Spacing of healthy moles hints at site-specific melanoma risk

Naevi (moles) are benign brown or black spots on your skin that sometimes look similar to melanomas, but are completely harmless.

However, it turns that they can tell us a lot about a person’s melanoma risk. Having many moles is the strongest visible risk factor for melanoma, but where the moles are on your body can also tell us about your overall risk of developing melanoma.

Two recent studies looked at whether moles of people’s backs were clustered together, dispersed to be roughly equally spread across the back, or arranged totally randomly. Jayasinghe et al studied a general Australian population, drawn from the electoral roll to ensure a relatively random selection (all Australian citizens aged 18 and over are on the roll.) Chousokas et al studied an American population at high risk of melanoma, including people with a personal or family history of melanoma, or a high total body mole count. 51% had already had at least one melanoma.

Both groups used a mathematical model of spatial randomness called CSR, originally developed for geographical imaging systems, and applied the Clark-Evans test, which measures the extent of deviation from a random dispersal of lesions – that is, whether the moles are more clustered or more evenly spread out than a truly random layout would suggest.

If the distribution is truly random, then over all a person’s moles, the distances from each mole to its nearest neighbour should follow a normal distribution.

In Chousokas et al’s high melanoma risk group, 20% of melanoma patients who had had their melanomas on the back (the study site) had significantly clustered naevi, and none had significantly dispersed naevi. The sites of previously-excised melanomas were also more likely to be clustered with other melanoma sites and naevi. In contrast, in patients without a melanoma on the back, only 8.9% had clustered naevi on that site.

In the low melanoma risk general population sample, naevi tended much more towards completely random dispersal. The sole melanoma patient in the group had clustered naevi.

The general population sample was much smaller than the high-risk samples, partly because relatively few people had enough moles on their back to measure, so more studies will be required to confirm these findings. If confirmed, assessing the clusteredness or dispersal of a person’s moles as part of a holistic risk score might help assess who is at risk of developing melanoma and where on the body clinicians should be especially vigilant during screening.