Basal cell carcinoma is slow-growing and occurs mostly in people over age 55. It's more common in men than in women, although the incidence in younger women is rising.

Sun exposure is the biggest risk factor. We don't know precisely why one person gets squamous cell carcinoma and another gets basal cell, but it probably has to do with greater and lesser degrees of sun damage over time and the intensity of the sun's rays.

People with fair skin, blond or red hair, and blue, grey, or green eyes are at greatest risk. Other risk factors include a history of ionizing radiation therapy to treat acne, eczema, or psoriasis; chronic immunosuppression for organ transplantation; long-term use of oral glucocorticoids (steroid drugs); and a history of previous skin cancer of any type. People who have had multiple basal cell cancers are at greater risk for squamous cell carcinoma and melanoma.

Scientists are only beginning to understand how sunlight causes basal cell carcinoma. One possibility is that the sun's ultraviolet rays cause inflammation and increased levels of COX-2, an enzyme implicated in the development of cancer. Certain gene mutations may also be involved.

The goal of treatment is to remove the cancer completely with the least possible cosmetic damage. The options, which include cryosurgery, surgical removal, radiation and topical creams, have rarely been compared in controlled trials. But in properly selected patients, each has a cure rate of 90 percent or more for first-time cancers.

Surgical approaches such as aimple excision, cryosurgery (freezing), and electrosurgery (which involves scraping and burning) work well for low-risk types. A low-risk skin cancers are a mostly small nodular or superficial ones that appear almost anywhere except the face (and possibly the neck and ears) and aren't aggressive.

Larger cancers, morpheaform ones of any size, and those on the face and other structurally or cosmetically sensitive areas are considered high-risk. These are best treated with surgical removal that allows the surgeon to check during the procedure to make sure she or he has removed the entire cancer and just enough healthy tissue to get cancer-free margins. This cuts the risk of a recurrence while giving the best possible cosmetic result.

These days, the usual first choice for removing high-risk basal cell skin cancers is Mohs micrographic surgery. The surgeon removes the cancer layer by layer, examining each one microscopically until the margin around the cancer is free of cancer cells. It's technically exacting and takes longer than other procedures, but the cure rate approaches 100 percent.

Radiation therapy is noninvasive, painless, and often the best choice if you're not a candidate for surgery. But it requires repeat visits, and there's some risk that the cancer will be more aggressive if it recurs. Radiation therapy generally isn't recommended for patients younger than 50 because of the risk of developing a new skin cancer at the site in 10-15 years.

Low-risk superficial basal cell carcinoma can be treated with ointments, including 5-FU and imiquimod (Aldara), which are applied daily for several weeks or months. Cure rates are mixed.

The sun's rays prematurely age the skin and cause wrinkles as well as skin cancer. For these reasons, a broad-spectrum sunscreen is the most important skin product you can buy. "Broad spectrum" means that it provides protection from the two main types of ultraviolet (UV) radiation, UVA and UVB. UVB is the chief cause of sunburn. The sun protection factor (SPF) number on the label indicates how well a sunscreen protects you from sunburn.

But sunburn protection doesn't equal skin cancer prevention. UVB was once considered the main skin cancer culprit, but experts now believe UVA is just as important.

Unfortunately, not all sunscreens contain UVA blocking agents, and for those that do, there's no rating system like the SPF numbers. (The U.S. Food and Drug Administration is working on this.) For now, make sure your sunscreen contains agents to help protect you from both UVA and UVB. These include titanium dioxide, zinc oxide, avobenzone (Parsol 1789 or methoxydibenzone), dioxybenzone, oxybenzone, sulisobenzone, and methyl anthranailate. Mexoryl, which some experts say provides the best UVA protection, is under FDA review and not yet available in the United States.

Our most common mistake with sunscreen is not using enough. Most people apply only a quarter of the amount they need, which "converts an SPF of 30 into an SPF of about 6," says Carl Schanbacher, a Mohs surgeon at Dana-Farber Cancer Institute in Boston.

How much sunscreen is enough? Schanbacher suggests the following amounts: 1 teaspoon for the head and neck, 1 teaspoon for each arm, 1-3 teaspoons for the trunk, and at least 1 teaspoon for each leg. That may seem like a lot, but that's what you need.

Apply sunscreen - use an SPF of at least 15, and preferably 30 - about 30 minutes before going out, so it absorbs well and becomes fixed to the skin. Reapply it every two hours and after swimming (some of the active ingredients break down in the sun).

But sunscreen isn't a carte blanche to sunbathe all day. Try to avoid the sun when UV radiation is most intense (10 a.m. to 4 p.m.). Also, wear a broad-brimmed hat that protects your ears and neck (a baseball cap isn't a good choice), sunglasses, and long-sleeved clothing made of tightly woven fabric.

You can buy sun-protective clothing. Some carry an ultraviolet protection factor (UPF) rating that indicates how much UV radiation the fabric blocks. You can also wash UV protection into your clothes with a laundry additive called SunGuard.

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