Sun exposure and melanoma risk at different latitudes: a pooled analysis of 5700 cases and 7216 controls

Sun exposure and melanoma risk at differentlatitudes: a pooled analysis of 5700 casesand 7216 controlsYu-mei Chang,1* Jennifer H Barrett,1 D Timothy Bishop,1 Bruce K Armstrong,2 Veronique Bataille,3,4

Wilma Bergman,5 Marianne Berwick,6 Paige M Bracci,7 J Mark Elwood,8 Marc S Ernstoff,9

Richard P Gallagher,8 Adele C Green,10 Nelleke A Gruis,5 Elizabeth A Holly,7 Christian Ingvar,11

Peter A Kanetsky,12 Margaret R Karagas,13 Tim K Lee,8 Loıc Le Marchand,14 Rona M Mackie,15

Hakan Olsson,16 Anne Østerlind,17 Timothy R Rebbeck,12 Peter Sasieni,18 Victor Siskind,10

Anthony J Swerdlow,19 Linda Titus-Ernstoff,20 Michael S Zens13 and Julia A Newton-Bishop1

Accepted 25 February 2009

Background Melanoma risk is related to sun exposure; we have investigated riskvariation by tumour site and latitude.

Methods We performed a pooled analysis of 15 case–control studies (5700melanoma cases and 7216 controls), correlating patterns of sunexposure, sunburn and solar keratoses (three studies) with mela-noma risk. Pooled odds ratios (pORs) and 95% Bayesian confidenceintervals (CIs) were estimated using Bayesian unconditional poly-tomous logistic random-coefficients models.

Results Recreational sun exposure was a risk factor for melanoma on thetrunk (pOR¼ 1.7; 95% CI: 1.4–2.2) and limbs (pOR¼ 1.4; 95% CI:1.1–1.7), but not head and neck (pOR¼ 1.1; 95% CI: 0.8–1.4),across latitudes. Occupational sun exposure was associatedwith risk of melanoma on the head and neck at low latitudes(pOR¼ 1.7; 95% CI: 1.0–3.0). Total sun exposure was

* Corresponding author. Cancer Genetics Building, St James’sUniversity Hospital, Beckett Street, Leeds LS9 7TF, UK.E-mail: [email protected]

1 Section of Epidemiology and Biostatistics, Leeds Institute ofMolecular Medicine, Leeds, UK.

2 School of Public Health, The University of Sydney, Sydney,Australia.

3 Twin Research and Genetic Epidemiology Unit, St Thomas’Campus, Kings College London, London, UK.

4 Dermatology Department, West Herts NHS Trust, HemelHempstead General Hospital, Herts, UK.

5 Department of Dermatology, Leiden University MedicalCenter, Leiden, the Netherlands.

6 Department of Internal Medicine, University of New Mexico,Albuquerque, NM, USA.

7 Department of Epidemiology and Biostatistics, University ofCalifornia, San Francisco, San Francisco, CA, USA.

8 Cancer Control Research Program, British Columbia CancerAgency, Vancouver, BC, Canada.

9 Department of Medicine, Dartmouth-Hitchcock MedicalCenter, Lebanon, NH, USA.

10 Queensland Institute of Medical Research, PO Royal BrisbaneHospital, Brisbane, Australia.

11 Department of Surgery, University Hospital, Lund, Sweden.

12 Department of Biostatistics and Epidemiology and Center forClinical Epidemiology and Biostatistics, University ofPennsylvania, Philadelphia, PA, USA.

13 Department of Community and Family Medicine, DartmouthMedical School and the Norris Cotton Cancer Center,Lebanon, NH, USA.

14 Cancer Research Center of Hawaii, University of Hawaii,Honolulu, HI, USA.

15 Department of Public Health and Health Policy, Faculty ofMedicine, University of Glasgow, UK.

16 Department of Oncology, University Hospital, Lund, Sweden.17 Kobenhaausevej 25, DK 3400 Hillerød, Denmark.18 Wolfson Institute of Preventive Medicine, Barts & The

London School of Medicine, London, UK.19 Section of Epidemiology, Institute of Cancer Research, Sir

Richard Doll Building, Sutton, Surrey, UK.20 Department of Community and Family Medicine and

Department of Pediatrics, Dartmouth Medical School andthe Norris Cotton Cancer Center, Lebanon, NH, USA.

