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

The lifetime risk of cutaneous melanoma in Australia is the highest in the world. The most important melanoma risk factor is the number of acquired cutaneous melanocytic naevi (AMN) on a person, the majority of these forming in adolescence. Childhood exposure to ultraviolet radiation (UVR) is a strong determinant of naevus count.

To examine childhood AMN and its risk factors over a 25-year period in the high-UVR environment of South-East Queensland.

The Brisbane Twin Nevus Study recorded sun behaviours and counted naevi on annual new samples of 12-year-old twins (and siblings nearest in age) from 1992 to 2016. Participants were re-examined 2 years later, and a subset was seen 20 years after the initial exam.

Among 3957 participants (158 per year), we saw an approximate halving of naevus counts over the 25-year period, and examined multiple explanations for this trend. As this trend was seen for both large (> 5 mm) and smaller naevi, 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 naevus count.

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 naevi in populations at different latitudes, we estimate the observed fall in naevus counts would be consistent with a 11.7% fall in average annual UVR dose (clear sky erythemal spectrum, from 1503 kJ m-2 to 1327 kJ m-2). Based on published risk prediction equations, the fall in mean naevus number over time should lead to a fourfold drop in lifetime melanoma risk for those born after 2000 compared with those born in the 1980s.