Vernon Beveridge, age 13, of Kallispell, Mont., for his question:

IS SKIN CANCER A COMMON PROBLEM?

Skin cancers are the most common of all malignant tumors. A recent study estimates that there are more than 300,000 new cases each year, nearly three times as many as the most prevalent cancers of other origin.

In contrast to other types of cancers, skin cancers are readily seen and detected, and therefore can be treated quickly.

The two most common types of skin cancer are called "basal cell carcinoma" and 11squamous cell carcinoma." They account for 29 out of 30 cases of skin cancer, but they are usually discovered while still localized and rarely lead to death.

Malignant melanoma, the rarest form of skin cancer, is always potentially fatal unless detected early and treated promptly.

Prolonged sun exposure increases the risk of developing the basal cell and squamous cell forms of cancer. Persons most susceptible are those who sunburn easily and tan poorly, such as those of Irish, Scottish or Welsh background, and persons who work outdoors and receive prolonged sun exposure over many years, such as sailors and farmers.

Persons who have dark skins or who keep well covered have lower incidence of skin cancer.

Although sun exposure is the major cause of these skin cancers, there are several other factors that can make someone susceptible to them. Certain inherited conditions carry the tendency to develop these tumors. And X ray therapy, given in the past for acne and thought to be safe, has sometimes been followed years later by the appearance of multiple skin cancers.

Also, the prolonged exposure to arsenic, which used to be used in certain medicines and also in agricultural insecticides, increases the likelihood of skin cancer.

Basal cell carcinoma accounts for more than 75 percent of all skin cancers. The tumor arises when the basal cells of the epidermis proliferate abnormally and invade the dermis.    

The most common type of basal cell cancer appears as a small nodule on the skin which eventually ulcerates in the center, intermittently bleeding and forming a crust. Because it is painless, the tumor may be neglected for some time. Bleeding and failure of the ulcer to heal usually prompts the person to see a doctor.

Squamous cell carcinoma, less common than basal cell carcinoma, arises from uncontrolled growth of the prickle or spinous cells in the epidermis. It may be a small growth with an irregular surface which bleeds, but the appearance varies.

Malignant melanoma causes more deaths in the United States than any other disease that arises in the skin. The malignant melanoma usually starts as a brown spot on the skin that first appears during adulthood.

The lesion gradually enlarges and assumes characteristic features: an irregular border, variation in color and an irregular surface. Often the easiest characteristic for a patient to notice is the irregular border, which is clearly different from the smooth, regular borders of benign moles.