814

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Published by Oxford University Press on behalf of the International Epidemiological Association.

� The Author 2009; all rights reserved. Advance Access publication 8 April 2009

International Journal of Epidemiology 2009;38:814–830

doi:10.1093/ije/dyp166

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associated with increased risk of melanoma on the limbs at lowlatitudes (pOR¼ 1.5; 95% CI: 1.0–2.2), but not at other bodysites or other latitudes. The pORs for sunburn in childhood were1.5 (95% CI: 1.3–1.7), 1.5 (95% CI: 1.3–1.7) and 1.4 (95% CI:1.1–1.7) for melanoma on the trunk, limbs, and head and neck,respectively, showing little variation across latitudes. The presenceof head and neck solar keratoses was associated with increased riskof melanoma on the head and neck (pOR¼ 4.0; 95% CI: 1.7–9.1)and limbs (pOR¼ 4.0; 95% CI: 1.9–8.4).

Conclusion Melanoma risk at different body sites is associated with differentamounts and patterns of sun exposure. Recreational sun exposureand sunburn are strong predictors of melanoma at all latitudes,whereas measures of occupational and total sun exposure appearto predict melanoma predominately at low latitudes.

Keywords Melanoma, recreational sun exposure, occupational sun exposure,total sun exposure, sunburn, solar keratoses

IntroductionSun exposure has been identified in epidemiologicalstudies as the leading environmental cause of mela-noma, but the lack of a simple dose–response rela-tionship between total sun exposure and risk ofmelanoma has been perplexing. In general, studieshave reported a positive association for recreational(intermittent) sun exposure and an inverse associa-tion with occupational (more continuous) expo-sure.1–4 It has been noted that differing odds ratios(ORs) for melanoma resulting from the use of differ-ing statistical methods and adjustment for differentconfounding factors have made the pooling of studiesproblematic in meta-analysis.2,3 Moreover, studieshave been carried out at a range of different latitudeswhere people are exposed to very different levels ofsolar ultraviolet radiation. Detailed comparisonsbetween studies carried out at different latitudeshave the potential to shed light on these complicatedissues. The primary aim of this pooled analysis was toinvestigate the complex relationship between sunexposure and melanoma risk at different latitudes.An understanding of this relationship will be veryimportant for developing health promotion messagesfor different countries.

Recent studies have suggested that there are multi-ple pathways to melanoma5–8 involving differentbody sites, different pathology (naevus remnants,solar damage and genetic mutations) and differentsun exposure patterns (intermittent or more continu-ous). An analysis based on a large number of studyparticipants may help to further clarify this complexrelationship, and this was a secondary aim of theseanalyses. Elwood et al.9 suggested that the relation-ship between occupational sun exposure and mela-noma might be non-linear with some beneficialeffect related to long continued exposure. A further

aim of this pooled analysis was to consider melanomarisk by tumour site and by latitude to investigate thishypothesis.

MethodsSummary of studies included in the pooledanalysesInformation regarding study location, time period,number of participating cases and controls, and detaileddefinitions of collected sun-exposure risk factors weretabulated for each of the 15 studies included in theseanalyses9–23 (Table 1). Wide inclusion criteria wereadopted to allow more data in the pooled analyses.Selection procedures were described in a previouspooled analysis paper on naevus phenotype.24 Thepooled data set consisted of seven studies fromEurope, five studies from North America, one studyfrom Hawaii and two studies from Australia. Therewere a total of 5700 melanoma cases and 7216 controls.

