What Are the Chances That a Mole Will Turn to the Dark Side?

Abstract & Commentary

Synopsis: Melanocytic nevi rarely transform into cutaneous melanoma.

Source: Tsao H, et al. Arch Dermatol. 2003;139:282-288.

Although the phenomenon of cutaneous melanoma (CM) arising from melanocytic nevi (MN) is known, the transformation rate of MN into CM is not. Tsao and associates set out to calculate the rate, using published data and making some assumptions to ease the calculation. Their first assumption is that the number of MN in any year is stable. The second assumption is that the number of CM arising from MN is the same as the number of MN that transform into CM per year. (This may appear to be an intuitive equivalence, but some MN are obliterated in the process of becoming CM, leaving no histologic trace.) They examined the records of a community-based general dermatopathology practice and identified 1615 CM. Of these, 425 (26%) had an associated MN. They also calculated rates based on gender and age stratification. The percentages ranged from 20 to 65 for men and 9 to 47 for women with a peak in the third decade for both. They used data from the National Cancer Institute’s Surveillance, Epidemiology, and End Result Study (SEER, available at seer.cancer.gov) to derive the number of CM in the US white population by sex and age group and data from the US Census Bureau to calculate incident rates. Previous studies of mole counts from British and Australian studies were used to establish the number of MN per person, again with sex and age stratified. Their calculations yielded annual transformation rates of 0.0005% or less for both sexes aged 40 years and younger to 0.003% for men older than 60. They also calculated cumulative risk rates. The risk of a mole on a 20 year old becoming malignant by age 80 is 0.03% (1 in 3164) in men and 0.009% (1 in 10,800) in women.

Comment by Allan J. Wilke, MD

I know what you’re thinking—GIGO! (Garbage In, Garbage Out, a derisive term from the early days of computer programming). Of course, if you really want to know the transformation rate, the gold standard would be to conduct a prospective cohort study in an unbiased population. However, this would involve following a very large group of people for many years. Until that study is done (not likely), this will have to do. In this article, Tsao et al set out to estimate the rate at which MN transform into CM. They cobbled together data from a collection of disparate sources and using a mathematical formula, estimated that the risk of any one MN becoming malignant is approximately 1 in 200,000 before age 40. The lifetime chance of developing a CM in the US is 1 in 75.1 The American Cancer Society (ACS) estimates that in 2003 there will be 54,200 (29,900 men, 24,300 women) new cases of CM.2 Early detection, which should be easy considering that they are pigmented and lie on the skin surface, and excision usually means cure. There is a well-established, albeit complex, association between sun exposure and melanoma, and the ACS recommends sun protection behaviors, including avoiding sunburn during childhood and intense intermittent sun exposure.3 A particularly vexing association with CM is MN, or in the vernacular, moles. MN are ubiquitous; almost every young adult has 20-40 of them. When CM are excised, MN are often found in histologic continuity with them. Along with excessive exposure to ultraviolet radiation (including tanning booths), fair skin, exposure to coal tar, pitch, creosote, arsenic, or radium, family history, and multiple or dysplastic nevi are all risk factors for CM. The ACS recommends that patients regularly perform self-examination and report suspicious lesions to their physicians. A suspicious lesion is scaling, oozing, bleeding, or changing in appearance and may be associated with a spread of pigmentation or itching, tenderness, or pain. Physicians are reminded of the ABCD rule. Be suspicious when A, the lesion is Asymmetric; B, the Border is irregular or blurred; C, the Color is not uniform; and D, the Diameter is greater than 6 mm. What should a harried primary care physician take from this study? First, it is a rare event for a mole to transform into a melanoma. Second, most people are covered with moles, making surveillance (even with a camera) a high-cost, low-yield venture. Third, excising moles isn’t likely to make a dent in the incidence of melanoma. Fourth, pay closer attention to older patients with traditional risk factors. Fifth, educate your patients in self-examination and self-restraint when it come to sun exposure.

Dr. Wilke is Assistant Professor of Family Medicine, Medical College of Ohio, Toledo, OH.


1. Hall HI, et al. J Am Acad Dermatol. 1999;40:35-42.

2. American Cancer Society. Cancer Facts and Figures 2003. Atlanta (GA): American Cancer Society; 2003.

3. American Cancer Society. Cancer Prevention & Early Detection, Facts & Figures 2003. Atlanta (GA): American Cancer Society; 2003.