This is the first published study of sunbed use with risk of early-onset melanoma. Our findings indicate that sunbed use is a risk factor for early-onset melanoma, with evidence of a dose-response relation between the number of lifetime sessions and melanoma risk. Compared with participants who had never used a sunbed, the risk of melanoma was 41% higher for those who had ever used a sunbed, and was approximately doubled for those who reported more than 10 lifetime sessions. We also found that the risk of melanoma was greater with earlier age at first use of sunbeds and for earlier onset disease. Those who reported more than 10 lifetime sessions appeared some six times more likely to be diagnosed with melanoma before 30 years of age compared with never-users, but the same exposure was associated with less than two-fold increased risk of melanoma diagnosed after 30 years of age.
The weaker association of sunbeds with later age at onset of melanoma suggests addition of risk from sunbed use to accumulating risk from sun exposure, with the relative effect of sunbeds becoming smaller when superimposed on increasing, accumulated sun exposure. This would indicate that sun exposure and sunbed use increase melanoma risk through the same mechanism. This additivity of risk with sun exposure is supported by our observation that sunbed use was associated with an increased risk of melanoma for those with lower but not higher levels of lifetime total sun exposure, although this difference was only evident with more than 10 lifetime sunbed sessions. We observed a similar pattern when stratified by ambient UV irradiance, although the interaction was not statistically significant.
Another explanation for the age at diagnosis interaction and the increasing risk associated with earlier age at first use could be that younger people might be more biologically susceptible to the carcinogenic effects of artificial UV radiation. This rationale is consistent with the hypothesis that UV radiation exposures during childhood are important contributors to the initiation and promotion of biological events involved in the development of adult melanoma.20-24
Similarly, risks of ionizing radiation-related cancers have been shown to be greatest for those exposed early in life.25,26
However, a recent study suggested that early age at first use of a sunbed is most likely a marker for cumulative sunbed exposure, and not an indication of increased susceptibility for younger people.27
In our study, mutual adjustment of these variables did indicate that cumulative sunbed exposure is probably more important than age at first use, although cumulative exposure did not fully account for the association of earlier age at first use with melanoma risk. Moreover, age at first use was not related to the number of lifetime sessions. The difference in risk by age at diagnosis was also unlikely to be explained by differences in sunbed exposure between these two age-groups because the number of sunbed sessions and age at first use were similar for both age-groups at diagnosis.
Our findings are broadly consistent with a systematic review1
that reported relative risks for melanoma diagnosed at any age of 1.15 (95% CI 1.00-1.31) for ever-use of a sunbed and 1.75 (95% CI 1.35-2.26) when first use of sunbeds was before 35 years of age. However, in contrast to our study, the review found no evidence of a dose-response relation. A dose-response relation was found in some,27-29
but not all,30,31
more recent studies. A recent prospective analysis of a Norwegian-Swedish cohort of women reported a relative risk of melanoma of 1.31 (95% CI 1.03-1.66) associated with ever-use of sunbeds at ages 10 to 39 years, and a clear positive trend with cumulative sunbed use; however, they did not observe a higher relative risk associated with an earlier age at first use.29
Previous studies that have performed subgroup analyses to examine the association of sunbeds with melanoma diagnosed at different ages have been limited by small numbers of younger cases, but similar to our findings, most have observed stronger associations for those diagnosed at younger ages (defined as younger than age 30 years,32
or 45 years28,34
). However, two recent studies27,31
found no indication of heterogeneity by age-group.
We found that sunbed use was more associated with melanoma on the trunk than on the limbs or head and neck. Although these differences could easily have been due to chance, other studies have also found that melanoma risk at different body sites reflects different patterns of sun exposure; in particular that more continuous exposures are associated with melanomas on the head and neck and more intermittent exposures, such as sunbed use, with melanomas on the trunk and limbs.32,35-37
Our findings that sunbed use was more prevalent among females and those with good tanning ability are similar to previous studies.4
Our data suggest that individuals are more likely to use sunbeds if they lived in cooler climates and had high levels of sun exposure during summer holidays, which is consistent with a large survey in Germany that found tanning and preparation for sunny holidays were the main reasons for sunbed use.38
Similarly, studies in the US have observed lower prevalence of sunbed use in warmer regions.3,5
Several studies have found sunbed use to be positively correlated with sun-seeking behaviours,3,30,33
although associations with sun exposure were not always consistent.4,31
Similar to previous studies,1,29-31
we found that adjustment for different measures of sun exposure or skin type had minimal effect on the association of sunbeds with melanoma.
Strengths of this study include the multi-centre, population-based design and a large sample of young cases and controls. A young study population is likely to have better recall of sunbed use and early-life sun exposure, which appears to be of particular etiological importance to melanoma. Our cases and controls were of similar socioeconomic status, cases’ melanoma characteristics were representative of the population, and demographic characteristics of controls were generally similar to the population.14
Our study also has several limitations. We cannot exclude the possibility of recall bias in our study, as the accuracy of reporting past sunbed exposures might differ between cases and controls. However, previous studies of sunbeds and melanoma have found minimal evidence of recall bias27,39
and good reliability for cases and controls when reporting sunbed use.40,41
Selection bias might also be a problem due to poor participation by cases and controls, if participation of cases and controls was influenced by their past or current sunbed use. However, collection of data on sunbed use was part of a larger study examining genetic and environmental risk factors for melanoma and thus sunbed use might be less likely to have influenced participation. Data collection was also completed well before local news media coverage of cases of melanoma in young people who had used sunbeds.42
We did not collect information on the specific type of indoor tanning device that was used, nor were we able to collect data on the duration of each sunbed session. Before 1980, indoor tanning devices emitted greater amounts of UVB radiation, whereas devices after 1980 emitted mainly UVA, however, there is little evidence that the association between artificial UV exposure and melanoma differs by the type of indoor tanning appliance used.1,27
In our young study population, it is likely that most participants used more modern UVA-emitting devices. Due to the high UV intensity of indoor tanning devices, it is estimated that the annual UVA doses received by frequent indoor tanners may be 1.2–4.7 times those received from the sun, in addition to sun exposure.1
Our findings indicate that UV radiation exposure from sunbeds is a risk factor for early-onset melanoma, particularly melanoma diagnosed between ages 18 and 29 years. The increasing risk associated with an earlier age at first use adds further support to efforts to restrict minors and discourage young adults from using sunbeds.1,43-45
Sunbed use is a completely avoidable but increasingly prevalent risk behaviour2,4
and, on the evidence of this study, causes some 76% of melanomas occurring in people 18-29 years of age who have ever used a sunbed. At the population level, we estimate that 16% of melanoma cases aged 18-29 years at diagnosis and 3% of melanoma cases aged 30-39 years at diagnosis would be prevented in Australia by avoiding sunbed exposure.18,19
Since Australia is reported to have the lowest frequency of sunbed use of developed countries4
and perhaps the highest sun exposure, the proportion of all cases of early-onset melanoma attributable to sunbed use is almost certainly higher in other developed countries.