Categorization of sun-exposure dataacross studiesInformation collected on recreational sun exposurevaried greatly between studies. Several studies hadcollected detailed information on all leisure-relatedsun-exposure activities during vacations and week-ends, whereas others recorded average non-workinghours spent outdoors during the summer or at week-ends. Many European studies had only asked specificquestions on sunbathing and/or activities wearingswimsuits during vacations (abroad and/or in thecountry of residence). We derived two measures:first, total hours (or weeks or years) of recreationalsun exposure were calculated as the sum of allreported outdoor recreational activities weighted

SUN EXPOSURE AND MELANOMA RISK AT DIFFERENT LATITUDES 815



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by frequency and duration; secondly, total hours (orweeks or years) of all reported sunbathing activitiesand/or activities wearing swimsuits were calculated,weighted by their frequency and duration. A recrea-tional exposure index was used for the Connecticutstudy as described in the original article.18

Many studies have collected comprehensive calendardata on type of job, duration and clothing worn for alloutdoor employment. Some studies have recordedonly average hours spent outdoors on weekdays. Foroccupational sun exposure analyses, outdoor weekdayhours (weeks or years) were summed across all jobsand weighted by duration. Information collected onrecreational and occupational sun exposure was inthe same age periods and measured in the sametime-units in seven studies, so for these studies totalsun exposure was estimated by summing across allrecreational and occupational exposures.

Recreational, occupational and total sun-exposuredata each were classified into sex- and study-specificquarters based on the distribution in the controlpopulation. If 425% of the controls had not engagedin the relevant sun-exposure activities (occupational,recreational or total), then controls in this ‘non-exposed’ group formed the lowest exposure level,and the remaining controls were equally dividedinto three groups representing thirds of controlswith non-zero exposure of that type. The quantilecut-off points obtained from the controls were thenapplied to melanoma cases of the same sex in thesame study. The resulting four study-specific sunexposure groups represent low, intermediate–low,intermediate–high and high sun-exposure categories.For studies using pre-defined categories, controlswere classified into four groups distributed as evenlyas possible, and these groupings were also applied tothe corresponding cases.

Many studies had collected sunburn information inseveral age periods, and pooled analyses on sunburnsin childhood and in adulthood were conducted.Childhood was defined as younger than 15 years ofage, and adulthood was defined as aged 20 years andolder. Evaluation of the risk of sunburn was based onpresence or absence of any reported painful sunburnexperience within the considered age intervals.

Solar keratoses (SK) are believed to be causedby excessive long-term exposure to sunlight in fair-skinned people. They are therefore markers of highepidermal sun exposure given a certain level of sus-ceptibility to sun-induced skin damage. Three stud-ies15,16,22 had recorded the presence of SK on theface and neck by examination, which was coded aspresence or absence in the pooled analysis.

The amount of ultraviolet (UV) in ambient sunlightvaries greatly by latitude. Individual-level latitudedata were not available and some studies coveredregions with a large latitude range. Average latitudewas thus used as an approximate index of the latituderange of a study, and studies were classified into

three categories. Those carried out at average latitudesabove 458 north were classed as ‘high latitude’,between 458 and 358 north as ‘middle latitude’ andbetween 348 north/south and 208 north/south as the‘low latitude’.

Melanoma tumour-site information was availablefor all except the New Hampshire study.17 Most stu-dies included cases of superficial spreading melanoma(SSM), nodular melanoma (NM) and lentigo malignamelanoma (LMM). Seven studies also included acrallentiginous melanoma (AM, with a total of 61 cases)(see Table 1). LMM and AM were excluded in theEast Midlands, UK, and the Western Canadastudies.9,14

Statistical methodsBayesian unconditional polytomous logistic randomcoefficient regression models were employed tostudy the overall effects of sun exposure on the riskof developing melanoma. Analyses were conducted toevaluate the risk of melanoma anywhere on the bodyand melanoma occurring specifically on the trunk,limbs, and head and neck. Heterogeneity wasaccounted for by allowing the effects to vary betweenstudies in a structured manner, and the variance ofeach effect was estimated using random coefficientsmodels. The estimated variances of the study-specificlog ORs quantified the degree of heterogeneity in rela-tive risk estimates among studies. For each risk factorits ‘relative heterogeneity’ was measured by the ratioof the estimated among-study standard deviation tothe range of log OR estimates.25 This measure allowscomparisons between the degrees of heterogeneityassociated with different risk factors.

