Editorials

Sunburn and melanoma: how strong is the evidence?

Preventing disease relies on knowing both its cause and practical and cheap ways of either avoiding the cause or blocking its effect. Most people attribute the rising incidence of melanoma to social and behavioural changes this century that have resulted in increased exposure to sunlight.⁴ The fashion for a suntan, associated with increasing public acceptance of decreasing proportions of the body being covered while outdoors, has led to large numbers of people being exposed to sufficient sunlight to cause sunburn during leisure hours.⁵

Exposures sufficient to cause sunburn, particularly in childhood, have been targeted by most educational programmes as the main changeable component in the process that leads to melanoma in people who are constitutionally predisposed to develop it.⁶ But just how strong is the evidence that exposure that is sufficient to cause sunburn is a risk factor for melanoma?

The epidemiological evidence linking sunburn with melanoma is complex. During the past decade more than a dozen studies have compared the histories of sunburn of patients with melanoma and controls. Because of considerable variations in design and method and differences in the definition of sunburn it is difficult to quantify a single estimate of risk from all these studies. Moreover, the problems of recall bias and misclassification of exposure inherent in retrospective studies demand cautious interpretation of the results.

Six large, population based, case-control studies conducted in Australia,^7,8 Europe,^9,10 and North America^11,12 provide the most complete and comparable data on sunburn and melanoma. Each study had a well defined base and appropriate controls and ascertained only incident cases of melanoma. In addition, the similarity of definition of sunburn provides a reasonable basis for comparison.

The unadjusted relative risk of melanoma for people reporting frequent sunburns throughout life was increased two to threefold in five of the six studies (no crude data were available for the Western Australian study⁸). Adjusted relative risks were less consistent across the studies, but these depended on the choice of confounding variables used in the final analysis. Skin type (that is, the susceptibility of the skin to burning) was included as a confounder in three studies, which substantially reduced the strength of the association. Green et al⁷ and Weinstock et al,¹² however, asserted that sunburn is a biological marker of high dose ultraviolet radiation penetrating to the melanocytes at the base of the epidermis, regardless of the degree of pigmentation in the epidermis. If this argument is pursued it follows that skin type is a determinant of sunburn and therefore should not confound the observed relation between melanoma and sunburn. The studies by Green et al,⁷ Weinstock et al,¹² and Osterlind et al⁹ each retained a clear association between sunburn and melanoma after adjustment for factors other than skin type.

These studies therefore suggest an association between sunburn and melanoma, but the increase in risk is modest. What cannot be determined from these studies is whether the recall of sunburn is merely a marker of other substantial, but unremembered, exposures to sunlight. Thus the association may reflect heavy exposure in general that did not necessarily result in sunburn. Also the possibility of greater reporting of sunburn by people with melanoma compared with controls cannot be ruled out.

Another important question is whether the risk related to sunburn is constant throughout life or whether a critical period exists during which exposure is more harmful. In Italy Zanetti et al reported a fivefold increase in risk associated with sunburn in childhood after controlling for episodes of sunburn later in life.¹⁰ Significantly increased risks associated with sunburn in childhood have also been reported in the United States, England, and Denmark.^9,12,13 On the other hand, a Scottish study suggested that sunburn in adulthood may also be a contributing factor.¹⁴ (This study did not consider childhood exposure in its analysis.)

Once again, it cannot be determined from these data whether the explanation is a specific and critical age related sensitivity to sunlight in childhood or a difference in dose. Recent memory of sunburn in adults is likely to be more accurate than past memory of sunburn in childhood. Past memories may merely reflect a global recall of considerable exposure in childhood; children have more leisure time than adults and are more likely than adults to have their clothes off during that time.

What do these data mean for the programmes that are being developed and delivered at present? They offer some support for the view that we should recommend reduced exposure to sunlight sufficient to cause sunburn, particularly in childhood. Migration studies showing a substantially reduced risk of melanoma in people who were protected from strong sunlight in childhood also support special precautions in childhood.¹⁵ Whether preventing only sunburn severe enough to be remembered many years later will reduce appreciably the incidence of melanoma is debatable. Nevertheless, in most European countries and the northern United States there seems to be a case for targeting programmes at behaviours that lead to episodic exposure to strong sunlight, particularly on such occasions as holidays in the Mediterranean, Caribbean, or Asian-Pacific regions.

R Marks, D Whiteman 

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