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Most adults have between 10 and 40 moles. Some people, especially those with lighter skin, have many more. These small clusters of pigment-producing skin cells may change in size and appearance over the years; in rare instances, a mole becomes a melanoma, the most potentially deadly skin cancer.

If you’re like most people, you don’t examine your moles or have a doctor check them periodically to see whether changes may signal a possible cancer. In fact, a survey of 476 people published in the Journal of Clinical and Aesthetic Dermatology found that only 25 percent reported checking their skin monthly; 17 percent reported that they did it just once a year.

That may be due, in part, to confusion over how effective regular skin checks are. So should you or a dermatologist check your skin? If so, how often? Here’s what you need to know about changing moles and the risk of skin cancer:

How moles change over time

Common moles are those we’re born with or develop until about age 40. They can change or even disappear over the years, and they very rarely become malignant. But according to the National Cancer Institute, having 50 or more moles puts a person at higher risk of melanoma — although research published in JAMA Dermatology has called that into question.

Moles that are called atypical often appear during puberty, but they can pop up throughout life. They’re considered benign but may have some features of melanoma, such as irregular borders.

Most atypical moles don’t become melanomas, but they are more likely than common moles to become cancerous. Having five or more atypical moles is linked to a higher-than-normal risk of “thick,” or more advanced, melanoma. (Many melanomas start as pigmented moles, but the two more common skin cancers, basal cell and squamous cell carcinoma, don’t. They arise on their own from normal skin.)

The mechanism behind a mole’s change from benign to cancerous may be genetic in some cases. And although melanoma can occur in areas that are usually shielded from sunlight, ultraviolet radiation seems to be a significant factor. In a study published in the New England Journal of Medicine, Boris C. Bastian, a professor of dermatology and pathology at the University of California at San Francisco, and colleagues found that a mole exposed to UV radiation (like that from the sun or tanning beds) can mutate, triggering the abnormally rapid cell division that characterizes cancer.

Skin checks: Pros and cons

While it might seems as if it would make sense to have a doctor check your skin for skin cancers, the U.S. Preventive Services Task Force, an independent expert panel that advises the government on screening tests, concluded in 2009 that there was insufficient evidence to recommend for or against regular skin checks by physicians. In July, the task force reaffirmed that conclusion, saying that there still is not clear evidence that regular professional skin exams saves lives by identifying cancers early.

But that doesn’t mean that you should avoid those exams, says Jessica Krant, a clinical assistant professor of dermatology at SUNY Downstate Medical Center in Brooklyn and a member of Consumer Reports’ medical advisory board.

For one thing, the task force didn’t look at people at high risk of skin cancers. And it looked only at exams from primary-care providers, not dermatologists. Moreover, catching melanomas early is key: Unnoticed, the cancers can grow and spread quickly. “A melanoma the size of a dime has a 50 percent chance of having spread,” says Darrell S. Rigel, a skin cancer expert at the NYU School of Medicine.

For those reasons, many experts and organizations such as the American Academy of Dermatology recommend that you regularly do skin self-checks and have screenings by a physician to monitor moles.

Exactly how often you should do that is unclear, and depends in part on your chance of developing the disease. People at high risk should probably see a physician, preferably a dermatologist, at least once a year, Krant says.

Key risk factors include a history of sunburns, fair skin, light eyes, red or blonde hair, a family history of melanoma or a personal history of basal cell or squamous cell cancer. (Screenings can also help pinpoint the latter two skin cancers, which are more common but not as deadly as melanoma.) People at very high risk or with a history of melanoma should be screened even more often.

Self-checks are also important. Krant recommends a monthly or every-other-month schedule. (A study in the journal Archives of Dermatology found that 44 percent of melanomas were discovered by patients.)

If you spot new moles or are unsure whether a mole’s changes may be meaningful, see a dermatologist. Sometimes, a melanoma may not resemble others in shape, color or size, says Scott W. Fosko, chair of dermatology at the Mayo Clinic in Jacksonville, Fla. For example, he says, the amelanotic melanoma has little to no color.

Copyright 2016. Consumers Union of United States Inc.

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