Three dummy variables were created for allparticipants to define the sun-exposure groups (inter-mediate–low, intermediate–high, high vs low cate-gory) in the analysis. If only three originally pre-defined categories (low, middle, high) were availablein a study, then the middle class was treated as theaverage of the intermediate–low and intermediate–high categories. Pooled ORs (pORs) adjusted onlyfor age and sex (referred to as pOR1 in the tables/figures) were reported, as well as pORs adjusted forage, sex, hair colour, ability to tan (or propensity toburn if ability to tan was not recorded) and freckling(if available) (referred to as pOR2 in the tables/figures).

To examine the potential influence of latitude on theeffects of sun exposure [without assuming a linearrelationship between log(OR) and latitude], weallowed the pORs to vary among the three pre-definedgeographical regions. However, the among-study var-iance was assumed to be the same across all regions.

The Markov chain Monte Carlo (MCMC) methodwas used to estimate the risk of melanoma in relationto sun exposure using WinBUGS software.26

Flat priors with low precisions were assigned for allparameters. Detailed specifications of the models and




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prior distributions were described in the supplemen-tary data of our previous report.24

ResultsAge distribution, histological subtypeand site of melanomaThe age distribution of cases corresponded reasonablywell with that of controls except for cases with headand neck melanoma who were much older than othercases and controls (P < 0.0001 for both comparisons).

Of the 3035 confirmed SSM cases with tumour siteinformation, 1180 (38.9%), 1599 (52.7%) and 256(8.4%) had melanoma on the trunk, limbs and headand neck, respectively. Within the 662 NM cases, theproportions were 222 (33.5%), 343 (51.8%) and 97(14.7%), and within the 445 LMM cases, the propor-tions were 64 (14.4%), 107 (24.0%) and 274 (61.6%).The site distribution of LMM cases was significantlydifferent from those of SSM and NM (P < 0.0001 forboth comparisons).

Recreational sun exposureThirteen studies recorded sunbathing and activitieswearing a swimsuit, and nine studies recorded alloutdoor recreational activities. In the recreationalsun-exposure analyses, 5567 melanoma cases and7033 controls were included (Table 2). Sunbathingdata were aggregated across latitude regions, sincemany studies recorded information on sunbathingduring holiday abroad. The among-study variancefor the effects of sunbathing activities was 0.032,and the relative heterogeneity was 0.07. Comparedwith the relative heterogeneity of other measures(see below), the slightly higher relative heterogeneityfor sunbathing could be due to the different periodsof life for which sunbathing data were collected indifferent studies and to combining sunbathing withother activities requiring a swimsuit. Forest plotsof study- and tumour-site-specific fully adjustedORs and pORs for the highest category of sunbathingexposure compared with the lowest group are shownin Figure 1. The pORs for melanoma on the trunk andlimbs were significantly41 for the three highest cate-gories of sunbathing exposure compared with lowestexposure and were little changed by adjustment formeasures of susceptibility such as ability to tan andfreckling (Table 2).

The among-study variance for effects of outdoorrecreational sun exposure was 0.007, and the relativeheterogeneity was 0.03. Similar to the finding forsunbathing activities, the fully adjusted pORs forthe highest category of all recreational sun exposurecompared with low exposure were significantly41 forboth trunk and limb melanoma (Table 2). The pOR2sfor the highest recreational sun exposure category com-pared with the lowest group were 1.7 (95% CI: 1.4–2.2),1.4 (95% CI: 1.1–1.7) and 1.1 (95% CI: 0.8–1.4) for

trunk, limb and head and neck melanoma, respectively,across all latitude regions. There was no evidence of asystematic relationship of risk to latitude (Figure 1). Inaddition, total recreational sun exposure correlated rea-sonably well with sunbathing frequency. Spearman cor-relations coefficients were 0.34 and 0.81 in the high–middle and low latitudes, respectively. Omission ofthe two high-latitude studies that excluded LMM hadlittle impact on the high latitude pOR2s (data notshown).

Occupational sun exposureIn the occupational sun-exposure analyses, 5578melanoma cases and 7024 healthy controls from 15studies were included (Table 3). The majority of theparticipants living in the high latitudes did not reportany outdoor work exposure (58%). Similarly,450% ofthe participants from the middle latitudes were in thelow and intermediate–low categories. The distributionof occupational sun exposure among those living atlow latitudes was more evenly distributed betweencategories (Table 3).

