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6.1 Melanoma Precursors

Revised January 2008

Link to Skin Cancer Atlas

Common Acquired Melanocytic Nevi
An average adult has 20-40 nevi. An individual with an increased number of common acquired melanocytic nevi has an increased risk of developing melanoma and should be counseled to avoid excessive sun exposure and to have regular skin examinations. As the risk of melanoma transformation in a common acquired melanocytic nevus is extremely low, prophylactic removal is not warranted.

Atypical Nevi
The exact risk of melanoma transformation in atypical nevi has not been established. Not all atypical nevi will evolve into melanoma. In fact, most atypical nevi will remain clinically stable and never progress to melanoma. It is recommended that those atypical nevi that appear clinically very atypical be excised (i.e., lesions with very irregular shape, marked pigment variegation, asymmetry in shape or pigment distribution). These atypical nevi may be clinically indistinguishable from melanoma. Nevi with slightly atypical features (i.e. symmetrical lesions with two-shades of brown, slightly fuzzy borders, or slightly irregular shape) generally do not warrant prophylactic excision. When in doubt, a Dermatologist should be consulted.

As discussed, atypical nevi are both potential precursors and markers for melanoma. In people with these lesions, melanoma can develop both from atypical nevi or de novo in normal appearing skin. These individuals should be advised to have regular follow-up skin examinations at 6 to 12 month intervals and to avoid excessive sun exposure. Atypical nevi may occur sporadically or be inherited in an autosomal dominant fashion. Therefore, it is suggested that first degree relatives be examined to determine if they too have atypical nevi.

Congenital Melanocytic Nevi (CMN)
It is recommended that large CMN be excised as early as possible, as the risk of melanoma transformation in infancy is high. Frequently, the complete excision will be carried out in several stages. Surgical excision significantly decreases, but does not completely eliminate the risk of melanoma as the CMN cells may be deep to the fascia or in leptomeninges that are not surgically accessible. Therefore, these patients should be kept under surveillance. Follow-up examination at 6 to 12 month intervals is generally recommended.

The management of small and medium CMN is controversial because the exact magnitude of the melanoma risk is unknown. Some experts recommend prophylactic excision while others recommend periodic observation only.