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Giver sol modermærkekræft?

Sol giver ikke modermærkekræft, mener professor emeritus, dermatolog og forsker Sam Shuster. I denne artikel fra British Medical Journal (27/7-2008) gennemgår han sine grunde. Der er en sikker forbindelse mellem sol og hudkræft − der typisk kan helbredes – men forbindelsen mellem sol og den farlige modermærkekræft højst usikker. Artiklen gengives med forfatterens tilladelse.

Is sun exposure a major cause of melanoma? No

The list of harmful things grows daily, freshly mined by descriptive epidemiology, a substitute for research that confuses association with cause. Although most disappear under the weight of their own inconsequence, the alleged increase in melanoma from ultraviolet radiation has survived on the life support of regular promotion. I am therefore setting out what is known, which is rather different from what is believed.

Does ultraviolet light cause melanoma?

There is solid descriptive, quantitative, and mechanistic proof that ultraviolet rays cause the main skin cancers (basal and squamous). They develop in pale, sun exposed skin, (1) are related to degree of exposure and latitude, (2) are fewer with avoidance and protection, (3,4) are readily produced experimentally, (4) and are the overwhelmingly predominant tumour in xeroderma pigmentosum, where DNA repair of ultraviolet light damage is impaired.

None of these is found with melanoma. Variation is more ethnic (5,6,7) than pigmentary, (8) and 75% occur on relatively unexposed sites, (9) especially the feet of dark skinned Africans. (6.7) The relation to latitude is small and inconsistent in, for example, Europe (10) and the United States (11); incidence and mortality fall with greater exposure (7,8,9,10,11,12,13,14,15,16,17); incidence is unaffected or increased by use of sunscreens (18); and the effect of sun bed exposure is small and inconsistent. (19) In addition, melanomas are difficult to produce experimentally with ultraviolet (20) light and are far less common than non-melanoma cancers in xeroderma pigmentosum.

Therefore, the effect of ultraviolet light can only be minimal, and the case against a major role is clear. Attempts to relate light exposure to surface area and site are irrelevant, since the cell of origin of melanoma and its distribution are unknown. The suggestion that the poor correlation of melanoma to ultraviolet light is because the causal event is sunburn from intermittent exposure in early life (13,14,15,16,17,21) is easily excluded, because the melanomas would then occur at the burn sites; there is no evidence for this, and it is unlikely that any will be found, because sunburn occurs in sun exposed sites, and these are not the sites at which melanomas occur. (7,8)

There is an association between melanoma and number of naevi, (13,22) and naevi increase after exposure to ultraviolet light (22,23); but this does not implicate ultraviolet light in the aetiology of melanoma, for the same reasons related to site. The likely explanation of the association is that stimulation of naevus growth by ultraviolet light simply increases the number of visible (and therefore countable) lesions. The associated histological changes can be indistinguishable from melanoma, as is the case with the benign lesions of lentigo maligna in elderly people, sun bed users, and psoriasis patients treated with psoralen and ultraviolet A; benign naevi stimulated by shave excision; and juvenile melanoma. Thus, unlike for squamous and basal cell cancers, there is no proof that ultraviolet light exposure is a significant cause of melanoma.

Is the reported increase in melanoma real?

In the past, naevi were left untreated and usually caused no harm. Then, fear of litigation and the search for early lesions led to removal of benign lesions; this introduced an ambiguity into histological classification, which eventually changed the definition of malignancy. Those who observed the process believe misdiagnosis of benign naevi explains the melanoma epidemic. (24) This view is supported by the findings of the Eastern region of England that the increase in new “melanomas” during 1991-2004 was entirely due to benign naevi (Levell et al, personal communication); a melanoma mountain in Australia has also been attributed to confusion with a benign disease. (25) The relation between incidence of new melanomas and higher social class (26) is best explained by removal of benign naevi after health warnings and encouragement to attend “pigmented lesion clinics”—the middle classes are always first on the scene. (27)

The subjective histopathological criteria used to diagnose melanoma have become too vague for use and are commonly found in benign disease. This problem can be resolved only by research, including a blind re-examination of histological slides used for past and present diagnoses, and a better distinction between benign and malignant changes in naevi. (24,27) Meanwhile, it can only be concluded that the reported increase in melanoma is probably an erroneous reclassification of benign naevi. Thus the question of whether ultraviolet light causes melanoma becomes irrelevant, because there is no case to answer.

Balancing the effects of ultraviolet light

Of course we know that ultraviolet light causes the common, virtually benign, and mostly trivial skin cancers and that, like smoking, it makes the skin look as if it has been well lived in. But is this enough to justify blanketing the sun when balanced against the possible advantages? We know the sun makes us feel better, although not how (28); we need skin synthesis of vitamin D for our bones; ultraviolet light may protect against some forms of cancer (29) including melanoma (14); and it has important, unexplained immunological effects. (30) We need to know much more before we can balance the biological books on ultraviolet radiation, even if we can now close the chapter on melanoma. (24)


Competing interests: None declared.

References

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  1. Harrison SL, MacLennan R, Speare R, Wronski I. Sun exposure and melanocytic naevi in young Australian children Lancet 1994;344:1529-32.
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Published 22 July 2008, doi:10.1136/bmj.a764
Cite this as: BMJ 2008;337:a764

Sam Shuster is Emeritus Professor of Dermatology at the University of Newcastle Upon Tyne, and Honorary Consultant to the Department of Dermatology, Norfolk and Norwich University Hospital.

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