The among-study variation for effects of occupa-tional sun exposure was 0.023, and the relative het-erogeneity was 0.05. Forest plots of study-specificoccupational sun-exposure risks for the highestversus lowest category are shown in Figure 2. Thefully adjusted pORs for the highest category ofoccupational sun exposure compared with the lowestgroup across all latitude regions were 1.0 (95% CI:0.8–1.2), 0.9 (95% CI: 0.8–1.1) and 1.2 (95% CI:0.9–1.6) for melanoma on the trunk, limbs andhead and neck, respectively. Increased occupationalsun exposure was not associated with melanomarisk on the trunk and limbs regardless of latitude(Table 3). There was evidence of increased riskfor occupational sun exposure for melanoma on thehead and neck at low latitudes, with a pOR2 of1.7 (95% CI: 1.0–2.0) for the highest occupationalsun exposure category compared with the lowest.Omission of the two high-latitude studies thatexcluded LMM had little impact on the adjustedpOR2s (data not shown). Similar pORs were observedwhen the models for occupational sun exposure wereadjusted for recreational sun exposure, and vice versa(results not shown).

Fair-skinned people were less likely to work out-doors, especially at low latitudes (P < 0.0001 com-pared with darker skinned people). For controlparticipants at low latitudes who worked at least 4 hper day outdoors during any period of their life, 17%had skin type I/II compared with 29% of indoor work-ers. In the two studies that recorded detailed outdooroccupation in the middle latitudes, 13% of controlswho pursued outdoor work had skin type I/II com-pared with 20% who mainly worked indoors(P¼ 0.015). There was little difference in distributionof skin types between control participants whoworked outdoors and indoors at high latitudes.




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Total sun exposureSeven studies recorded total outdoor sun exposure(sum of occupational and recreational sun exposure)(Table 4). The among-study variation for effects oftotal sun exposure was 0.012, and the relative hetero-geneity was 0.05. Overall, increased total sun expo-sure was not associated with melanoma risk atany site in the high latitudes or with melanoma onthe trunk and head and neck at any latitudes(Table 4). It was, however, associated with melanomaon the limbs and, more weakly, the head and neck,at low latitudes: the fully adjusted pORs were1.5 (95% CI: 1.0–2.2) and 1.3 (95% CI: 0.8–2.2). Thelack of LMM in two of the three studies9,14 in thehigh latitudes could affect the estimated associationbetween total sun exposure and head and neckmelanoma. The forest plots of study-specific adjustedORs for total sun exposure are shown in Figure 3.

Solar keratosesOf the 1035 controls from three studies15,16,22 thatrecorded SK on the face and neck, 159 (15.4%)

participants had at least one SK present; the corre-sponding percentages for cases with melanoma on thetrunk, limbs and head and neck were 14.8, 23.2 and39.0%, respectively (Table 4). Within those 159 con-trols who had at least one SK, only 15 of them were<50 years old. The among-study variance was 0.12with a relative heterogeneity of 0.20, whichwas much larger than for other measures of sunexposure, probably because of the smaller number ofstudies that were included and possibly because ofdifferences in phenotyping between studies. ThepORs for melanoma with at least one SK adjustedfor age and sex only were 1.9 (95% CI: 0.9–4.1), 4.0(95% CI: 1.9–8.4) and 4.0 (95% CI: 1.7–9.1), respec-tively, for melanoma on the trunk, limbs and headand neck. For all sites together, the pOR was 3.2(95% CI: 1.0–8.2). The pORs were slightly attenuatedafter additional adjustment for hair colour, ability totan and freckling.

Cases with SK had similar recreational sun exposurecompared with other cases (P¼ 0.16), but controlswith SK had higher recreational sun exposure than

Figure 1 Forest plots of the association between (A–C) the highest sun-bathing exposure and (D–F) the highest totalrecreational sun exposure and melanoma risk. Each line represents results from an individual study, with the length of thehorizontal line indicating the 95% CIs, and the square box indicating the study-specific adjusted OR (OR2) for the ‘High vsLow’ recreational sun-exposure category. Adjusted pOR2s and 95% CIs are represented by grey diamonds




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other controls (P¼ 0.04). Participants with SK hadhigher occupational sun exposure in both cases andcontrols (P < 0.0001 in both groups).

