Melanoma, accounting for less than 5% of skin cancers, is one of the most dangerous of all cancers and is the most rapidly increasing form of cancer in the United States. Melanoma arises from pigment cells in the skin and usually takes the form of a very dark, irregularly-colored spot or nodule . The risk of melanoma is increased with occasional, intense exposure, such as blistering sunburns, especially in childhood, rather than long-term sun exposure.
Common locations for melanoma include the back and scalp in men and legs in women, areas not usually exposed extensively to the sun, and it is more common in younger individuals. Melanoma may occur by itself, or arise from a pre-melanoma mole or skin growth.
Unlike other cancers, melanoma may remain inactive for years, may even regress temporarily, and then suddenly metastasize to lymph nodes or vital organs. The tendency of melanoma to metastasize is fairly predictable and is related to the thickness rather than the size of the tumor.
In the United States, the risk of melanoma is about 15 cases for every 100,000 people, and at the time of writing there will be 40,000 new cases with about 8,000 deaths expected in the next year.
Australia has the world's highest incidence of melanoma, because of its location near the equator and population composed largely of people of Celtic extraction. The fact that melanoma occurs mainly on sites not overly sun-exposed indicates that other factors are involved.