Case-control study of melanocytic nevi on the buttocks in atypical mole syndrome: role of solar radiation in the pathogenesis of atypical moles.

Background: It is not known why melanocytic nevi (MN) become dysmorphic (atypical) in patients with the atypical mole syndrome (AMS). A complex origin for acquired MN has been postulated. Genetic predisposition, solar radiation, and/or the formation of a sun-induced circulating mitogenic factor may contribute to the formation of MN.
Objective: This study was undertaken to help elucidate the pathogenesis of atypical MN in patients with AMS.
Methods: The number of common and atypical MN was determined for a defined sun-protected area on the buttocks in 150 patients with AMS (cases) and 150 control subjects without AMS. Patients and control subjects were matched for age and sex and were classified into risk groups for the development of malignant melanoma according to the Rigel classification.
Results: MN on the buttocks were found in 23% of patients with AMS and 9% of control subjects (p < 0.003). In patients versus control subjects the mean number (1.3 vs 1.2, respectively) and mean diameter (5.7 vs 5.9 mm, respectively) of MN on the buttocks did not differ significantly. The MN in both patients and control subjects were not atypical clinically. The odds ratio for having AMS if MN were present on the buttocks was calculated to be 1.56 (95% confidence interval, 1.25 to 1.95).
Conclusion: Although the patients were 2.6 times more likely to have MN on their buttocks than the control subjects, clinically the MN did not differ significantly in number or appearance from those found on the buttocks of control subjects. It is hypothesized that the formation of some atypical MN requires direct solar radiation for their phenotypic expression.