SunburnFourteen studies had collected data on sunburn inchildhood (<15 years of age), and 13 studies hadinformation on sunburn in adulthood (420 years ofage) (Table 5). The among-study variances were 0.003and 0.005 (relative heterogeneities were 0.03 and0.04) for effects of childhood and adult sunburn,respectively, suggesting negligible heterogeneity.Forest plots of study-specific ORs adjusted for ageand sex only for childhood and adult sunburn areshown in Figure 4.

Sunburn before the age of 15 was a consistentlysignificant risk factor for all three latitude regions.There was no evidence of a systematic relationshipof risk to latitude. The pORs for melanoma in relationto any sunburn before the age of 15 across three lati-tude regions, adjusted for age and sex only (pOR1s)were 1.5 (95% CI: 1.3–1.7), 1.5 (95% CI: 1.3–1.7) and1.4 (95% CI: 1.1–1.7) for melanoma on the trunk,limbs, and head and neck, respectively. Excludingthe two high latitude studies that left out LMMhad little effect on the pORs for childhood sunburn(data not shown).

Sunburn after the age of 20 years was significantlyassociated with melanoma on the trunk and limbs,but less strongly than was childhood sunburn

(Table 5 and Figure 4); the pOR1s across latituderegions were 1.4 (95% CI: 1.2–1.6), 1.2 (95% CI:1.1–1.4) and 1.1 (95% CI: 0.9–1.3) for melanoma onthe trunk, limbs and head and neck, respectively.Sunburn after the age of 20 was also less stronglyassociated with melanoma in the middle and low lati-tudes than in the high latitudes. The pOR1 for sun-burn after the age of 20 in the high latitudes changedlittle when the two studies that did not include LMMwere left out (data not shown).

There was a strong correlation between sunburnbefore age 15 and sunburn after age 20 (P < 0.0001),and the association of sunburn after age 20 with mela-noma diminished if sunburn before age 15 wasincluded in the models. However, sunburn after age20 remained significantly associated with melanomaon the trunk in the high and middle latitudes afteradjusting for childhood sunburn: the adjusted pORswere both 1.3 (95% CI: 1.0–1.7). Furthermore, sunburnas an adult remained significantly associated withmelanoma on the limbs in the high latitudesafter adjusting for childhood sunburn with a pOR of1.3 (95% CI: 1.0–1.6).

The pORs for sunburn before age 15 and sunburnafter age 20 adjusted for age, sex, hair colour, abilityto tan and freckling were slightly lower than theircorresponding pORs adjusted for age and sex only.

There were strong correlations between sunburnat any age and more frequent sunbathing for bothcases and controls (P < 0.0001 in both groups).

Figure 2 Forest plots of the association between the highest occupational sun exposure and melanoma risk. Each linerepresents results from an individual study, with the length of the horizontal line indicating the 95% CIs, and thesquare box indicating the study-specific adjusted OR (OR2) for the ‘High vs Low’ occupational sun-exposure category.Adjusted pOR2s and 95% CIs are represented by grey diamonds




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However, there was no association between sunburnand total recreational sun exposure. There was also asignificant inverse association between sunburn afterage 20 and occupational sun exposure in the controlparticipants (P < 0.0001), which echoed the associa-tion of skin type and outdoor occupation.

DiscussionWe used original data from 15 case–control studiesacross a range of latitudes to examine the relationshipof risk of melanoma to recreational (9 studies), occu-pational sun exposure, total sun exposure (7 studies),reported sunburn and the presence of SK on the face(3 studies). Our analysis has the limitations inherentin all pooled analyses of studies done by differentinvestigators according to different protocols. Theaccurate quantification of sun exposure is a difficulttask at any time and is made more difficult in thisanalysis by the use of different questionnaires and byprobable differences in the sun-exposure habits in thepopulations studied. The analysis also carried with itthe inevitable weakness of retrospective studies, recallerror. If differential between cases and controls,which would be plausible for reported sun-exposureif melanoma diagnosis affects recall, recall error,would have a largely unpredictable effect on estimatesof association. If non-differential, it would lead toweakening of associations and thus was probablyan important contributor to the weakness in theassociations observed. In addition, our use of thequantile method to categorize exposure for eachstudy could reduce our ability to detect an associationif melanoma risks were higher only above somethreshold sun-exposure level and only a few studieshad a substantial number of participants exposedabove this threshold. That a relatively objective

