Halving of Australian children's naevus counts 1992-2016 and change in sun behaviour

Abstract

Background: In Australia, the lifetime risk of cutaneous melanoma is the highest in the world. The most important melanoma risk factor is the number of acquired cutaneous melanocytic nevi (AMN) on a person, the majority of these forming in adolescence. Childhood exposure to ultraviolet (UV) light is a strong determinant of nevus count.

Objective: Examine childhood AMN and its risk factors over a 25 year period in the high UV environment of South-East Queensland.

Methods: The Brisbane Twin Nevus Study recorded sun behaviours and counted nevi on annual new samples of twelve-year old twins (and siblings nearest in age) from 1992 to 2016. Participants were reexamined two years later, and a subset seen twenty years after the initial exam.

Results: Among 3957 participants (158 per year), we saw an approximate halving of nevus counts over the 25 year period, and examined multiple explanations for this trend. As this trend was seen for both large (>5mm) and smaller nevi, we inferred errors in counting were unlikely. There was an increase in the number of participants reporting non-European ancestry, but this explained only a small proportion of the change in nevus count.

Conclusions: We propose that, in Queensland through the 1990s and 2000s, children's sun exposure has been altered by changes in behaviour. Looking at studies counting nevi at in populations at different latitudes, we estimate the observed fall in nevus counts would be consistent with a 11.7% fall in average annual UV dose (clear sky erythemal spectrum 1503 kJ/m2 to 1327 kJ/m2). Based on published risk prediction equations, the fall in mean nevus number over time should lead to a four-fold drop in lifetime melanoma risk for those born after 2000 compared to those born in the 1980s.