medications applied to your skin, such as imiquimod (Aldara), enhance your immune reaction to the presence of skin cancer.

Most skin cancers are preventable. To protect yourself:

• Avoid the sun between 10 a.m. and 4 p.m. Because the sun's rays are strongest during this period, try to schedule outdoor activities for other times of the day, even in winter or when the sky is cloudy. You absorb UV radiation year-round, and clouds offer little protection from damaging rays. Remember, sunburns and suntans cause skin damage that can increase your risk of developing skin cancer. Sun exposure accumulated over time also may cause skin cancer.

• Wear sunscreen year-round. Sunscreens don't filter out all harmful UV radiation, especially the radiation that can lead to melanoma. But they play a major role in an overall sun-protection program. Be sure to use a broad-spectrum sunscreen with a sun protection factor (SPF) of at least 15 when you go outside, year-round. Use a generous amount of sunscreen on all exposed skin, including your lips, the tips of your ears, and the backs of your hands and neck.

  For the most protection, apply sunscreen 20 to 30 minutes before sun exposure and reapply it every two hours throughout the day, as well as after swimming or exercising. Apply sunscreen to young children before they go outdoors, and teach older children and teens how to use sunscreen to protect themselves. Keep a bottle of sunscreen in your car as well as with your gardening tools and sports and camping gear to remind yourself and your family to use it.

  In July 2006, the Food and Drug Administration (FDA) approved a new over-the-counter sunscreen that will be marketed in the United States as Anthelios SX. The new sunscreen offers better protection from UVA rays than do traditional broad-spectrum sunscreens, according to the manufacturer. This may help reduce the risk of various types of skin cancer — including melanoma and basal and squamous cell carcinomas. Better UVA protection also may reduce sun-related skin wrinkling. But the added protection may come at a cost. Although U.S. prices aren't yet available, similar products sold in Canada cost about twice as much as traditional sunscreens — or even more.

• Wear protective clothing. Sunscreens don't provide complete protection from UV rays. That's why it's a good idea to also wear dark, tightly woven clothing that covers your arms and legs, and a broad-brimmed hat, which provides more protection than a baseball cap or visor does. Some companies also sell photoprotective clothing. Your dermatologist can recommend an appropriate brand. Don't forget sunglasses. Look for those that block both UVA and UVB rays.
• Avoid tanning beds and tan-accelerating agents. Tanning beds emit UVA rays, which may be as dangerous as UVB rays — especially since UVA light penetrates deeper into your skin and causes precancerous skin lesions.
• Be aware of sun-sensitizing medications. Some common prescription and over-the-counter drugs — including antibiotics; certain cholesterol, high blood pressure and diabetes medications; birth control pills; nonsteroidal anti-inflammatories such as ibuprofen (Advil, Motrin, others); and the acne medicine isotretinoin (Accutane) — can make your skin more sensitive to sunlight. Ask your doctor or pharmacist about the side effects of any medications you take. If they increase your sensitivity to sunlight, be sure to take extra precautions.
• Check your skin regularly and report changes to your doctor. Examine your skin often for new skin growths or changes in existing moles, freckles, bumps and birthmarks. With the help of mirrors, check your face, neck, ears and scalp. Examine your chest and trunk, and the tops and undersides of your arms and hands. Examine both the front and back of your legs, and your feet, including the soles and the spaces between your toes. Also check your genital area, including between your buttocks.
• Have regular skin exams. Consult your doctor for a complete skin exam every year if you're older than 40, or more often if you're at high risk of developing skin cancer.

To detect melanomas or other skin cancers, use the A-B-C-D skin self-examination guide, adapted from the American Academy of Dermatology:

• A is for asymmetrical shape. Look for moles with irregular shapes, such as two very different-looking halves.
• B is for irregular border. Look for moles with irregular, notched or scalloped borders — the characteristics of melanomas.
• C is for changes in color. Look for growths that have many colors or an uneven distribution of color.
• D is for diameter. Look for growths that are larger than about 1/4 inch (6 millimeters).