measure of high total sun exposure of usually exposedskin, the presence of SK on the face, was the factormost strongly associated with melanoma, even afteradjustment for measured phenotypic factors, may givesome indication of the impact of measurement erroron our other risk estimates: phenotype-adjusted pORof melanoma in the presence of SK was more thandouble that for the highest vs lowest level of recalledtotal sun exposure.

In spite of the probable bias towards the nullinduced by non-differential measurement error, thestatistical power of this pooled analysis is sufficientto suggest some important patterns. First, highsunbathing and total recreational sun exposureincrease risk of melanoma of the trunk and limbsbut not melanoma of the head and neck (Figure 1and Table 2). The relative risk associated with theseintermittent pattern sun exposures appears largelyuninfluenced by latitude of residence and, by infer-ence, ambient UVB radiation. Secondly, occupationalsun exposure appeared neither to increase nordecrease risk of melanoma on the trunk and limbs,but may increase risk of melanoma on the headand neck especially at low latitudes (Figure 2 andTable 3). Thirdly, high total sun exposure, as inferredfrom SK on the face, increases risk of melanomason the limbs and head and neck but increases riskof melanoma of the trunk less, if at all (Figure 3and Table 4). The results from recalled total sun expo-sure suggest that its effect may be more evident inlow than in high latitudes. Fourthly, reasonably con-sistent with the pattern for sunbathing and recrea-tional sun exposure, sunburn in childhood increasesthe risk of melanoma at all body sites but increasesrisk on the trunk and limbs more than it does onthe head and neck, and these patterns are largelyconsistent across latitudes (Figure 4 and Table 5).

Figure 3 Forest plots of the association between total sun exposure and melanoma risk by tumour sites. Eachline represents results from an individual study, with the length of the horizontal line indicating the 95% CIs, andthe square box indicating the study-specific adjusted OR (OR2). Adjusted pOR2 and 95% CIs are represented by greydiamonds




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Sunburn in adulthood shows a similar pattern butwith little evidence of an increase in risk of melanomaof the head and neck.

The pooled analyses were consistent in identifyingsunbathing, recreational sun exposure and reportedsunburn as being important risk factors for melanomaon body sites that are not usually exposed to the sun.

Although the relative risks for these exposures weresimilar across latitude bands, the baseline risk in theleast-exposed categories, and hence the absolute risk,would be expected to increase with increasing ambi-ent UV. Sunburn history has been considered asan important risk factor for melanoma, consistentwith the view that host factors including sensitivity

Figure 4 Forest plots of the association between (A–C) ever sunburn before age 15 and (D–F) ever sunburn afterage 20 and melanoma risk by tumour sites. Each line represents results from an individual study, with the length of thehorizontal line indicating the 95% CIs, and the square box indicating the study-specific OR (OR1). pOR1s and 95% CIsare represented by grey diamonds




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to excessive sun exposure are important in melanoma.Our pooled analysis confirmed this associationfor tumours on the trunk and limbs, and less conclu-sively, for the head and neck. Our pORs for everhaving had sunburn as a child and as an adult weresimilar to results adjusted for publication biasreported in the meta-analysis by Gandini et al.3

Whiteman and Green27 reviewed 16 case–control stu-dies regarding sunburn history and estimated a 2-foldincreased risk of melanoma in those ever sunburned,with a 3.7-fold increase risk among those in the high-est category of sunburn exposure, compared withthose never sunburned.

Meta-analyses of published data have shown aninverse association between occupational sun expo-sure and risk of melanoma overall.1–4 We did notfind a significant inverse association in this pooledanalysis. In addition, there was some evidence thatoccupational sun exposure increased risk, but onlyfor melanoma of the head and neck at low latitudes(Figure 2). It is possible that participants living in thehigh- or middle-latitude regions were not exposed tosufficiently high ambient UV radiation to increasemelanoma risk, even when they were in the highestoccupational sun-exposure category. Self-selectionagainst outdoor work by fair-skinned people livingat low latitudes, which we have demonstrated, couldalso lower the estimates of melanoma risk in thosewho had high occupational exposure.

