How does sunlight cause melanomas on the soles of the feet?

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This article in the Daily Mail caught my eye today. It concerns, as often seems to be the case at this time of year, the perils of sun exposure. It includes the comments of a consultant dermatologist who tells us that she wears factor 30 or higher sunscreen on her face throughout the year, “even in November when it’s raining”, and that she never sits with the sun in my face. Well, you can’t be too careful. Expect, maybe you can, seeing as there is evidence linking the use of sunscreens with an increased risk of skin cancers, including malignant melanoma
. But only whisper these findings as, well, they seem to put a major spanner in the works of the ‘sunlight causes melanoma’ rhetoric.

And I say rhetoric because, actually, quite a lot of evidence points away from the idea that melanoma is chiefly caused by sun exposure. Some of the most salient research was summarised by emeritus professor of dermatology Sam Shuster in the British Medical Journal [1]. You can read the full text of this piece here. You can read the counter arguments here.

In his piece, Sam Shuster points out that about 75 per cent of melanomas occur in relatively unexposed sites of the body. Elsewhere, he draws our attention to a form of melanoma known as acral lentiginous melanoma, where typical sites include the soles of the feet, palms of the hand and the inside of the mouth. The Daily Mail piece today even draws our attention to this paradox, including in its title which references ‘hotspots’ for cancers including the soles of the feet.

In his BMJ piece, Sam Schuster also draws our attention to the evidence showing that in Europe and the US, melanoma incidence and deaths due to melanoma fall as sun exposure increases.

Some say that it’s not sunlight per se that causes melanoma but intermittent sun exposure and/or burning, especially in early life (the counter-argument piece argues this position). Sam Shuster pours cold water on this theory though, by writing that this theory: “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.”

The BMJ allows people to comment on articles on-line in the form of ‘rapid responses’. One of the rapid responses comes from surgeon who suggests that Sam Shuster has taken leave of his senses. Here’s an extract:

As a newly qualified doctor who spent 8 weeks last year studying at the Sydney Melanoma centre I was dumbfounded by the claims made against a link between sun exposure and melanoma. Having spent time with endless patients reporting hours of sun exposure and sunburn in earlier years and now presenting with cancerous lesions I believe that Dr Shuster may benefit from a similar “elective” in order to change his mind on the melanoma theory! Rubbishing the claims that the high incidences of melanoma do not occur in sun bathed areas seems bizarre.

Oh, dear. Notice the complete absence of reference to relevant science here. The observations this doctor made fit his pre-conceived beliefs so, voila, the answer is obvious (to him). There is a term for this sort of (faulty) thinking: confirmation bias. This doctor suggests Sam Shuster could do with an educational trip down-under, but I suggest he himself might take a trip to a relevant textbook or the internet to read about the scientific method.

But don’t lose complete faith in medical professionals just yet. Compare and contrast that first response with a later one which comes from another surgeon:

We most certainly do NOT know for sure that sunlight exposure is directly responsible for melanoma. As Schuster so rightly states, debate based on opinion is precisely what we don’t need in this area. We need hard evidence, and our profession isn’t always the best at offering advice on this basis.

Good to see this doctor thinking straight, and not falling into the trap of propagating ideas that seem to be based more on folklore than fact.


1. Shuster S. Is sun exposure a major cause of melanoma? No BMJ 2008;337:a764 [hr]

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