There’s another reason to keep an eye on your moles. Two new studies suggest that women with more moles may have a higher risk of breast cancer.
Moles are already linked with a higher risk of melanoma — the deadliest form of skin cancer. The two new studies cannot explain how moles might be related to breast cancer, but there’s a lot of research showing that moles, like breast cancer, can be influenced by hormone levels.
Moles are formed naturally by cells in the skin, and the vast majority of moles are completely harmless. But dermatologists say it’s a good idea for people to keep an eye on their moles, and report any changes such as sudden growth itching or bleeding.
In the new studies, two teams of researchers, one in France and one in the United States, looked at large surveys of tens of thousands of women. In both, women were asked about how many moles they had, in addition to a batch of other questions. They were then followed for years to see what happened to their health.
In the U.S. study, Jiali Han and colleagues from Indiana University and Harvard University studied 74,523 female nurses who had been watched for 24 years. Those who had 15 or more moles were 35 percent more likely to be diagnosed with breast cancer than women who said they didn’t have any.
After 24 years, women with no moles had about an 8 percent chance of developing breast cancer but the women with 15 or more had an 11 percent risk.
That doesn't mean that women who have lots of moles need to worry, says Dr. Deanna Attai, a breast surgeon in Burbank, California, who was not involved in the studies.
"A cause for alarm in women that have lots of (moles?). No, I don't think so," she said. "I would still put more emphasis on obesity and family history."
The women with more moles (their medical name is nevi) also seemed to have higher levels of hormones, Han’s team reported in the Public Library of Science journal PLoS Medicine.
“Postmenopausal women with six or more nevi had a 45.5 percent higher level of free estradiol and a 47.4 percent higher level of free testosterone compared to those with no nevi,” they wrote.
Older women with higher circulating levels of sex hormones such as estrogen and testosterone have more moles. They also have higher risks of breast cancer, and melanocytes, the cells involved in melanoma and in making moles, have built in docking points for hormones, called receptors.
And moles often darken or grow when women are pregnant, another piece of evidence showing they can be influenced by hormones.
“Nevi may be a marker of exposure to sex hormones,” Barbara Fuhrman and Victor Cardenas of the University of Arkansas wrote in a commentary.
And it was only so-called hormone receptor positive breast cancer — the type fueled by estrogen — that was more common in the women with many moles, another piece of evidence pointing to hormones.
But Attai notes while this holds true in large populations, it doesn't mean much for an individual woman. "You can't always make a direct link between hormone levels and the risk of breast cancer in an individual patient," she said.
The French study shed some more light on the question. They looked at a similar large study of 89,000 French women who were aged 40 to 65 when they enrolled in 1990. The women who said they had “very many” moles at the beginning were 13 percent more likely to develop breast cancer.
"We found a modest association between number of nevi and overall breast cancer risk," they wrote.
Not only are moles linked with melanoma, but some studies suggest the risk of melanoma may be related to breast cancer, Marina Kvaskoff of the French research institute Inserm and colleagues said.
And some people with many moles and with melanoma have a mutation in the CDKN2A gene. There may be an association with mutations in this gene and some cases of breast cancer. And there may be links between having a lot of moles and other hormone-related conditions such as thyroid disease and endometriosis, Kvaskoff's team said.
Fuhrman and Cardenas said it might be possible to use moles as one more factor in the calculations used to advise women about their risk of breast cancer — calculations that include questions such as whether a mother or sister has ever had breast cancer and the number of biopsies that have been done because of suspicious-looking breast lumps.
But, they added, the associations between moles and breast cancer are really pretty weak. As is so often the case with medical reports, more study is needed, the researchers agree.