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The skin's dynamic outer layer, or epidermis, serves as the staging ground for all three of the major skin cancers. Both basal-cell and squamous-cell carcinomas arise from the most common skin cells, the keratinocytes, which form at the base of the epidermis and work their way toward the surface. Near the base, they are plump and are called basal cells. But as they move outward, they flatten to become the squamous cells that form the skin's tough, protective surface. Melanomas spring from melanocytes, cells that produce pigment.
Epidermal cells become malignant when the DNA in their nuclei is altered, causing them to divide uncontrollably and form tumors. The transformation of DNA can be caused by repeated X-ray exposure, burns, infectious disease or frequent contact with certain chemicals. But by far the most common culprit is the sun's ultraviolet light. After years of exposure to sunlight, the damage becomes visible first as small, scaly, precancerous spots called keratoses, usually on middle-aged or older people and in areas of the skin generally not protected by clothing. These spots can turn malignant, becoming translucent basal-cell nodules that slowly expand into adjoining tissue.
Unlike other cancers, basal-cell carcinomas rarely metastasize, or migrate to form tumors in other parts of the body. For that reason, many people regard these carcinomas lightly and unwisely put off corrective surgery. Doctors excising basal tumors that have gone too long without treatment must often remove large chunks of their patients' noses or ears, which then must be reconstructed surgically. Worse consequences can occur. "I've heard of only a few deaths due to basal-cell carcinomas," says Dr. Lamberg. "But if an unattended tumor on the head grew into the brain, for example, it could cause considerable damage or even death."
Squamous-cell carcinomas also develop from keratoses on long-exposed areas of the skin, affecting about 100,000 Americans each year. They take the form of red or pink warty growths that may scale or open in the center and ooze. Squamous tumors are more dangerous than basals; they grow more rapidly and can metastasize, sometimes with fatal results.
But by far the most fearsome form of skin cancer is malignant melanoma, which sometimes emerges from an existing mole or simply appears in an area of previously unblemished skin. Melanomas are asymmetrically shaped, usually begin as mottled light brown or black blotches that eventually can turn red, white or blue in spots, become crusty and bleed. They grow rapidly, and once they have expanded to about the thickness of a dime, they have probably metastasized and become lethal.
Here too the sun's ultraviolet radiation plays a role, but apparently a different one. Many melanoma victims have had three or more episodes of severe sunburn and blistering, usually as children or teenagers. Those experiences seem to set off a still mysterious process that results in the development of melanomas years later, often on parts of the body seldom exposed to the sun. Some evidence also exists that heredity plays a more important role in melanoma than in other skin cancers.