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Transformation of Moles into Melanoma
Elucidation of the risks for malignant transformation of nevi confirms that some prophylactic practices are unnecessary.
Retrospective histologic study shows that 21% to 35% of melanomas are associated with transformed melanocytic nevi. Prospectively, the risk that a given nevus will transform into melanoma is unknown. In this epidemiologic study, researchers counted the number of melanomas associated with melanocytic nevi in specimens sent to a community pathology laboratory over a 4-year period. From published data, they ascertained the number of melanomas diagnosed in the U.S. in a given year and the average number of melanocytic nevi that occur in a fair-skinned person. Assuming that the number of moles that will turn into cutaneous melanoma each year is approximately similar to the annual number of melanomas found to be associated with nevi, the authors calculated the risk for transformation in an individual nevus.
The histologic association of a nevus with melanoma decreased in men from 65% at age 20 to 20% at age older than 70 and in women from 47% at age 20 to 9% at age older than 70. The average number of melanocytic nevi in a white person decreased from 28 at age 20 to 6 at age older than 70. The annual risk for transformation of a single nevus into melanoma ranged from 1 in 200,000 for men and women younger than 40 to 1 in 33,000 for men older than 60. For a 20-year-old, the risk that a single nevus will transform into melanoma by age 80 was 1 in 3164 for men and 1 in 10,800 for women.
Comment: Even though the histologic association of melanoma with nevi decreases with age, this effect is more than offset by increases in melanoma incidence and decreases in the number of nevi with age. Ultimately there is an increased risk that a given nevus will transform; however, even in older people, the risk is very low. Therefore, prophylactic excision of benign-appearing nevi is unproductive and should not be done. Detailed photography and mole mapping should be reserved for patients with known elevated melanoma risks, such as strong personal or family history or presence of atypical nevi. In the general population, these measures would render a very low yield at high cost.
— George J. Hruza, MD
Published in Journal Watch Dermatology April 15, 2003
Citation(s):
Tsao H et al. The transformation rate of moles (melanocytic nevi) into cutaneous melanoma: A population-based estimate. Arch Dermatol 2003 Mar; 139:282-8.
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