Melanocytic naevi, melanoma and sun exposure.

The naevus profile was examined in 379 randomly selected Swedes (30-50 years) and in 121 melanoma patients in the same age-range selected from the Regional Cancer Register. The total body count of common naevi (CN) greater than or equal to 2 mm in the population was high (mean 67, range 1-300). Even so, the melanoma patients had almost twice as many CN as the controls (mean 113, range 13-347). The prevalence of clinical dysplastic naevi (DN) was 18% in the population and 56% in the patients. The corresponding figures for histologically diagnosed DN were 8% and 40% respectively. Subjects with dysplastic naevi had a significantly larger number of CN than those without DN. Subjects with a sun-sensitive skin, greater than or equal to 150 naevi and presence of DN had a 50 times higher melanoma risk than those without these characteristics. For identifying subjects at risk of developing cutaneous malignant melanoma (CMM), clinically diagnosed DN was as good a discriminator as histologically diagnosed DN. The numbers of naevi in different skin areas were tested for their power in predicting the total body naevus count. The strongest correlations were found between total counts and counts on the anterior surface of the thighs and the lateral aspect of the arms. Counts from either of these areas will provide a practical and satisfactory estimate of the total number of naevi. To study the possible link between sun exposure, naevus formation and melanoma development, the distribution of CN, DN and CMM over the body surface was studied in the 121 melanoma patients and in 310 consecutive controls. The number of naevi was four times as high in a sun-exposed area on the lower back compared with in an adjacent sun-protected area on the buttocks, indicating that sunlight plays a role in naevus development. Both CN and CMM were found to have a general distribution pattern over the body surface consistent with the idea that sun exposure is important for their formation. The number of CN and CMM per unit body surface area was significantly higher in intermittently sun-exposed than in rarely or chronically exposed skin. The distribution pattern of DN was quite different from that of CN, with few DN on the sun-exposed upper chest and face and many DN in protected areas such as the buttocks and lower abdomen, indicating that DN may develop independently of sun exposure.(ABSTRACT TRUNCATED AT 400 WORDS)