Researchers have found that most melanomas form as new spots, rather than from existing moles.
Researchers from Italy and Greece describe their findings, gleaned from 38 melanoma studies published in recognized academic databases, in the Journal of the American Academy of Dermatology.
Melanoma is a particularly aggressive skin cancer that starts as mutations in melanocytes - the cells that give skin its color. The mutated cells grow abnormally and form tumors.
The most common cause of melanoma is exposure to ultraviolet (UV) light, such as from sunbathing or indoor tanning. UV rays can alter skin cells and damage their DNA.
Melanoma is much easier to treat successfully if it is found early, while the growth is confined to the outer layer of the skin or epidermis. If not found early, it can invade deeper layers and spread to other parts of the body (metastasis).
Once melanoma has spread, it is very hard to treat and the chances of survival are much diminished.
In 2014, the most recent year for national statistics, 76,665 people in the United States received a diagnosis of melanoma of the skin. In that year, 9,324 people also died of the disease.
Existing mole or new growth?
In their study paper, the researchers explain that the evidence about how melanoma starts - whether from an existing mole, or "nevus,' or from a new growth - is mixed.
They note that since the late 1940s, studies of melanoma have stated a wide range of rates. Some have said that melanomas that start in existing moles account for only 4 percent of cases, while others have said that they account for 72 percent. Also, a recent analysis of 25 studies suggests that they account for 36 percent of cases.
However, the authors argue that the main reason for this disparity is that the studies have looked at different aspects of the cancer and its origins, such as the thickness of the tumor and the underlying features of the mole or site.
They note, for example, that features of any underlying mole or nevus are often hard to describe, as melanoma tumors can deform or even destroy the tissue of the site.
"Thus, it is extremely difficult or even impossible to determine if the lesion had originally been associated with a nevus or not," they write.
Most melanomas are new growth
So, for their review, the team pooled and analyzed data from studies of patients with melanoma where the researchers had specified whether the melanomas were associated with moles ("nevus-associated" melanomas) or had arisen from new growths ("de novo" melanomas).
Altogether, the team analyzed 38 observational and case-control studies accounting for a total of 20,126 melanomas.
The main finding of the study was that the majority (70.9 percent) of melanomas develop from new growths, and only a minority (29.1 percent) arise from an existing mole or nevus.
The authors write, "This result provides further evidence that most melanomas do not originate from malignant transformation of nevus cells."
They found no link between the rate of mole-associated melanomas and the presence of abnormal (dysplastic) tissue in the mole.
The researchers also found that melanomas that arise from existing moles tend to be thinner than those that develop from new spots, suggesting that patients with this kind of tumor are likely to have a better prognosis.
Self-exams and sun protection
The team suggests that this finding means that patients who regularly check their moles for signs of suspicious changes might stand a better chance of finding melanoma in its early stage, when treatment is more likely to succeed.
The American Academy of Dermatology recommend that people regularly perform self-exams on their skin and get their partners to check their backs and any other hard-to-see areas.
"Because the disease [melanoma] is more likely to appear as a new growth, however, it's important for everyone to familiarize themselves with all the moles on their skin and look for not only changes to those moles, but also any new spots that may appear."
Dr. Caterina Longo, University of Modena and Reggio Emilia, Italy
They also recommend that people shield themselves from harmful UV rays when outdoors by staying in the shade, wearing protective clothing and applying a water-resistant, "broad-spectrum" sunscreen with a minimum sun protection factor (SPF) of 30.
In 2011, the U.S Food and Drug Administration (FDA) added the term "broad-spectrum" on sunscreen labeling. A broad-spectrum sunscreen protects against both ultraviolet A (UVA) and ultraviolet B (UVB) radiation. SPF only indicates level of protection against UVB.
'Broad-spectrum confuses consumers'
However, another study just reported in Journal of the American Academy of Dermatology has found that most people in the U.S. do not consider broad-spectrum when choosing their sunscreen.
The researchers say that the current labeling on sunscreen products, "specifically the broad-spectrum designation, is confusing to consumers."
Study author Roopal V. Kundu, an associate professor of dermatology at Northwestern University Feinberg School of Medicine in Chicago, IL, and colleagues suggest that sunscreen labels should show UVA and UVB protection levels separately.
In their research, they found that only 39 percent of participants took broad-spectrum into account when buying sunscreens, despite most of them saying that level of protection against skin cancer should feature on labels.
When the team presented participants with a label that clearly showed UVA and UVB protection levels separately, they were better able to understand how the product protected against the two types of UV rays.
The researchers say that further studies should be done to assess how well consumers understand the differences between UVA and UVB and their effects on the skin. These should also find out if showing protection levels separately on sunscreen labels really does influence buying decisions.
However, they conclude that apart from clearer labeling, there is also a need to improve public education so that people make well-informed decisions when buying sunscreens.