Melanoma is a cancer that typically starts in skin cells known as melanocytes that make the skin's pigment. Compared with more common skin cancers that begin in squamous or basal cells, melanoma is more likely to spread to other parts of the body.

"It truly has a very aggressive behavior and biology behind it," said Dr. Michael Davies, chairman of the melanoma medical oncology department at the University of Texas MD Anderson Cancer Center. Most melanomas appear as flat or slightly elevated blotches of dark color on skin that has been frequently exposed to ultraviolet light, such as the scalp and face, arms, back and legs (though they can occur on areas that have never been exposed to the sun, too).

In a smaller share of cases, the growth may appear as a dark- or red-colored bump and grow down into the skin, which can make it more difficult to detect. Early detection and treatment are crucial for improving outcomes.

Risk factors include excessive sun exposure, fair skin, and family history. Prevention strategies involve sun protection and regular skin checks.