Whilst the ‘two pathways to melanoma’ hypothesissuggests that more continuous sun exposure is morerelevant for melanoma on the head and neck andintermittent exposure to melanoma on the trunkand limbs, we found only a weak relationship bet-ween occupational sun exposure and risk of mela-noma on the head and neck, particularly intemperate climates. However, increased total sunexposure was associated with melanoma on thelimbs at low latitudes, which is probably due to thefact that distal parts of the limbs are usually exposedto the sun in many people, particularly in areas ofhigh ambient UV.

The presence of SK has been reported as a riskfactor for melanoma in a few studies.28,29 Green andO’Rourke28 reported an OR of 2.8 for SK on the face,and in a joint UK and Australia study, the presence of10 or more SK compared with none on the left fore-arm was associated with an OR of 4.7.29 In our pooleddata analysis we found that SK are a risk factor formelanoma overall and particularly for primaries inusually sun-exposed sites, although the analysis islimited by the fact that only three studies includedSK as a measure. Our estimated pORs for presenceof any SK compared with none for melanoma onthe head and neck or limbs are similar to those ofother studies. We have found no indication of anassociation between presence of SK and melanomaon the trunk.

SK are postulated to be caused by damage to theskin by solar UV radiation over a long period oftime. In addition, they probably reflect inherent sus-ceptibility to sun-caused skin damage and individualDNA repair deficiency.30,31 Our analyses suggestedthat their presence was associated both with recrea-tional and occupational sun exposure, but morestrongly and consistently with the latter.

In conclusion, these pooled data analyses suggestmelanoma risk at different body sites is associatedwith different amounts and patterns of sun exposure.Recreational sun exposure and sunburns are strongpredictors of melanoma on less frequently sun-exposed body sites, at all latitudes. It is known thatintermittent sun exposure is associated with DNAdamage and induced immunosuppression,32–35 and itseems likely that this more acute sun-induceddamage is relevant to melanoma at all latitudes. Inaddition, more continuous sun exposure is importantwhen exposure level is high, as occupational and totalsun exposure at low latitudes and SK across latitudesshowed a relationship to melanoma on more fre-quently sun-exposed body sites. These observationsare consistent with the ‘two pathways to melanoma’hypothesis recently explored.5–8,36,37

FundingEuropean Commission, 6th Framework Programme(LSHC-CT-2006-018702); Cancer Research UK (C588/A4994, C569/A5030); National Cancer Institute(RO1-CA52345 to E.A.H., P0-1 CA42101 to M.B.,RO1-CA66032 to L.T.); National Institutes of Health(R01-CA92428 to P.A.K.); University of SydneyMedical Foundation Program Grant (to B.A.).

AcknowledgementsThe authors thank the funders of the contributingstudies, who are acknowledged in the original studypublications listed in the references to this paper, andother investigators for those studies, who are authorsof the original study publication. Dr J.N. BouwesBavinck is thanked for putting the melanoma data-base together for Leiden University Medical Center,the Netherlands. Lund Melanoma Study Group isthanked for compiling the Swedish data. Mr JohnTaylor is thanked for recoding the New Hampshirestudy for the pooled analysis. We thank also DrM.R.K. who provided original data from the EastDenmark, Scotland, East Midlands, San Francisco,Queensland and Western Australian studies, whichshe had compiled for pooled analysis of othervariables.

Conflict of interest: None declared.




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KEY MESSAGES

� Melanoma risk at different body sites is associated with different patterns of sun exposure.

� Recreational sun exposure and sunburn are strong predictors of melanoma on less frequently sun-exposed body sites, at all latitudes.

� More continuous sun exposure is important when exposure level is high, as occupational and total sunexposure at low latitudes and SK across latitudes showed a relationship to melanoma on more fre-quently sun-exposed body sites.




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Sun exposure and melanoma risk at different latitudes: a pooled analysis of 5700 cases and 7216 controls

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